Discussion in 'The Lounge' started by Fishers of Men, Apr 11, 2008.

  1. Fishers of Men

    Fishers of Men Senior Member

    ATTENTION: You will need to go through part 1 before attempting this thread this is a continuation of part 1.

    PART 1

    CONTINUATION of Bowditch:

    CHAPTER 30
    3000. Introduction
    Because the nautical chart is so essential to safe navigation, it is important for the mariner to understand the capabilities and limitations of both digital and paper charts. Previous chapters have dealt with horizontal and vertical datums, chart projections, and other elements of cartographic science. This chapter will explain some basic concepts of hydrography and cartography which are important to the navigator, both as a user and as a source of data. Hydrography is the science of measurement and description of all of the factors which affect navigation, including depths, shorelines, tides, currents, magnetism, and other factors. Cartography is the final step in a long process which leads from raw data to a usable chart for the mariner. The mariner, in addition to being the primary user of hydrographic data, is also an important source of data used in the production and correction of nautical charts. This chapter discusses the processes involved in producing a nautical chart, whether in digital or paper form, from the initial planning of a hydrographic survey to the final printing. With this information, the mariner can better evaluate the information which comes to his attention and can forward it in a form that will be most useful to charting agencies, allowing them to produce more accurate and useful charts.

    3001. Planning The Survey
    The basic documents used to produce nautical charts are hydrographic surveys. Much additional information is included, but the survey is central to the compilation of a chart. A survey begins long before actual data collection starts. Some elements which must be decided are:
    • Exact area of the survey.
    • Type of survey (reconnaissance or standard) and scale to meet standards of chart to be produced.
    • Scope of the survey (short or long term).
    • Platforms available (ships, launches, aircraft, leased vessels, cooperative agreements).
    • Support work required (aerial or satellite photography, geodetics, tides).
    • Limiting factors (budget, political or operational constraints, positioning systems limitations, logistics).
    Once these issues are decided, all information available in the survey area is reviewed. This includes aerial photography, satellite data, topographic maps, existing nautical charts, geodetic information, tidal information, and anything else affecting the survey. The survey planners then compile sound velocity information, climatology, water clarity data, any past survey data, and information from lights lists, sailing directions, and notices to mariners. Tidal information is thoroughly reviewed and tide gauge locations chosen. Local vertical control data is reviewed to see if it meets the expected accuracy standards, so the tide gauges can be linked to the vertical datum used for the survey. Horizontal control is reviewed to check for accuracy and discrepancies and to determine sites for local positioning systems to be used in the survey.

    Line spacing refers to the distance between tracks to be run by the survey vessel. It is chosen to provide the best coverage of the area using the equipment available. Line spacing is a function of the depth of water, the sound footprint of the collection equipment to be used, and the complexity of the bottom. Once line spacing is chosen, the hydrographer can compute the total miles of survey track to be run and have an idea of the time required for the survey, factoring in the expected weather and other possible delays. The scale of the survey, orientation to the shorelines in the area, and the method of positioning determine line spacing. Planned tracks are laid out so that there will be no gaps between sound lines and sufficient overlaps between individual survey areas.

    Lines with spacing greater than the primary survey’s line spacing are run at right angles to the primary survey development to verify data repeatability. These are called cross check lines.

    Other tasks to be completed with the survey include bottom sampling, seabed coring, production of sonar pictures of the seabed, gravity and magnetic measurements (on deep ocean surveys), and sound velocity measurements in the water column.

    3002. Echo Sounders In Hydrographic Surveying
    Echo sounders were developed in the early 1920s, and compute the depth of water by measuring the time it takes for a pulse of sound to travel from the source to the sea bottom and return. A device called a transducer converts electrical energy into sound energy and vice versa. For basic hydrographic surveying, the transducer is mounted permanently in the bottom of the survey vessel, which then follows the planned trackline, generating soundings along the track. The major difference between different types of echo sounders is in the frequencies they use. Transducers can be classified according to their beam width, frequency, and power rating. The sound radiates from the transducer in a cone, with about 50% actually reaching to sea bottom. Beam width is determined by the frequency of the pulse and the size of the transducer. In general, lower frequencies produce a wider beam, and at a given frequency, a smaller transducer will produce a wider beam. Lower frequencies also penetrate deeper into the water, but have less resolution in depth. Higher frequencies have greater resolution in depth, but less range, so the choice is a trade-off. Higher frequencies also require a smaller transducer. A typical low frequency transducer operates at 12 kHz and a high frequency one at 200 kHz.


    where D is depth from the water surface, V is the average velocity of sound in the water column, T is round-trip time for the pulse, K is the system index constant, and Dr is the depth of the transducer below the surface (which may not be the same as vessel draft). V, Dr, and T can be only generally determined, and K must be determined from periodic calibration. In addition, T depends on the distinctiveness of the echo, which may vary according to whether the sea bottom is hard or soft. V will vary according to the density of the water, which is determined by salinity, temperature, and pressure, and may vary both in terms of area and time. In practice, average sound velocity is usually measured on site and the same value used for an entire survey unless variations in water mass are expected. Such variations could occur, for example, in areas of major currents. While V is a vital factor in deep water surveys, it is normal practice to reflect the echo sounder signal off a plate suspended under the ship at typical depths for the survey areas in shallow waters. The K parameter, or index constant, refers to electrical or mechanical delays in the circuitry, and also contains any constant correction due to the change in sound velocity between the upper layers of water and the average used for the whole project. Further, vessel speed is factored in and corrections are computed for settlement and squat, which affect transducer depth. Vessel roll, pitch, and heave are also accounted for. Finally, the observed tidal data is recorded in order to correct the soundings during processing.

    Tides are accurately measured during the entire survey so that all soundings can be corrected for tide height and thus reduced to the chosen vertical datum. Tide corrections eliminate the effect of the tides on the charted waters and ensure that the soundings portrayed on the chart are the minimum available to the mariner at the sounding datum. Observed, not predicted, tides are used to account for both astronomically and meteorlogically induced water level changes during the survey.

    3003. Collecting Survey Data
    While sounding data is being collected along the planned tracklines by the survey vessel(s), a variety of other related activities are taking place. A large-scale boat sheet is produced with many thousands of individual soundings plotted. A complete navigation journal is kept of the survey vessel’s position, course and speed. Side-scan sonar may be deployed to investigate individual features and identify rocks, wrecks, and other dangers. Time is the parameter which links the ship’s position with the various echograms, sonograms, journals, and boat sheets that make up the hydrographic data package.

    3004. Processing Hydrographic Data
    During processing, echogram data and navigational data are combined with tidal data and vessel/equipment corrections to produce reduced soundings. This reduced data is combined on a plot of the vessel’s actual track the boat sheet data to produce a smooth sheet. A contour overlay is usually made to test the logic of all the data shown. All anomolous depths are rechecked in either the survey records or in the field. If necessary, sonar data are then overlayed to analyze individual features as related to depths. It may take dozens of smooth sheets to cover the area of a complete survey. The smooth sheets are then ready for cartographers, who will choose representative soundings manually or using automated systems from thousands shown, to produce a nautical chart. Documentation of the process is such that any individual sounding on any chart can be traced back to its original uncorrected value. See Figure 3004.


    3005. Recent Developments In Hydrographic Surveying
    The evolution of echo sounders has followed the same pattern of technological innovation seen in other areas. In the 1940s low frequency/wide beam sounders were developed for ships to cover larger ocean areas in less time with some loss of resolution. Boats used smaller sounders which usually required visual monitoring of the depth. Later, narrow beam sounders gave ship systems better resolution using higher frequencies, but with a corresponding loss of area. These were then combined into dual-frequency systems. All echo sounders, however, used a single transducer, which limited surveys to single lines of soundings. For boat equipment, automatic recording became standard.

    The last three decades have seen the development of multiple-transducer, multiple-frequency sounding systems which are able to scan a wide area of seabed. Two general types are in use. Open waters are best surveyed using an array of transducers spread out athwartships across the hull of the survey vessel. They may also be deployed from an array towed behind the vessel at some depth to eliminate corrections for vessel heave, roll, and pitch. Typically, as many as 16 separate transducers are arrayed, sweeping an arc of 90°. The area covered by these swath survey systems is thus a function of water depth. In shallow water, track lines must be much closer together than in deep water. This is fine with hydrographers, because shallow waters need more closely spaced data to provide an accurate portrayal of the bottom on charts. The second type of multiple beam system uses an array of vertical beam transducers rigged out on poles abeam the survey vessel with transducers spaced to give overlapping coverage for the general water depth. This is an excellent configuration for very shallow water, providing very densely spaced soundings from which an accurate picture of the bottom can be made for harbor and small craft charts. The width of the swath of this system is fixed by the distance between the two outermost transducers and is not dependent on water depth.


    A recent development is Airborne Laser Hydrography (ALH). An aircraft flies over the water, transmitting a laser beam. Part of the generated laser beam is reflected by the water’s surface, which is noted by detectors. The rest penetrates to the sea bottom and is also partially reflected; this is also detected. Water depth can be computed from the difference in times of receipt of the two reflected pulses. Two different wavelength beams can also be used, one which reflects off the surface of the water, and one which penetrates and is reflected off the sea bottom. The obvious limitation of this system is water clarity. However, no other system can survey at 200 or so miles per hour while operating directly over shoals, rocks, reefs, and other hazards to boats. Both polar and many tropical waters are suitable for ALH systems. Depth readings up to 40 meters have been made, and at certain times of the year, some 80% of the world’s coastal waters are estimated to be clear enough for ALH.

    3006. Chart Accuracies
    The chart results from a hydrographic survey can be no more accurate than the survey; the survey’s accuracy, in turn, is limited by the positioning system used. For many older charts, the positioning system controlling data collection involved using two sextants to measure horizontal angles between signals established ashore. The accuracy of this method, and to a lesser extent the accuracy of modern, shore based electronic positioning methods, deteriorates rapidly with distance. This often determined the maximum scale which could be considered for the final chart. With the advent of the Global Positioning System (GPS) and the establishment of Differential GPS networks, the mariner can now navigate with greater accuracy than could the hydrographic surveyor who collected the chart source data. Therefore, exercise care not to take shoal areas or other hazards closer aboard than was past practice because they may not be exactly where charted. This is in addition to the caution the mariner must exercise to be sure that his navigation system and chart are on the same datum. The potential danger to the mariner increases with digital charts because by zooming in, he can increase the chart scale beyond what can be supported by the source data. The constant and automatic update of the vessels position on the chart display can give the navigator a false sense of security, causing him to rely on the accuracy of a chart when the source data from which the chart was compiled cannot support the scale of the chart displayed.

    3007. Navigational And Oceanographic Information
    Mariners at sea, because of their professional skills and location, represent a unique data collection capability unobtainable by any government agency. Provision of high quality navigational and oceanographic information by government agencies requires active participation by mariners in data collection and reporting. Examples of the type of information required are reports of obstructions, shoals or hazards to navigation, sea ice, soundings, currents, geophysical phenomena such as magnetic disturbances and subsurface volcanic eruptions, and marine pollution. In addition, detailed reports of harbor conditions and facilities in both busy and out-of-the way ports and harbors helps charting agencies keep their products current. The responsibility for collecting hydrographic data by U.S. Naval vessels is detailed in various directives and instructions. Civilian mariners, because they often travel to a wider range of ports, also have an opportunity to contribute substantial amounts of information.

    3008. Responsibility For Information
    The Defense Mapping Agency, the U.S. Naval Oceanographic Office (NAVOCEANO), the U.S. Coast Guard and the Coast and Geodetic Survey (C&GS) are the primary agencies which receive, process, and disseminate marine information in the U.S.
    DMA provides charts and chart update (Notice to Mariners) and other nautical materials for the U.S. military services and for navigators in general in waters outside the U.S. NAVOCEANO conducts hydrographic and oceanographic surveys of primarily foreign or international waters, and disseminates information to naval forces, government agencies, and civilians.

    The Coast and Geodetic Survey (C&GS) conducts hydrographic and oceanographic surveys and provides charts for marine and air navigation in the coastal zones of the United States and its territories.
    The U.S. Coast Guard is charged with protecting safety of life and property at sea, maintaining aids to navigation, and improving the quality of the marine environment. In the execution of these duties, the Coast Guard collects, analyzes, and disseminates navigational and oceanographic data. Modern technology allows contemporary navigators to contribute to the body of hydrographic and oceanographic information.

    Navigational reports are divided into four categories:
    1. Safety Reports
    2. Sounding Reports
    3. Marine Data Reports
    4. Port Information Reports

    The seas and coastlines continually change through the actions of man and nature. Improvements realized over the years in the nautical products published by DMAHTC, NOS, and U.S. Coast Guard have been made possible largely by the reports and constructive criticism of seagoing observers, both naval and merchant marine. DMAHTC and NOS continue to rely to a great extent on the personal observations of those who have seen the changes and can compare charts and publications with actual conditions. In addition, many ocean areas and a significant portion of the world’s coastal waters have never been adequately surveyed for the purpose of producing modern nautical charts. Information from all sources is evaluated and used in the production and maintenance of DMAHTC, NOS and Coast Guard charts and publications. Information from surveys, while originally accurate, is subject to continual change. As it is impossible for any hydrographic office to conduct continuous worldwide surveys, reports of changing conditions depend on the mariner. Such reports provide a steady flow of valuable information from all parts of the globe. After careful analysis of a report and comparison with all other data concerning the same area or subject, the organization receiving the information takes appropriate action. If the report is of sufficient urgency to affect the immediate safety of navigation, the information will be broadcast as a SafetyNET or NAVTEX message. Each report is compared with others and contributes in the compilation, construction, or correction of charts and publications. It is only through the constant flow of new information that charts and publications can be kept accurate and up-to-date.
    A convenient Data Collection Kit is available free from
    DMAHTC and NOS sales agents and from DMAHTC Representatives.
    The stock number is HYDRODATAKIT.

    3009. Safety Reports
    Safety reports are those involving navigational safety which must be reported and disseminated by message. The types of dangers to navigation which will be discussed in this section include ice, floating derelicts, wrecks, shoals, volcanic activity, mines, and other hazards to shipping. 1. Ice—Mariners encountering ice, icebergs, bergy bits, or growlers in the North Atlantic should report to Commander, International Ice Patrol, Groton, CT through a U.S. Coast Guard Communications Station. Direct printing radio teletype (SITOR) is available through USCG Communications Stations Boston or Portsmouth.
    Satellite telephone calls may be made to the Ice Patroloffice in Groton, Connecticut throughout the season at (203) 441-2626 (Ice Patrol Duty Officer). Messages can also be sent through Coast Guard Operations Center, Boston at (617) 223-8555.

    When sea ice is observed, the concentration, thickness, and position of the leading edge should be reported. The size, position, and, if observed, rate and direction of drift, along with the local weather and sea surface temperature, should be reported when icebergs, bergy bits, or growlers are encountered. Ice sightings should also be included in the regular synoptic ship weather report, using the five-figure group following the indicator for ice. This will assure the widest distribution to all interested ships and persons. In addition, sea surface temperature and weather reports should be made to COMINTICEPAT every 6 hours by vessels within latitude 40°N and 52°N and longitude 38°W and 58°W, if a routine weather report is not made to METEO Washington. 2. Floating Derelicts—All observed floating and drifting dangers to navigation that could damage the hull or propellers of a vessel at sea should be immediately reported by radio. The report should include a brief description of the danger, the date, time (GMT) and the location (latitude and longitude).
    3.Wrecks/Man-Made Obstructions—
    Information is needed to assure accurate charting of wrecks, man-made obstructions, other objects dangerous to surface and submerged navigation, and repeatable sonar contacts that may be of interest to the U.S. Navy. Man-made obstructions not in use or abandoned are particularly hazardous if unmarked and should be reported immediately. Examples include abandoned wellheads and pipelines, submerged platforms and pilings, and disused oil structures. Ship sinkings, strandings, disposals. or salvage data are also reportable, along with any large amounts of debris, particularly metallic. Accuracy, especially in position, is vital: therefore, the date and time of the observation of the obstruction as well as the method used in establishing the position, and an estimate of the fix accuracy should be included. Reports should also include the depth of water, preferably measured by soundings (in fathoms or meters). If known, the name, tonnage, cargo, and cause of casualty should be provided. Data concerning wrecks, man-made obstructions, other sunken objects, and any salvage work should be as complete as possible. Additional substantiating information is encouraged.
    4. Shoals—When a vessel discovers an uncharted or erroneously charted shoal or an area that is dangerous to navigation, all essential details should be immediately reported to DMAHTC WASHINGTON DC via radio. An uncharted depth of 300 fathoms or less is considered an urgent danger to submarine navigation. Immediately upon receipt of messages reporting dangers to navigation, DMAHTC issues appropriate NAVAREA warnings. The information must appear on published charts as “reported” until sufficient substantiating evidence (i.e. clear and properly annotated echograms and navigation logs, and any other supporting information) is received.

    Therefore, originators of shoal reports are requested to verify and forward all substantiating evidence to DMAHTC at the earliest opportunity. It cannot be overemphasized that clear and properly annotated echograms and navigation logs are especially important in shoal reports.
    5. Volcanic Activity—Volcanic disturbances may be observed from ships in many parts of the world. On occasion, volcanic eruptions may occur beneath the surface of the water. These submarine eruptions may occur more frequently and be more widespread than has been suspected in the past. Sometimes the only evidence of a submarine eruption is a noticeable discoloration of the water, a marked rise in sea surface temperature, or floating pumice. Mariners witnessing submarine activity have reported steams with a foul sulfurous odor rising from the sea surface, and strange sounds heard through the hull, including shocks resembling a sudden grounding. A subsea volcanic eruption may be accompanied by rumbling and hissing as hot lava meets the cold sea. In some cases, reports of discolored water at the sea surface have been investigated and found to be the result of newly formed volcanic cones on the sea floor. These cones can grow rapidly (within a few years) to constitute a hazardous shoal. It is imperative that a mariner report evidence of volcanic activity immediately to DMAHTC by message. Additional substantiating information is encouraged.
    6. Mines—All mines or objects resembling mines should be considered armed and dangerous. An immediate radio report to DMAHTC should include (if possible):
    1. Greenwich Mean Time and date.
    2. Position of mine, and how near it was approached.
    3. Size, shape, color, condition of paint, and presence of marine growth.
    4. Presence or absence of horns or rings.
    5. Certainty of identification.

    3010. Instructions For Safety Report Messages
    The International Convention for the Safety of Life at Sea (1974), which is applicable to all U.S. flag ships, requires:
    “The master of every ship which meets with dangerous ice, dangerous derelict, or any other direct danger to navigation, or a tropical storm, or encounters subfreezing air temperatures associated with gale force winds causing severe ice accretion on superstructures, or winds of force 10 or above on the Beaufort scale for which no storm warning has been received, is bound to communicate the information by all means at his disposal to ships in the vicinity, and also to the competent authorities at the first point on the coast with which he can communicate.” The report should be broadcast first on 2182 kHz prefixed by the safety signal “SECURITE.” This should be followed by transmission of the message on a suitable working frequency to the proper shore authorities. The transmission of information regarding ice, derelicts, tropical storms, or any other direct danger to navigation is obligatory. The form in which the information is sent is not obligatory. It may be transmitted either in plain language (preferably English) or by any means of International Code of Signals (wireless telegraphy section). It should be issued CQ to all ships and should also be sent to the first station with which communication can be made with the request that it be transmitted to the appropriate authority. A vessel will not be charged for radio messages to government authorities reporting dangers to navigation.
    Each radio report of a danger to navigation should answer briefly three questions:
    1. What? A description to of the object or phenomenon.
    2. Where? Latitude and longitude.
    3. When? Greenwich Mean Time (GMT) and date.
    SECURITE. DERELICT: OBSERVED WOODEN 25 METER DERELICT ALMOST SUBMERGED AT 4406N, 1243W AT 1530 GMT, APRIL 21. The report should be addressed to one of the following shore authorities as appropriate:
    1. U.S. Inland Waters—Commander of the Local Coast Guard District.
    2. Outside U.S. Waters—DMAHTC WASHINGTON, DC.
    Whenever possible, messages should be transmitted via the nearest government radio station. If it is impractical to use a government station, a commercial station may be used. U.S. government navigational warning messages should invariably be sent through U.S. radio stations, government or commercial, and never through foreign stations.
    Detailed instructions for reporting via radio are contained
    in DMAHTC Pub. 117, Radio Navigation Aids.

    3011. Sounding Reports
    Acquisition of reliable sounding data from all ocean areas of the world is a continuing effort of DMAHTC, NAVOCEANO, and NOS. There are vast ocean areas where few soundings have ever been acquired. Much of the bathymetric data shown on charts has been compiled from information submitted by mariners. Continued cooperation in observing and submitting sounding data is absolutely necessary to enable the compilation of accurate charts. Compliance with sounding data collection procedures by merchant ships is voluntary, but for U.S. Naval vessels compliance is required under various fleet directives.

    3012. Areas Where Soundings Are Needed
    Prior to a voyage, navigators can determine the importance of recording sounding data by checking the charts for the route. Any ship crossing a densely sounded shipping lane perpendicular or nearly perpendicular to the lane can obtain very useful sounding data despite the density. Such tracks provide cross checks for verifying existing data. Other indications that soundings may be particularly useful are:
    1. Old sources listed on source diagram or source note on chart.
    2. Absence of soundings in large areas.
    3. Presence of soundings, but only along well-defined lines indicating the track of the sounding vessel, with few or no sounding between tracks.
    4. Legends such as “Unexplored area.”

    3013. Fix Accuracy
    A realistic goal of open ocean positioning for sounding reports is ±1 nautical mile with the continuous use of GPS. However, depths of 300 fathoms or less should always be reported regardless of the fix accuracy. When such depths are uncharted or erroneously charted, they should be reported by message to DMAHTC WASHINGTON DC, giving the best available positioning accuracy. Echograms and other supporting information should then be forwarded by mail to DMAHTC.

    The accuracy goal noted above has been established to enable DMAHTC to create a high quality data base which will support the compilation of accurate nautical charts. It is particularly important that reports contain the navigator’s best estimate of his fix accuracy and that the positioning aids being used (GPS, Loran C, etc.) be identified.

    3014. False Shoals
    Many poorly identified shoals and banks shown on charts are probably based on encounters with the Deep Scattering Layer (DSL), ambient noise, or, on rare occasions, submarine earthquakes. While each appears real enough at the time of its occurrence, a knowledge of the events that normally accompany these incidents may prevent erroneous data from becoming a charted feature. The DSL is found in most parts of the world. It consists of a concentration of marine life which descends from near the surface at sunrise to an approximate depth of 200 fathoms during the day. It returns near the surface at sunset. Although at times the DSL may be so concentrated that it will completely mask the bottom, usually the bottom return can be identified at its normal depth at the same time the DSL is being recorded.
    Ambient noise or interference from other sources can cause erroneous data. This interference may come from equipment on board the ship, from another transducer being operated close by, or from waterborne noise. Most of these returns can be readily identified on the echo sounder records and should cause no major problems; however, on occasion they may be so strong and consistent as to appear as the true bottom. Finally, a volcanic disturbance beneath the ship or in the immediate vicinity may give erroneous indications of a shoal. The experience has at times been described as similar to running aground or striking a submerged object. Regardless of whether the feature is an actual shoal or a submarine eruption the positions, date/time, and other information should be promptly reported to DMAHTC.

    3015. Doubtful Hydrographic Data
    Navigators are strongly requested to assist with the confirmation and proper charting of actual shoals and the removal from the charts of doubtful data which was erroneously reported.
    The classification or confidence level assigned to doubtful hydrographic data is indicated by the following standard symbols:
    Many of these reported features are sufficiently deep that if valid, a ship can safely navigate across the area. Confirmation of the existence of the feature will result in proper charting. On the other hand, properly collected and annotated sounding reports of the area may enable DMAHTC to accumulate sufficient evidence to justify the removal of the sounding from the chart.
    Abbreviation Meaning
    Rep (date) Reported (year)
    E.D. Existence Doubtful
    P.A. Position Approximate
    P.D. Position Doubtful

    3016. Preparation Of Sounding Reports
    The procedures for preparing sounding reports have been designed to minimize the efforts of the shipboard observers, yet provide the essential information needed by DMAHTC. Blank OCEANIC SOUNDING REPORT forms are available from DMAHTC as a stock item or through DMA Representatives in Los Angeles/Long Beach, New Orleans, and Washington, D.C. Submission of plotted sounding tracks is not required. Annotated echograms and navigation logs are preferred. The procedure for collecting sounding reports is for the ship to operate a recording echo sounder while transiting an area where soundings are desired. Fixes and course changes are recorded in the log, and the event marker is used to note these events on the echogram. Both the log and echogram can then be sent to DMAHTC whenever convenient. The following annotations or information should be clearly written on the echogram to ensure maximum use of the recorded depths:
    1. Ship’s name—At the beginning and end of each roll of echogram or portion.
    2. Date—Annotated at 1200 hours each day and when starting and stopping the echo sounder, or at least once per roll.
    3. Time—The echogram should be annotated at the beginning of the sounding run, at least once each hour thereafter, at every scale change, and at all breaks in the echogram record. Accuracy of these time marks is critical for correlation with ship’s position. 4.Time Zone—Greenwich Mean Time (GMT) should be used if practicable. In the event local zone times are used, annotate echogram whenever clocks are reset and identify zone time in use. It is most important that the echogram and navigation log use the same time basis.


    5. Phase or scale changes—If echo sounder does not indicate scale setting on echogram automatically, clearly label all depth phase (or depth scale) changes and the exact time they occur. Annotate the upper and lower limits of the echogram if necessary. Figure 3016a and Figure 3016b illustrates the data necessary to reconstruct a sounding track. If ship operations dictate that only periodic single ping soundings can be obtained, the depths may be recorded in the Remarks column. A properly annotated echogram is always strongly preferred by DMAHTC over single ping soundings whenever operations permit. The navigation log is vital to the reconstruction of a sounding track. Without the position information from the log, the echogram is virtually useless. The data received from these reports is digitized and becomes part of the digital bathymetric data library of DMAHTC. This library is used as the basis of new chart compilation. Even in areas where numerous soundings already exist, sounding reports allow valuable cross-checking to verify existing data and more accurately portray the sea floor. This is helpful to our Naval forces and particularly to the submarine fleet, but is also useful to geologists, geophysicists, and other scientific disciplines.
    A report of oceanic soundings should contain the following:
    1. A completed Oceanic Sounding Report, Form DMAHTC 8053/1.
    2. A detailed Navigation Log.
    3. The echo sounding trace, properly annotated.
    Each page of the report should be clearly marked with the ship’s name and date, so that it can be identified if it becomes separated. Mail the report to:
    DMA Hydrographic/Topographic Center
    MC, D-40
    4600 Sangamore Rd.
    Bethesda, MD, 20816-5003
    3017. Marine Information Reports
    Marine Information Reports are reports of items of navigational interest such as the following:
    1. Discrepancies in published information.
    2. Changes in aids to navigation.
    3. Electronic navigation reports.
    4. Satellite navigation reports.
    5. Radar navigation reports.
    6. Magnetic disturbances.
    Report any marine information which you believe may be useful to charting authorities or other mariners. Depending on the type of report, certain items of information are absolutely critical for a correct evaluation. The following general suggestions are offered to assist in reporting information that will be of maximum value:
    1. The geographical position included in the report may be used to correct charts. Accordingly, it should be fixed by the most exact method available, more than one if possible.
    2. If geographical coordinates are used to report position, they should be as exact as circumstances permit. Reference should be made to the chart by number, edition number, and date.
    3. The report should state the method used to fix the position and an estimate of fix accuracy.
    4. When reporting a position within sight of charted objects, the position may be expressed as bearings and ranges from them. Bearings should preferably be reported as true and expressed in degrees.
    5. Always report the limiting bearings from the ship toward the light when describing the sectors in which a light is either visible or obscured. Although this is just the reverse of the form used to locate objects, it is the standard method used on DMAHTC nautical charts and in Light Lists.
    6. A report prepared by one person should, if possible, be checked by another.

    In most cases marine information can be adequately reported on one of the various forms printed by DMAHTC or NOS. It may be more convenient to annotate information directly on the affected chart and mail it to DMAHTC. As an example, it may be useful to sketch uncharted or erroneously charted shoals, buildings, or geological features directly on the chart. Appropriate supporting information should also be provided.
    DMAHTC forwards reports applicable to NOS, NAVOCEANO, or Coast Guard products to the appropriate agency.
    Reports by letter are just as acceptable as those prepared on regular forms. A letter report will often allow more flexibility in reporting details, conclusions, or recommendations concerning the observation. When reporting on the regular forms, if necessary use additional sheets to complete the details of an observation.
    Reports are required concerning any errors in information published on nautical charts or in nautical publications. The ports should be as accurate and complete as possible. This will result in corrections to the information including the issuance of Notice to Mariners changes when appropriate. Report all changes, defects, establishment or discontinuance of navigational aids and the source of the information. Check your report against the light list, list of lights, Radio Aids to Navigation, and the largest scale chart of the area. If it is discovered that a new light has been established, report the light and its characteristics in a format similar to that carried in light lists and lists of lights. For changes and defects, report only elements that differ with light lists. If it is a lighted aid, identify by number. Defective aids to navigation in U.S. territorial waters should be reported immediately to the Commander of the local Coast Guard District.

    3018. Electronic Navigation Reports
    Electronic navigation systems such as GPS and LORAN have become an integral part of modern navigation. Reports on propagation anomalies or any unusual reception while using the electronic navigation system are desired.
    Information should include:
    1. Type of electronic navigation system and channel or frequency used.
    2. Type of antenna: whip, vertical or horizontal wire.
    3. Transmitting stations, rate or pair used.
    4. Nature and description of the reception.
    5. Type of signal match.
    6. Date and time.
    7. Position of own ship.
    8. Manufacturer and model of receiver.

    Calibration information is being collected in an effort to evaluate and improve the accuracy of the DMAHTC derived Loran signal propagation corrections incorporated in National Ocean Service Coastal Loran C charts. Loran C monitor data consisting of receiver readings with corresponding well defined reference positions are required. Mariners aboard vessels equipped with Loran C receiving units and having precise positioning capability independent of the Loran C system (i.e., docked locations or visual bearings, radar, GPS, Raydist, etc.) are requested to provide information to DMAHTC.

    3019. Radar Navigation Reports
    Reports of any unusual reception or anomalous propagation by radar systems caused by atmospheric conditions are especially desirable. Comments concerning the use of radar in piloting, with the locations and description of good radar targets, are particularly needed. Reports should include:
    1. Type of radar, frequency, antenna height and type.
    2. Manufacturer and model of the radar.
    3. Date, time and duration of observed anomaly.
    4. Position.
    5. Weather and sea conditions.

    Radar reception problems caused by atmospheric parameters are contained in four groups. In addition to the previously listed data, reports should include the following specific data for each group:
    1. Unexplained echoes—Description of echo, apparent velocity and direction relative to the observer, and range.
    2. Unusual clutter—Extent and Sector.
    3. Extended detection ranges—Surface or airborne target, whether point or distributed target, such as a coastline or landmass.
    4. Reduced detection ranges—Surface or airborne target, whether point or distributed target, such as a coastline or landmass.

    3020. Magnetic Disturbances
    Magnetic anomalies, the result of a variety of causes, exist in many parts of the world. DMAHTC maintains a record of such magnetic disturbances and whenever possible attempts to find an explanation. A better understanding of this phenomenon can result in more detailed charts which will be of greater value to the mariner. The report of a magnetic disturbance should be as specific as possible, for instance: “Compass quickly swung 190° to 170°, remained offset for approximately 3 minutes and slowly returned.” Include position, ship’s course, speed, date, and time.
    Whenever the readings of the standard magnetic compass are unusual, an azimuth check should be made as soon as possible and this information forwarded to DMAHTC.

    3021. Importance Of Port Information Reports
    Port Information Reports provide essential information obtained during port visits which can be used to update and improve coastal, approach, and harbor charts as well as nautical publications including Sailing Directions, Coast Pilots, and Fleet Guides. Engineering drawings, hydrographic surveys and port plans showing new construction affecting charts and publications are especially valuable. Items involving navigation safety should be reported by message. Items which are not of immediate urgency, as well as additional supporting information may be submitted by the Port Information Report (DMAHTC Form 8330-1), or the Notice to Mariners Marine Information Report and Suggestion Sheet found in the back of each Notice to Mariners. Reports by letter are completely acceptable and may permit more reporting flexibility.
    In some cases it may be more convenient and more effective to annotate information directly on a chart and mail it to DMAHTC. As an example, new construction, such as new port facilities, pier or breakwater modifications, etc., may be drawn on a chart in cases where a written report would be inadequate.

    Specific Navy reporting requirements exist for ships visiting foreign ports. These reports are primarily intended to provide information for use in updating the Navy Port Directories. A copy of the navigation information resulting from port visits should be provided directly to DMAHTC by including DMAHTC WASHINGTON DC/MCC// as an INFO addressee on messages containing hydrographic information.

    3022. What To Report
    Coastal features and landmarks are almost constantly changing. What may at one time have been a major landmark may now be obscured by new construction, destroyed, or changed by the elements. Sailing Directions (Enroute) and Coast Pilots utilize a large number of photographs and line sketches. Photographs, particularly a series of overlapping views showing the coastline, landmarks, and harbor entrances are very useful. Photographs and negatives can be used directly as views or in the making of line sketches. The following questions are suggested as a guide in preparing reports on coastal areas that are not included or that differ from the Sailing Directions and Coast Pilots.
    1. What is the first landfall sighted?
    2. Describe the value of soundings, radio bearings, GPS, LORAN, radar and other positioning systems in making a landfall and approaching the coast. Are depths, curves, and coastal dangers accurately charted?
    3. Are prominent points, headlands, landmarks, and aids to navigation adequately described in Sailing Directions and Coast Pilots? Are they accurately charted?
    4. Do land hazes, fog or local showers often obscure the prominent features of the coast?
    5. Do discolored water and debris extend offshore?
    How far? Were tidal currents or rips experienced along the coasts or in approaches to rivers or bays?
    6. Are any features of special value as radar targets?
    Tides and Currents
    1. Are the published tide and current tables accurate?
    2. Does the tide have any special effect such as river bore? Is there a local phenomenon, such as double high or low water interrupted rise and fall?
    3. Was any special information on tides obtained from local sources?
    4. What is the set and drift of tidal currents along coasts, around headlands among islands, in coastal indentations?
    5. Are tidal currents reversing or rotary? If rotary, do they rotate in a clockwise or counterclockwise direction?
    6. Do subsurface currents affect the maneuvering of surface craft? If so, describe.
    7. Are there any countercurrents, eddies, overfalls, or tide rips in the area? If so, locate.
    River and Harbor Entrances
    1. What is the depth of water over the bar, and is it subject to change? Was a particular stage of tide necessary to permit crossing the bar?
    2. What is the least depth in the channel leading from sea to berth?
    3. If the channel is dredged, when and to what depth and width? Is the channel subject to silting?
    4. What is the maximum draft, length, and width of a vessel that can be taken into port?
    5. If soundings were taken, what was the stage of tide? Were the soundings taken by echo sounder or lead line? If the depth information was received from other sources, what were they?
    6. What was the date and time of water depth observations?
    Hills, Mountains, and Peaks
    1. Are hills and mountains conical, flat-topped, or of any particular shape?
    2. At what range are they visible in clear weather?
    3. Are they snowcapped throughout the year?
    4. Are they cloud-covered at any particular time?
    5. Are the summits and peaks adequately charted?
    Can accurate distances and/or bearings be obtained by sextant, pelorus, or radar?
    6. What is the quality of the radar return?
    1. Where is the signal station located?
    2. Where does the pilot board the vessel? Are special arrangements necessary before a pilot boards?
    3. Is pilotage compulsory? Is it advisable?
    4. Will a pilot direct a ship in at night, during foul weather, or during periods of low visibility?
    5. Where does the pilot boat usually lie?
    6. Does the pilot boat change station during foul weather?
    7. Describe the radiotelephone communication facilities available at the pilot station or pilot boat. What is the call-sign, frequency, and the language spoken?
    1. What cautionary advice, additional data, and information on outstanding features should be given to a mariner entering the area for the first time?
    2. At any time did a question or need for clarification arise while using DMAHTC, NOS, or Coast Guard products?
    3. Were charted land contours useful while navigating using radar? Indicate the charts and their edition numbers.
    4. Would it be useful to have radar targets or topographic features that aid in identification or position plotting described or portrayed in the Sailing Directions and Coast Pilots?
    The overlapping photograph method for panoramic views should be used. On the back of the photograph (negatives should accompany the required information), indicate the camera position by bearing and distance from a fixed, charted object if possible, name of the vessel, the date, time of exposure, and height of tide. All features of navigational value should be clearly and accurately identified on an overlay, if time permits. Bearings and distances (from the vessel) of uncharted features, identified on the print, should be included.
    Radarscope Photography
    Because of the value of radar as an aid to navigation, DMAHTC desires radarscope photographs. Guidelines for radar settings for radarscope photography are given in Radar
    Navigation Manual, Pub. 1310. Such photographs, reproduced in the Sailing Directions and Fleet Guides, supplement textual information concerning critical navigational areas and assist the navigator in correlating the radarscope presentation with the chart. To be of the greatest value, radarscope photographs should be taken at landfalls, sea buoys, harbor approaches, major turns in channels, constructed areas and other places where they will most aid the navigator. Two glossy prints of each photograph are needed. One should be unmarked, the other annotated. Examples of desired photographs are images of fixed and floating navigational aids of various sizes and shapes as observed under different sea and weather conditions, and images of sea return and precipitation of various intensities. There should be two photographs of this type of image, one without the use of special anti clutter circuits and another showing remedial effects of these. Photographs of actual icebergs, growlers, and bergy bits under different sea conditions, correlated with photographs of their radarscope images are also desired.
    Radarscope photographs should include the following annotations:
    1. Wavelength.
    2. Antenna height and rotation rate.
    3. Range-scale setting and true bearing.
    4. Antenna type (parabolic, slotted waveguide).
    5. Weather and sea conditions, including tide.
    6. Manufacturer’s model identification.
    7. Position at time of observation.
    8. Identification of target by Light List, List of Lights, or chart.
    9. Camera and exposure data.
    Other desired annotations include:
    1. Beam width between half-power points.
    2. Pulse repetition rate.
    3. Pulse duration (width).
    4. Antenna aperture (width).
    5. Peak power.
    6. Polarization.
    7. Settings of radar operating controls, particularly use of special circuits.
    8. Characteristics of display (stabilized or unstabilized), diameter, etc.
    Port Regulations and Restrictions
    Sailing Directions (Planning Guides) are concerned with pratique, pilotage, signals, pertinent regulations, warning areas, and navigational aids. Updated and new information is constantly needed by DMAHTC. Port information is best reported on the prepared “Port Information Report”, DMAHTC form 8330-1. If this form is not available, the following questions are suggested as a guide to the requested data.
    1. Is this a port of entry for overseas vessels?
    2. If not a port of entry where must vessel go for customs entry and pratique?
    3. Where do customs, immigration, and health officials board?
    4. What are the normal working hours of officials?
    5. Will the officials board vessels after working hours?
    Are there overtime charges for after-hour services?
    6. If the officials board a vessel underway, do they remain on board until the vessel is berthed?
    7. Were there delays? If so, give details.
    8. Were there any restrictions placed on the vessel?
    9. Was a copy of the Port Regulations received from the local officials?
    10.What verbal instructions were received from the local officials?
    11.What preparations prior to arrival would expedite formalities?
    12. Are there any unwritten requirements peculiar to the port?
    13.What are the speed regulations?
    14.What are the dangerous cargo regulations? 15.What are the flammable cargo and fueling regulations?.
    16. Are there special restrictions on blowing tubes, pumping bilges. oil pollution, fire warps, etc.?
    17. Are the restricted and anchorage areas correctly shown on charts, and described in the Sailing Directions and Coast Pilots?
    18.What is the reason for the restricted areas; gunnery, aircraft operating, waste disposal, etc.?
    19. Are there specific hours of restrictions, or are local blanket notices issued?
    20. Is it permissible to pass through, but not anchor in, restricted areas?
    21. Do fishing boats, stakes, nets, etc., restrict navigation?
    22.What are the heights of overhead cables, bridges, and pipelines?
    23.What are the locations of submarine cables, their landing points, and markers?
    24. Are there ferry crossings or other areas of heavy local traffic?
    25.What is the maximum draft, length, and breadth of a vessel that can enter?
    Port Installations
    Much of the port information which appears in the Sailing Directions and Coast Pilots is derived from visit reports and port brochures submitted by mariners. Comments and recommendations on entering ports are needed so that corrections to these publications can be made. If extra copies of local port plans, diagrams, regulations, brochures, photographs, etc., can be obtained, send them to DMAHTC. It is not essential that they be printed in English. Local pilots, customs officials, company agents, etc., are usually good information sources.

    Much of the following information is included in the regular Port Information Report, but may be used as a check-off list when submitting a letter report.
    1. Name of the port.
    2. Date of observation and report.
    3. Name and type of vessel.
    4. Gross tonnage.
    5. Length (overall).
    6. Breadth (extreme).
    7. Draft (fore and aft).
    8. Name of captain and observer.
    9. U.S. mailing address for acknowledgment.
    Tugs and Locks
    1. Are tugs available or obligatory? What is their power?
    2. If there are locks, what is the maximum size and draft of a vessel that can be locked through?
    Cargo Handling Facilities
    1. What are the capacities of the largest stationary, mobile, and floating cranes available? How was this information obtained?
    2. What are the capacities, types, and number of lighters and barges available?
    3. Is special cargo handling equipment available (e.g.) grain elevators, coal and ore loaders, fruit or sugar conveyors, etc.?
    4. If cargo is handled from anchorage, what methods are used? Where is the cargo loaded? Are storage facilities available there?
    1. Are fuel oils, diesel oils, and lubricating oils available?
    If so, in what quantity?
    1. What are the dimensions of the pier, wharf, or basin used?
    2. What are the depths alongside? How were they obtained?
    3. Describe berth/berths for working containers or roll-on/ roll-off cargo.
    4. Does the port have berth for working deep draft tankers? If so, describe.
    5. What storage facilities are available, both dry and refrigerated?
    6. Are any unusual methods used when docking? Are special precautions necessary at berth?
    Medical, Consular, and Other Services
    1. Is there a hospital or the services of a doctor and dentist available?
    2. Is there a United States consulate? Where is it located?
    If none, where is the nearest?

    1. What are the limits of the anchorage areas?
    2. In what areas is anchorage prohibited?
    3. What is the depth, character of the bottom, types of holding ground, and swinging room avaiable?
    4. What are the effects of weather, sea, swell, tides, currents on the anchorages?
    5. Where is the special quarantine anchorage?
    6. Are there any unusual anchorage restrictions?
    Repairs and Salvage
    1. What are the capacities of drydocks and marine railways, if available?
    2. What repair facilities arc available? Are there repair facilities for electrical and electronic equipment?
    3. Are divers and diving gear available?
    4. Are there salvage tugs available? What is the size and operating radius?
    5. Are any special services, (e.g., compass compensation or degaussing,) available?

    3023. Ocean Current Reports
    The set and drift of ocean currents are of great concern to the navigator. Only with the correct current information can the shortest and most efficient voyages be planned. As with all forces of nature, most currents vary considerably with time at a given location. Therefore, it is imperative that DMAHTC receive ocean current reports on a continuous basis.
    The general surface currents along the principal trade routes of the world are well known; however, in other less traveled areas the current has not been well defined because of the lack of information. Detailed current reports from those areas are especially valuable.

    An urgent need exists for more inshore current reports along all coasts of the world because data in these regions are scarce. Furthermore, information from deep draft ships is needed as this type of vessel is significantly influenced by the deeper layer of surface currents.

    The CURRENT REPORT form, NAVOCEANO 3141/6, is designed to facilitate passing information to NAVOCEANO so that all mariners may benefit. The form is self-explanatory and can be used for ocean or coastal current information. Reports by the navigator will contribute significantly to accurate current information for nautical charts, Current Atlases, Pilot Charts, Sailing Directions and other special charts and publications.

    3024. Route Reports
    Route Reports enable DMAHTC, through its Sailing Directions (Planning Guides), to make recommendations for ocean passages based upon the actual experience of mariners. Of particular importance are reports of routes used by very large ships and from any ship in regions where, from experience and familiarity with local conditions, mariners have devised routes that differ from the “preferred track.” In addition, because of the many and varied local conditions which must be taken into account, coastal route information is urgently needed for updating both Sailing Directions and Coast Pilots.
    A Route Report should include a comprehensive summary of the voyage with reference to currents, dangers, weather, and the draft of the vessel. If possible, each report should answer the following questions and should include any other data that may be considered pertinent to the particular route. All information should be given in sufficient detail to assure accurate conclusions and appropriate recommendations.
    Some questions to be answered are:
    1. Why was the route selected?
    2. Were anticipated conditions met during the voyage?
  2. Fishers of Men

    Fishers of Men Senior Member

    I started part two because Reel suggested that "maybe the OGF software limits thread size to stop a person from spamming up the entire site ? ?

    Try removing your last post so we all can still read the entire thread then start a Mooglow part 2 starting with the removed last thread." Here's what was showing:

    *Fatal error*: Allowed memory size of 8388608 bytes exhausted (tried to
    allocate 1625207 bytes) in
    */home/ohiogame/public_html/community/includes/functions.php* on line *4840

    So, I did and it seems to be working again.

  3. Fishers of Men

    Fishers of Men Senior Member

    CHAPTER 31
    3100. The Importance Of Oceanography
    Oceanography is the application of the sciences to the phenomena of the oceans. It includes a study of their physical, chemical, and geological forms, and biological features. Thus, it embraces the widely separated fields of geography, geology, chemistry, physics, and biology, along with their many subdivisions, such as sedimentation, ecology, bacteriology, biochemistry, hydrodynamics, acoustics, and optics. The oceans cover 70.8 percent of the surface of the earth. The Atlantic covers 16.2 percent, the Pacific 32.4 percent (3.2 percent more than the land area of the entire earth), the Indian Ocean 14.4 percent, and marginal and adjacent areas (of which the largest is the Arctic Ocean) 7.8 percent. Their extent alone makes them an important subject for study. However, greater incentive lies in their use for transportation, their influence upon weather and climate, and their potential as a source of power, food, fresh water, minerals, and organic substances.

    3101. Origin Of The Oceans
    The structure of the continents is fundamentally different from that of the oceans. The rocks underlying the ocean floors are more dense than those underlying the continents. According to one theory, all the earth’s crust floats on a central liquid core, and the portions that make up the continents, being lighter, float with a higher freeboard. Thus, the thinner areas, composed of heavier rock, form natural basins where water has collected. The shape of the oceans is constantly changing due to continental drift. The surface of the earth consists of many different “plates.” These plates are joined along fracture or fault lines. There is constant and measurable movement of these plates at rates of 0.02 meters per year or more. The origin of the water in the oceans is unclear. Although some geologists have postulated that all the water existed as vapor in the atmosphere of the primeval earth, and that it fell in great torrents of rain as soon as the earth cooled sufficiently, another school holds that the atmosphere of the original hot earth was lost, and that the water gradually accumulated as it was given off in steam by volcanoes, or worked to the surface in hot springs.

    Most of the water on the earth’s crust is now in the oceans–about 1,370,000,000 cubic kilometers, or about 85 percent of the total. The mean depth of the ocean is 3,795 meters, and the total area is 360,000,000 square kilometers.

    3102. Chemical Description
    Oceanographic chemistry may be divided into three main parts: the chemistry of (1) seawater, (2) marine sediments, and (3) organisms living in the sea. The first is of particular interest to the navigator.
    Chemical properties of seawater are usually determined by analyzing samples of water obtained at various locations and depths. Samples of water from below the surface are obtained with special bottles designed for this purpose. The open bottles are mounted in a rosette which is attached to the end of a wire cable which contains insulated electrical wires. The rosette is lowered to the depth of the deepest sample, and a bottle is closed electronically. As the rosette is raised to the surface, other bottles are closed at the desired depths. Sensors have also been developed to measure a few chemical properties of sea water continuously. Physical properties of seawater are dependent primarily upon salinity, temperature, and pressure. However, factors like motion of the water, and the amount of suspended matter, affect such properties as color and transparency, conduction of heat, absorption of radiation, etc.

    3103. Salinity
    Salinity is a measure of the amount of dissolved solid material in the water. It has been defined as the total amount of solid material in grams contained in one kilogram of seawater when carbonate has been converted to oxide, bromine and iodine replaced by chlorine, and all organic material completely oxidized. It is usually expressed as parts per thousand (by weight), for example the average salinity of sea water is 35 grams per kilogram which would be written “35 ppt” or “35 ‰”. Historically the determination of salinity was a slow and difficult process, while the amount of chlorine ions (plus the chlorine equivalent of the bromine and iodine), called chlorinity, could be determined easily and accurately by titration with silver nitrate. From chlorinity, the salinity was determined by a relation based upon the measured ratio of chlorinity to total dissolved substances: Salinity = 1.80655 ´ Chlorinity

    This is now called the absolute salinity, (SA). (lowered "A") With titration techniques, salinity could be determined to about 0.02 parts per thousand.
    This definition of salinity has now been replaced by the Practical Salinity Scale, (S). Using this scale, the salinity of a seawater sample is defined as the ratio between the conducutivity of the sample and the conductivity of a standard potassium chloride (KCl) sample.
    As salinity on the practical scale is defined to be conservative with respect to addition and removal of water, the entire salinity range is accessible through precise weight dilution or evaporation without additional definitions. Since practical salinity is a ratio, it has no physical units but is designated practical salinity units, or psu.

    The Practical Salinity Scale, combined with modern conductivity cells and bench salinometers, provides salinity measurements which are almost an order of magnitude more accurate and precise, about 0.003 psu, than titration. Numerically, absolute salinity and salinity are nearly equal.

    It has also been found that electrical conductivity is better related to density than chlorinity. Since one of the main reasons to measure salinity is to deduce the density, this favors the Practical Salinity Scale as well.

    Salinity generally varies between about 33 and 37 psu. However, when the water has been diluted, as near the mouth of a river or after a heavy rainfall, the salinity is somewhat less; and in areas of excessive evaporation, the salinity may be as high as 40 psu. In certain confined bodies of water, notably the Great Salt Lake in Utah, and the Dead Sea in Asia Minor, the salinity is several times this maximum.

    3104. Temperature
    Temperature in the ocean varies widely, both horizontally and with depth. Maximum values of about 32°C are encountered at the surface in the Persian Gulf in summer, and the lowest possible values of about –2°C; the usual minimum freezing point of seawater) occur in polar regions.

    Except in the polar regions, the vertical distribution of temperature in the sea nearly everywhere shows a decrease of temperature with depth. Since colder water is denser (assuming the same salinity), it sinks below warmer water.
    This results in a temperature distribution just opposite to that of the earth’s crust, where temperature increases with depth below the surface of the ground.

    In the sea there is usually a mixed layer of isothermal water below the surface, where the temperature is the same as that of the surface. This layer is caused by two physical processes: wind mixing, and convective overturning as surface water cools and becomes more dense. The layer is best developed in the Arctic and Antarctic regions, and in seas like the Baltic and Sea of Japan during the winter, where it may extend to the bottom of the ocean. In the Tropics, the wind-mixed layer may exist to a depth of 125 meters, and may exist throughout the year. Below this layer is a zone of rapid temperature decrease, called the thermocline. At a depth greater than 400 m, the temperature everywhere is below 15°C. In the deeper layers, fed by cooled waters that have sunk from the surface in the Arctic and Antarctic, temperatures as low as –2°C exist.

    In the colder regions the cooling creates the convective overturning and isothermal water in the winter; but in the summer a seasonal thermocline is created as the upper water becomes warmer. A typical curve of temperature at various depths is shown in Figure 3110a.

    Temperature is commonly measured with either a platinum or copper resistance thermometer or a thermistor (devices that measure the change in conductivity of a semiconductor with change in temperature). The CTD (conductivity-temperature depth) is an instrument that generates continuous signals as it is lowered into the ocean; temperature is determined by means of a platinum resistance thermometer, salinity by conductivity, and depth by pressure. These signals are transmitted to the surface through a cable and recorded. Accuracy of temperature measurement is 0.005°C and resolution an order of magnitude better.

    A method commonly used to measure upper ocean temperature profiles from a vessel which is underway is the expendable bathythermograph (XBT). The XBT uses a thermistor and is connected to the vessel by a fine wire. The wire is coiled inside the probe, and as the probe freefalls in the ocean, the wire pays out. Depth is determined by elapsed time and a known sink rate. Depth range is determined by the amount of wire stored in the probe; the most common model has a depth range of 450 meters. At the end of the drop, the wire breaks and the probe falls to the ocean bottom. One instrument of this type is dropped from an aircraft; the data is relayed to the aircraft from a buoy to which the wire of the XBT is attached. The accuracy and precision of an XBT is about 0.1°C.

    3105. Pressure
    The appropriate international standard (SI) unit for pressure in oceanography is 1 kPa = 103 Pa where Pa is a Pascal and is equal to one Newton per square meter. A more commonly used unit is a bar, which is nearly equal to 1 atmosphere (atmospheric pressure is measured with a barometer and may be read as millibars). Water pressure is expressed in terms of decibars, 10 of these being equal to 1 bar. One decibar is equal to nearly 1 ½ pounds per square inch. This unit is convenient because it is very nearly the
    pressure exerted by 1 meter of water. Thus, the pressure in decibars is approximately the same as the depth in meters, the unit of depth.

    Although virtually all of the physical properties of seawater are affected to a measurable extent by pressure, the effect is not as great as those of salinity and temperature. Pressure is of particular importance to submarines, directly because of the stress it induces on the hull and structures, and indirectly because of its effect upon buoyancy.

    3106. Density
    Density is mass per unit of volume. The appropriate SI unit is kilograms per cubic meter. The density of seawater depends upon salinity, temperature, and pressure. At constant temperature and pressure, density varies with salinity. A temperature of 0°C and atmospheric pressure are considered standard for density determination. The effects of thermal expansion and compressibility are used to determine the density at other temperatures and pressures. Density changes at the surface generally do not affect the draft or trim of a ship. But density changes at a particular subsurface pressure affect the buoyancy of submarines because they are ballasted to be neutrally buoyant. For oceanographers, density is important because of its relationship to ocean currents.

    Open ocean values of density range from about 1,021 kilograms per cubic meter at the surface to about 1,070 kilograms per cubic meter at 10,000 meters depth. As a matter of convenience, it is usual in oceanography to define a density anomaly which is equal to the density minus 1,000 kilograms per cubic meter. Thus, when an oceanographer speaks of seawater with a density of 25 kilograms per cubic meter, the actual density is 1,025 kilograms per cubic meter. The greatest changes in density of seawater occur at the surface, where the water is subject to influences not present at depths. At the surface, density is decreased by precipitation, run-off from land, melting ice, or heating. When the surface water becomes less dense, it tends to float on top of the more dense water below. There is little tendency for the water to mix, and so the condition is one of stability. The density of surface water is increased by evaporation, formation of sea ice, and by cooling. If the surface water becomes more dense than that below, convection currents cause vertical mixing. The more dense surface water sinks and mixes with less dense water below. The resultant layer of water is of intermediate density. This process continues until the density of the mixed layer becomes less than that of the water below. The convective circulation established as part of this process can create very deep uniform mixed layers. If the surface water becomes sufficiently dense, it sinks all the way to the bottom. If this occurs in an area where horizontal flow is unobstructed, the water which has descended spreads to other regions, creating a dense bottom layer. Since the greatest increase in density occurs in polar regions, where the air is cold and great quantities of ice form, the cold, dense polar water sinks to the bottom and then spreads to lower latitudes. In the Arctic Ocean region, the cold, dense water is confined by the Bering Strait and the underwater ridge from Greenland to Iceland to Europe. In the Antarctic, however, there are no similar geographic restrictions and large quantities of very cold, dense water formed there flow to the north along the ocean bottom. This process has continued for a sufficiently long period of time that the entire ocean floor is covered with this dense water, thus explaining the layer of cold water at great depths in all the oceans.

    In some respects, oceanographic processes are similar to those occurring in the atmosphere. The convective circulation in the ocean is similar to that in the atmosphere.

    Masses of water of uniform characteristics are analogous to air masses.

    3107. Compressibility
    Seawater is nearly incompressible, its coefficient of compressibility being only 0.000046 per bar under standard conditions. This value changes slightly with changes in temperature or salinity. The effect of compression is to force the molecules of the substance closer together, causing it to become more dense. Even though the compressibility is low, its total effect is considerable because of the amount of water involved. If the compressibility of seawater were zero, sea level would be about 90 feet higher than it is now. Compressibility is inversely proportional to temperature, i.e., cold water is more compressible than warm water. Waters which flow into the North Atlantic from the Mediterranean and Greenland Seas are equal in density, but because the water from the Greenland Sea is colder, it is more compressible and therefore becomes denser at depth. These waters from the Greenland Sea are therefore found beneath those waters which derive their properties from the Mediterranean.

    3108. Viscosity
    Viscosity is resistance to flow. Seawater is slightly more viscous than freshwater. Its viscosity increases with greater salinity, but the effect is not nearly as marked as that occurring with decreasing temperature. The rate is not uniform, becoming greater as the temperature decreases.

    Because of the effect of temperature upon viscosity, an incompressible object might sink at a faster rate in warm surface water than in colder water below. However, for most objects, this effect may be more than offset by the compressibility of the object.

    The actual relationships existing in the ocean are considerably more complex than indicated by the simple explanation here, because of turbulent motion within the sea. The disturbing effect is called eddy viscosity.

    3109. Specific Heat
    Specific Heat is the amount of heat required to raise the temperature of a unit mass of a substance a stated amount. In oceanography, specific heat is stated, in SI units, as the number of Joules needed to raise 1 kilogram of a given substance 1°C. Specific heat at constant pressure is usually the quantity desired when liquids are involved, but occasionally the specific heat at constant volume is required.
    The ratio of these two quantities is directly related to the speed of sound in seawater.
    The specific heat of seawater decreases slightly as salinity increases. However, it is much greater than that of land. The ocean is a giant storage area for heat. It can absorb large quantities of heat with very little change in temperature. This is partly due to the high specific heat of water and partly due to mixing in the ocean that distributes the heat throughout a layer. Land has a lower specific heat and, in addition, all heat is lost or gained from a thin layer at the surface; there is no mixing. This accounts for the greater temperature range of land and the atmosphere above it, resulting in monsoons, and the familiar land and sea breezes of tropical and temperate regions.

    3110. Sound Speed
    The speed of sound in sea water is a function of its density, compressibility and, to a minor extent, the ratio of specific heat at constant pressure to that at constant volume. As these properties depend on the temperature, salinity and pressure (depth) of sea water, it is customary to relate the speed of sound directly to the water temperature, salinity and pressure. An increase in any of these three properties causes an increase in the sound speed; the converse is true also. Figure 3110a portrays typical mid-ocean profiles of temperature and salinity; the resultant sound speed profile is shown in Figure 3110b.



    The speed of sound changes by 3 to 5 meters per second per °C temperature change, by about 1.3 meters per second per psu salinity change and by about 1.7 meters per second per 100 m depth change. A simplified formula adapted from Wilson’s (1960) equation for the computation of the sound speed in sea water is:


    where U is the speed (m/s), T is the temperature (°C), S is the salinity (psu), and D is depth (m).

    3111. Thermal Expansion
    One of the more interesting differences between salt and fresh water relates to thermal expansion. Saltwater continues to become more dense as it cools to the freezing point; freshwater reaches maximum density at 4°C and then expands (becomes less dense) as the water cools to 0°C and freezes. This means that the convective mixing of freshwater stops at 4°C; freezing proceeds very rapidly beyond that point. The rate of expansion with increased temperature is greater in seawater than in fresh water. Thus, at temperature 15°C, and atmospheric pressure, the coefficient of thermal expansion is 0.000151 per degree Celsius for freshwater, and 0.000214 per degree Celsius for average seawater. The coefficient of thermal expansion increases not only with greater salinity, but also with increased temperature and pressure. At a salinity of 35 psu, the coefficient of surface water increases from 0.000051 per degree Celsius at 0°C to 0.000334 per degree Celsius at 31°C. At a constant temperature of 0°C and a salinity of 34.85 psu, the coefficient increases to 0.000276 per degree Celsius at a pressure of 10,000 decibars (a depth of approximately 10,000 meters).

    3112. Thermal Conductivity
    In water, as in other substances, one method of heat transfer is by conduction. Freshwater is a poor conductor of heat, having a coefficient of thermal conductivity of 582 Joules per second per meter per degree Celsius. For seawater it is slightly less, but increases with greater temperature or pressure.

    However, if turbulence is present, which it nearly always is to some extent, the processes of heat transfer are altered. The effect of turbulence is to increase greatly the rate of heat transfer. The “eddy” coefficient used in place of the still-water coefficient is so many times larger, and so dependent upon the degree of turbulence, that the effects of temperature and pressure are not important.

    3113. Electrical Conductivity
    Water without impurities is a very poor conductor of electricity. However, when salt is in solution in water, the salt molecules are ionized and become carriers of electricity. (What is commonly called freshwater has many impurities and is a good conductor of electricity; only pure distilled water is a poor conductor.) Hence, the electrical conductivity of seawater is directly proportional to the number of salt molecules in the water. For any given salinity, the conductivity increases with an increase in temperature.

    3114. Radioactivity
    Although the amount of radioactive material in seawater is very small, this material is present in marine sediments to a greater extent than in the rocks of the earth’s crust. This is probably due to precipitation of radium or other radioactive material from the water. The radioactivity of the top layers of sediment is less than that of deeper layers. This may be due to absorption of radioactive material in the soft tissues of marine organisms.

    3115. Transparency
    The two basic processes that alter the underwater distribution of light are absorption and scattering. Absorption is a change of light energy into other forms of energy; scattering entails a change in direction of the light, but without loss of energy. If seawater were purely absorbing, the loss of light with distance would be given by Ix = I0e-ax where Ix is the intensity of light at distance x, I0 is the intensity of light at the source, and “a” is the absorption coefficient in the same units with which distance is measured. In a pure scattering medium, the transmission of light is governed by the same power law only in this case the exponential term is I0e-bx, where “b” is the volume scattering coefficient. The attenuation of light in the ocean is defined as the sum of absorption and scattering so that the attenuation coefficient, c, is given by c = a + b. In the ocean, the attenuation of light with depth depends not only on the wavelength of the light but also the clarity of the water. The clarity is mostly controlled by biological activity although at the coast, sediments transported by rivers or resuspended by wave action can strongly attenuate light.

    Attenuation in the sea is measured with a transmissometer. Transmissometers measure the attenuation of light over a fixed distance using a monochromatic light source which is close to red in color. Transmissometers are designed for in situ use and are usually attached to a CTD. Since sunlight is critical for almost all forms of plant life in the ocean, oceanographers developed a simple method to measure the penetration of sunlight in the sea using a white disk 31 centimeters (a little less than 1 foot) in diameter which is called a Secchi disk. This is lowered into the sea, and the depth at which it disappears is recorded. In coastal waters the depth varies from about 5 to 25 meters. Offshore, the depth is usually about 45 to 60 meters. The greatest recorded depth at which the disk has disappeared is 79 meters in the eastern Weddell Sea. These depths, D, are sometimes reported as a diffuse attenuation (or “extinction”) coefficient, k, where k = 1.7/D and the penetration of sunlight is given by Iz = I0e-kz where z is depth and I0 is the energy of the sunlight at the ocean’s surface.

    3116. Color
    The color of seawater varies considerably. Water of the Gulf Stream is a deep indigo blue, while a similar current off Japan was named Kuroshio (Black Stream) because of the dark color of its water. Along many coasts the water is green. In certain localities a brown or brownish-red water has been observed. Colors other than blue are caused by biological sources, such as plankton, or by suspended sediments from river runoff.

    Offshore, some shade of blue is common, particularly in tropical or subtropical regions. It is due to scattering of sunlight by minute particles suspended in the water, or by molecules of the water itself. Because of its short wavelength, blue light is more effectively scattered than light of longer waves. Thus, the ocean appears blue for the same reason that the sky does. The green color often seen near the coast is a mixture of the blue due to scattering of light and a stable soluble yellow pigment associated with phytoplankton. Brown or brownish-red water receives its color from large quantities of certain types of algae, microscopic plants in the sea, or from river runoff.

    3117. Bottom Relief
    Compared to land, relatively little is known of relief below the surface of the sea. The development of an effective echo sounder in 1922 greatly simplified the determination of bottom depth. Later, a recording echo sounder was developed to permit the continuous tracing of a bottom profile. The latest sounding systems employ an array of echosounders aboard a single vessel, which continuously sound a wide swath of ocean floor. This has contributed immensely to our knowledge of bottom relief. By this means, many undersea mountain ranges, volcanoes, rift valleys, and other features have been discovered. Along most of the coasts of the continents, the bottom slopes gradually downward to a depth of about 130 meters or somewhat less, where it falls away more rapidly to greater depths. This continental shelf averages about 65 kilometers in width, but varies from nothing to about 1400 kilometers, the widest part being off the Siberian Arctic coast. A similar shelf extending outward from an island or group of islands is called an island shelf. At the outer edge of the shelf, the steeper slope of 2° to 4° is called the continental slope, or the island slope, according to whether it surrounds a continent or a group of islands. The shelf itself is not uniform, but has numerous hills, ridges, terraces, and canyons, the largest being comparable in size to the Grand Canyon.

    The relief of the ocean floor is comparable to that of land. Both have steep, rugged mountains, deep canyons, rolling hills, plains, etc. Most of the ocean floor is considered to be made up of a number of more-or-less circular or oval depressions called basins, surrounded by walls (sills) of lesser depth.
    A wide variety of submarine features has been identified and defined. Some of these are shown in Figure 3117. Detailed definitions and descriptions of such features can be found in Kennett (1982) or Fairbridge (1966). The term deep may be used for a very deep part of the ocean, generally that part deeper than 6,000 meters.

    The average depth of water in the oceans is 3795 meters (2,075 fathoms), as compared to an average height of land above the sea of about 840 meters. The greatest known depth is 11,524 meters, in the Marianas Trench in the Pacific. The highest known land is Mount Everest, 8,840 meters. About 23 percent of the ocean is shallower than 3,000 meters, about 76 percent is between 3,000 and 6,000 meters, and a little more than 1 percent is deeper than 6,000 meters.

    3118. Marine Sediments
    The ocean floor is composed of material deposited through the ages. This material consists principally of (1) earth and rocks washed into the sea by streams and waves, (2) volcanic ashes and lava, and (3) the remains of marine organisms. Lesser amounts of land material are carried into the sea by glaciers, blown out to sea by wind, or deposited by chemical means. This latter process is responsible for the manganese nodules that cover some parts of the ocean floor. In the ocean, the material is transported by ocean currents, waves, and ice. Near shore the material is deposited at the rate of about 8 centimeters in 1,000 years, while in the deep water offshore the rate is only about 1 centimeter in 1,000 years. Marine deposits in water deep enough to be relatively free from wave action are subject to little erosion. Recent studies have shown that some bottom currents are strong enough to move sediments. There are turbidity currents, similar to land slides, that move large masses of sediments. Turbidity currents have been known to rip apart large transoceanic cables on the ocean bottom. Because of this and the slow rate of deposit, marine sediments provide a better geological record than does the land. Marine sediments are composed of individual particles of all sizes from the finest clay to large boulders. In general, the inorganic deposits near shore are relatively coarse (sand, gravel, shingle, etc.), while those in deep water are much finer (clay). In some areas the siliceous remains of marine organisms or calcareous deposits of either organic or inorganic origin predominate on the ocean floor.

    A wide range of colors is found in marine sediments. The lighter colors (white or a pale tint) are usually associated with coarse-grained quartz or limestone deposits. Darker colors (red, blue, green, etc.) are usually found in mud having a predominance of some mineral substance, such as an oxide of iron or manganese. Black mud is often found in an area that is little disturbed, such as at the bottom of an inlet or in a depression without free access to other areas. Marine sediments are studied primarily through bottom samples. Samples of surface deposits are obtained by means of a “snapper” (for mud, sand, etc.) or “dredge” (usually for rocky material). If a sample of material below the bottom surface is desired, a “coring” device is used.
    This device consists essentially of a tube driven into the bottom by weights or explosives. A sample obtained in this way preserves the natural order of the various layers. Samples of more than 100 feet in depth have been obtained using coring devices.

    3119. Satellite Oceanography
    Weather satellites are able to observe ocean surface temperatures in cloud free regions by using infrared sensors. Although these sensors are only able to penetrate a few millimeters into the ocean, the temperatures that they yield are representative of upper ocean conditions except when the air is absolutely calm during daylight hours. For cloud covered regions, it is usually possible to wait a few days for the passage of a cold front and then use a sequence of infrared images to map the ocean temperature over a region.
    The patterns of warm and cold water yield information on ocean currents, the existence of fronts and eddies, and the temporal and spatial scales of ocean processes. Other satellite sensors are capable of measuring ocean color, ice coverage, ice age, ice edge, surface winds and seas, ocean currents, and the shape of the surface of the ocean. (The latter is controlled by gravity and ocean circulation patterns. See Chapter 2.) The perspective provided by these satellites is a global one and in some cases they yield sufficient quantities of data that synoptic charts of the ocean surface, similar to weather maps and pilot charts, can be provided to the mariner for use in navigation.

    The accuracy of satellite observations of the ocean surface depends, in many cases, on calibration procedures which use observations of sea surface conditions provided by mariners. These observations include marine weather observations, expendable bathythermograph soundings, and currents measured by electromagnetic logs or acoustic Doppler current profilers. Care and diligence in these observations will improve the accuracy and the quality of satellite data.

    3120. Synoptic Oceanography
    Oceanographic data provided by ships, buoys, and satellites are analyzed by the Naval Oceanographic Office and the National Meteorological Center. These data are utilized in computer models both to provide a synoptic view of ocean conditions and to predict how these conditions will change in the future. These products are available to the mariner via radio or satellite.

  4. Fishers of Men

    Fishers of Men Senior Member

    CHAPTER 32

    Here, you will find of interest, the similaritys of topics we discussed way in the beginning of the first thread. If you have access to a globe, you will see these defined currents. Then look at the magnetic fields of the earth in the pics from earlier posts. Then look at your jet stream currents. Then you should have a better understanding of why and how the OLD original sailors left home one way and returned another...But, how did they know all this???

    It's a wonder they ever made it home at all.

    The "?" mark showed up instead of degrees again, keep it in mind.

    3200. Definitions
    The movement of ocean water is one of the two principal sources of discrepancy between dead reckoned and actual positions of vessels. Water in motion is called a current; the direction toward which it moves is called set, and its speed is called drift. Modern shipping speeds have lessened the impact of currents on a typical voyage, and since electronic navigation allows continuous adjustment of course, there is less need to estimate current set and drift before setting the course to be steered. Nevertheless, a knowledge of ocean currents can be used in cruise planning to reduce transit times. Ocean current models are an integral part of ship routing systems.

    Oceanographers have developed a number of methods of classifying currents in order to facilitate descriptions of their physics and geography. Currents may be referred to according to their forcing mechanism as either wind driven or thermohaline.

    Alternatively, they may be classified according to their depth (surface, intermediate, deep or bottom). The surface circulation of the world ocean is mostly wind driven. Thermohaline currents are driven by differences in heat and salt and are associated with the sinking of dense water at high latitudes; the currents driven by thermohaline forcing are typically subsurface. Note that this classification scheme is not unambiguous; the circumpolar current, which is wind driven, extends from the surface to the bottom.
    A periodic current is one for which the speed or direction changes cyclically at somewhat regular intervals, such as a tidal current. A seasonal current is one which changes in speed or direction due to seasonal winds. The mean circulation of the ocean consists of semi-permanent currents which experience relatively little periodic or seasonal change.

    A coastal current flows roughly parallel to a coast, outside the surf zone, while a longshore current is one parallel to a shore, inside the surf zone, generated by waves striking the beach at an angle. Any current some distance from the shore may be called an offshore current, and one close to the shore an inshore current.

    3201. Causes Of Ocean Currents
    The primary generating forces are wind and differences in density of the water caused by variations in heat and salt. Currents generated by these forces are modified by such factors as depth of water, underwater topography including shape of the basin in which the current is running, extent and location of land, and deflection by the rotation of the earth.

    3202. Wind Driven Currents
    The stress of wind blowing across the sea causes a surface layer of water to move. Due to the low viscosity of water, this stress is not directly communicated to the ocean interior, but is balanced by the Coriolis force within a relatively thin surface layer, 10-200m thick. This layer is called the Ekman layer and the motion of this layer is called the Ekman transport. Because of the deflection by the Coriolis force, the Ekman transport is not in the direction of the wind, but is 90to the right in the Northern Hemisphere and 90toward the left in the Southern Hemisphere. The amount of water flowing in this layer depends only upon the wind and the Coriolis force and is independent of the depth of the Ekman layer and the viscosity of the water. The large scale convergence or divergence of Ekman transport serves to drive the general ocean circulation. Consider the case of the Northern Hemisphere subtropics. To the south lie easterly winds with associated northward Ekman transport. To the north lie westerly winds with southward Ekman transport. The convergence of these Ekman transports is called Ekman pumping and results in a thickening of the upper ocean and a increase in the depth of the thermocline. The resulting subsurface pressure gradients, balanced by the Coriolis force, give rise to the anticyclonic subtropical gyres found at mid latitudes in each ocean basin. In subpolar regions, Ekman suction produces cyclonic gyres.

    These wind driven gyres are not symmetrical. Along the western boundary of the oceans, currents are narrower, stronger, and deeper, often following a meandering course. These currents are sometimes called a stream. In contrast, currents in mid-ocean and at the eastern boundary, are often broad, shallow and slow-moving. Sometimes these are called drift currents.

    Within the Ekman layer, the currents actually form a spiral. At the surface, the difference between wind direction and surface wind-current direction varies from about 15along shallow coastal areas to a maximum of 45in the deep oceans. As the motion is transmitted to successively deep layers, the Coriolis force continues to deflect the current. At the bottom of the Ekman layer, the current flows in the opposite direction to the surface current. This shift of current directions with depth, combined with the decrease in velocity with depth, is called the Ekman spiral.

    The velocity of the surface current is the sum of the velocities of the Ekman, geostrophic, tidal, and other currents. The Ekman surface current or wind drift current depends upon the speed of the wind, its constancy, the length of time it has blown, and other factors. In general, however, wind drift current is about 2 percent of the wind speed, or a little less, in deep water where the wind has been blowing steadily for at least 12 hours.

    3203. Currents Related To Density Differences
    The density of water varies with salinity, temperature, and pressure. At any given depth, the differences in density are due only to differences in temperature and salinity. With sufficient data, maps showing geographical density distribution at a certain depth can be drawn, with lines connecting points of equal density. These lines would be similar to isobars on a weather map and serve an analogous purpose, showing areas of high density and those of low density. In an area of high density, the water surface is lower than in an area of low density, the maximum difference in height being about 1 meter in 100 km. Because of this difference, water tends to flow from an area of higher water (low density) to one of lower water (high density). But due to rotation of the earth, it is deflected by the Coriolis force or toward the right in the Northern Hemisphere, and toward the left in the Southern Hemisphere. This balance, between subsurface pressure fields and the Coriolis force, is called geostrophic equilibrium. At a given latitude, the greater the density gradient (rate of change with distance), the faster the geostrophic current.

    3204. Introduction
    A number of ocean currents flow with great persistence, setting up a circulation that continues with relatively little change throughout the year. Because of the influence of wind in creating current, there is a relationship between this oceanic circulation and the general circulation of the atmosphere. The oceanic circulation is shown on the chart following this page (winter N. hemisphere), with the names of the major ocean currents. Some differences in opinion exist regarding the names and limits of some of the currents, but those shown are representative. Speed may vary somewhat with the season. This is particularly noticeable in the Indian Ocean and along the South China coast, where currents are influenced to a marked degree by the monsoons.

    3205. Southern Ocean Currents
    The Southern Ocean has no meridional boundaries and its waters are free to circulate around the world. It serves as a conveyor belt for the other oceans, exchanging waters between them. The northern boundary of the Southern Ocean is marked by the Subtropical Convergence zone. This zone marks the transition from the temperate region of the ocean to the polar region and is associated with the surfacing of the main thermocline. This zone is typically found at 40S but varies with longitude and season.

    In the Antarctic, the circulation is generally from west to east in a broad, slow-moving current extending completely around Antarctica. This is called the Antarctic Circumpolar Current or the West Wind Drift, and it is formed partly by the strong westerly wind in this area, and partly by density differences. This current is augmented by the Brazil and Falkland Currents in the Atlantic, the East Australia Current in the Pacific, and the Agulhas Current in the Indian Ocean. In return, part of it curves northward to form the Cape Horn, Falkland, and most of the Benguela Currents in the Atlantic, and the Peru Current in the Pacific. In a narrow zone next to the Antarctic continent, a westward flowing coastal current is usually found. This current is called the East Wind Drift because it is attributed to the prevailing easterly winds which occur there.

    3206. Atlantic Ocean Currents
    The trade winds set up a system of equatorial currents which at times extends over as much as 50of latitude or more. There are two westerly flowing currents conforming generally with the areas of trade winds, separated by a weaker, easterly flowing countercurrent.

    The North Equatorial Current originates to the northward of the Cape Verde Islands and flows almost due west at an average speed of about 0.7 knot. The South Equatorial Current is more extensive. It starts off the west coast of Africa, south of the Gulf of Guinea, and flows in a generally westerly direction at an average speed of about 0.6 knot. However, the speed gradually increases until it may reach a value of 2.5 knots, or more, off the east coast of South America. As the current approaches Cabo de Sao Roque, the eastern extremity of South America, it divides, the southern part curving toward the south along the coast of Brazil, and the northern part being deflected northward by the continent of South America.

    Between the North and South Equatorial Currents, the weaker North Equatorial Countercurrent sets toward the east in the general vicinity of the doldrums. This is fed by water from the two westerly flowing equatorial currents, particularly the South Equatorial Current. The extent and strength of the Equatorial Countercurrent changes with the seasonal variations of the wind. It reaches a maximum during July and August, when it extends from about 50west longitude to the Gulf of Guinea. During its minimum, in December and January, it is of very limited extent, the western portion disappearing altogether.

    That part of the South Equatorial Current flowing along the northern coast of South America which does not feed the Equatorial Countercurrent unites with the North Equatorial Current at a point west of the Equatorial Countercurrent. A large part of the combined current flows through various passages between the Windward Islands and into the Caribbean Sea. It sets toward the west, and then somewhat north of west, finally arriving off the Yucatan peninsula.
    From there, the water enters the Gulf of Mexico and forms the Loop Current; the path of the Loop Current is variable with a 13-month period. It begins by flowing directly from Yucatan to the Florida Straits, but gradually grows to flow anticyclonically around the entire Eastern Gulf; it then collapses, again following the direct path from Yucatan to the Florida Straits, with the loop in the Eastern Gulf becoming a separate eddy which slowly flows into the Western Gulf.

    Within the Straits of Florida, the Loop Current feeds the beginnings of the most remarkable of American ocean currents, the Gulf Stream. Off the southeast coast of Florida this current is augmented by the Antilles Current which flows along the northern coasts of Puerto Rico, Hispaniola, and Cuba. Another current flowing eastward of the Bahamas joins the stream north of these islands. The Gulf Stream follows generally along the east coast of North America, flowing around Florida, northward and then northeastward toward Cape Hatteras, and then curving toward the east and becoming broader and slower. After passing the Grand Banks, it turns more toward the north and becomes a broad drift current flowing across the North Atlantic. The part in the Straits of Florida is sometimes called the Florida Current.
    A tremendous volume of water flows northward in the Gulf Stream. It can be distinguished by its deep indigo-blue color, which contrasts sharply with the dull green of the surrounding water. It is accompanied by frequent squalls. When the Gulf Stream encounters the cold water of the Labrador Current, principally in the vicinity of the Grand Banks, there is little mixing of the waters. Instead, the junction is marked by a sharp change in temperature. The line or surface along which this occurs is called the cold wall. When the warm Gulf Stream water encounters cold air, evaporation is so rapid that the rising vapor may be visible as frost smoke.

    Investigations have shown that the current itself is much narrower and faster than previously supposed, and considerably more variable in its position and speed. The maximum current off Florida ranges from about 2 to 4 knots. Northward, the speed is generally less, and it decreases further after the current passes Cape Hatteras. As the stream meanders and shifts position, eddies sometimes break off and continue as separate, circular flows until they dissipate. Boats in the Newport-Bermuda sailing yacht race have been known to be within sight of each other and be carried in opposite directions by different parts of the same current. This race is generally won by the boat which catches an eddy just right. As the current shifts position, its extent does not always coincide with the area of warm, blue water. When the sea is relatively smooth, the edges of the current are marked by ripples.

    A recirculation region exists adjacent to and southwest of the Gulf Stream. The flow of water in the recirculation region is opposite to that in the Gulf Stream and surface currents are much weaker, generally less than half a knot. As the Gulf Stream continues eastward and northeastward beyond the Grand Banks, it gradually widens and decreases speed until it becomes a vast, slow-moving current known as the North Atlantic Current, in the general vicinity of the prevailing westerlies. In the eastern part of the Atlantic it divides into the Northeast Drift Current and the Southeast Drift Current.

    The Northeast Drift Current continues in a generally northeasterly direction toward the Norwegian Sea. As it does so, it continues to widen and decrease speed. South of Iceland it branches to form the Irminger Current and the Norway Current. The Irminger Current curves toward the north and northwest to join the East Greenland Current southwest of Iceland. The Norway Current continues in a northeasterly direction along the coast of Norway. Part of it, the North Cape Current, rounds North Cape into the Barents Sea. The other part curves toward the north and becomes known as the Spitsbergen Current. Before reaching Svalbard (Spitsbergen), it curves toward the west and joins the cold East Greenland Current flowing southward in the Greenland Sea. As this current flows past Iceland, it is further augmented by the Irminger Current. Off Kap Farvel, at the southern tip of Greenland, the East Greenland Current curves sharply to the northwest following the coastline. As it does so, it becomes known as the West Greenland Current, and its character changes from that of an intense western boundary current to a weaker eastern boundary current. This current continues along the west coast of Greenland, through Davis Strait, and into Baffin Bay. In Baffin Bay the West Greenland Current generally follows the coast, curving westward off Kap York to form the southerly flowing Labrador Current. This cold current flows southward off the coast of Baffin Island, through Davis Strait, along the coast of Labrador and Newfoundland, to the Grand Banks, carrying with it large quantities of ice. Here it encounters the warm water of the Gulf Stream, creating the cold wall. Some of the cold water flows southward along the east coast of North America, inshore of the Gulf Stream, as far as Cape Hatteras. The remainder curves toward the east and flows along the northern edge of the North Atlantic and Northeast Drift Currents, gradually merging with them.

    The Southeast Drift Current curves toward the east, southeast, and then south as it is deflected by the coast of Europe. It flows past the Bay of Biscay, toward southeastern Europe and the Canary Islands, where it continues as the Canary Current. In the vicinity of the Cape Verde Islands, this current divides, part of it curving toward the west to help form the North Equatorial Current, and part of it curving toward the east to follow the coast of Africa into the Gulf of Guinea, where it is known as the Guinea Current. This current is augmented by the North Equatorial Countercurrent and, in summer, it is strengthened by monsoon winds. It flows in close proximity to the South Equatorial Current, but in the opposite direction. As it curves toward the south, still following the African coast, it merges with the South Equatorial Current.

    The clockwise circulation of the North Atlantic leaves a large central area between the recirculation region and the Canary Current which has no well-defined currents. This area is known as the Sargasso Sea, from the large quantities of sargasso or gulfweed encountered there. That branch of the South Equatorial Current which curves toward the south off the east coast of South America, follows the coast as the warm, highly-saline Brazil Current, which in some respects resembles a weak Gulf Stream. Off Uruguay it encounters the colder, less-salty Falkland or Malvinas Current forming a sharp meandering front in which eddies may form. The two currents curve toward the east to form the broad, slow-moving, South Atlantic Current in the general vicinity of the prevailing westerlies and the front dissipates somewhat. This current flows eastward to a point west of the Cape of Good Hope, where it curves northward to follow the west coast of Africa as the strong Benguela Current, augmented somewhat by part of the Agulhas Current flowing around the southern part of Africa from the Indian Ocean. As it continues northward, the current gradually widens and slows. At a point east of St. Helena Island it curves westward to continue as part of the South Equatorial Current, thus completing the counterclockwise circulation of the South Atlantic. The Benguela Current is also augmented somewhat by the West Wind Drift, a current which flows easterly around Antarctica. As the West Wind Drift flows past Cape Horn, that part in the immediate vicinity of the cape is called the Cape Horn Current. This current rounds the cape and flows in a northerly and northeasterly direction along the coast of South America as the Falkland or Malvinas Current.

    3207. Pacific Ocean Currents
    Pacific Ocean currents follow the general pattern of those in the Atlantic. The North Equatorial Current flows westward in the general area of the northeast trades, and the South Equatorial Current follows a similar path in the region of the southeast trades. Between these two, the weaker North Equatorial Countercurrent sets toward the east, just north of the equator.
    After passing the Mariana Islands, the major part of the North Equatorial Current curves somewhat toward the northwest, past the Philippines and Taiwan. Here it is deflected further toward the north, where it becomes known as the Kuroshio, and then toward the northeast past the Nansei Shoto and Japan, and on in a more easterly direction. Part of the Kuroshio, called the Tsushima Current, flows through Tsushima Strait, between Japan and Korea, and the Sea of Japan, following generally the northwest coast of Japan. North of Japan it curves eastward and then southeastward to rejoin the main part of the Kuroshio. The limits and volume of the Kuroshio are influenced by the monsoons, being augmented during the season of southwesterly winds, and diminished when the northeasterly winds are prevalent. The Kuroshio (Japanese for “Black Stream”) is so named because of the dark color of its water. It is sometimes called the Japan Current. In many respects it is similar to the Gulf Stream of the Atlantic. Like that current, it carries large quantities of warm tropical water to higher latitudes, and then curves toward the east as a major part of the general clockwise circulation in the Northern Hemisphere. As it does so, it widens and slows, continuing on between the Aleutians and the Hawaiian Islands, where it becomes known as the North Pacific Current. As this current approaches the North American continent, most of it is deflected toward the right to form a clockwise circulation between the west coast of North America and the Hawaiian Islands called the California Current. This part of the current has become so broad that the circulation is generally weak. Near the coast, the southeastward flow intensifies and average speeds are about 0.8 knot. But the flow pattern is complex, with offshore directed jets often found near more prominent capes, and poleward flow often found over the upper slope and outer continental shelf. It is strongest near land. Near the southern end of Baja California, this current curves sharply to the west and broadens to form the major portion of the North Equatorial Current. During the winter, a weak countercurrent flows northwestward, inshore of the southeastward flowing California Current, along the west coast of North America from Baja California to Vancouver Island. This is called the Davidson Current.

    Off the west coast of Mexico, south of Baja California the current flows southeastward during the winter as a continuation of part of the California Current. During the summer, the current in this area is northwestward as a continuation of the North Equatorial Countercurrent.

    As in the Atlantic, there is in the Pacific a counterclockwise circulation to the north of the clockwise circulation. Cold water flowing southward through the western part of Bering Strait between Alaska and Siberia, is joined by water circulating counterclockwise in the Bering Sea to form the Oyashio. As the current leaves the strait, it curves toward the right and flows southwesterly along the coast of Siberia and the Kuril Islands. This current brings quantities of sea ice, but no icebergs. When it encounters the Kuroshio, the Oyashio curves southward and then eastward, the greater portion joining the Kuroshio and North Pacific Current.

    The northern branch of the North Pacific Current curves in a counterclockwise direction to form the Alaska Current, which generally follows the coast of Canada and Alaska. When the Alaska Current turns to the southwest and flows along the Kodiak Island and the Alaska Peninsula, its character changes to that of a western boundary current and it is called the Alaska Stream. When this westward flow arrives off the Aleutian Islands, it is less intense and becomes known as the Aleutian Current. Part of it flows along the southern side of these islands to about the 180th meridian, where it curves in a counterclockwise direction and becomes an easterly flowing current, being augmented by the northern part of the Oyashio. The other part of the Aleutian Current flows through various openings between the Aleutian Islands, into the Bering Sea. Here it flows in a general counterclockwise direction. The southward flow along the Kamchatka peninsula is called the Kamchatka Current which feeds the southerly flowing Oyashio. Some water flows northward from the Bering Sea through the eastern side of the Bering Strait, into the Arctic Ocean. The South Equatorial Current, extending in width between about 4N latitude and 10S, flows westward from South America to the western Pacific. After this current crosses the 180th meridian, the major part curves in a counterclockwise direction, entering the Coral Sea, and then curving more sharply toward the south along the east coast of Australia, where it is known as the East Australian Current. The East Australian Current is the weakest of the subtropical western boundary currents and separates from the Australian coast near 34S. The path of the current from Australia to New Zealand is known as the Tasman Front, which marks the boundary between the warm water of the Coral Sea and the colder water of the Tasman Sea. The continuation of the East Australian Current east of New Zealand is the East Auckland Current. The East Auckland Current varies seasonally: in winter, it separates from the shelf and flows eastward, merging with the West Wind Drift, while in winter it follows the New Zealand shelf southward as the East Cape Current until it reaches Chatham Rise where it turns eastward, thence merging with the West Wind Drift. Near the southern extremity of South America, most of this current flows eastward into the Atlantic, but part of it curves toward the left and flows generally northward along the west coast of South America as the Peru Current or Humboldt Current. Occasionally a set directly toward land is encountered. At about Cabo Blanco, where the coast falls away to the right, the current curves toward the left, past the Galapagos Islands, where it takes a westerly set and constitutes the major portion of the South Equatorial Current, thus completing the counterclockwise circulation of the South Pacific.

    During the northern hemisphere summer, a weak northern branch of the South Equatorial Current, known as the New Guinea Coastal Current, continues on toward the west and northwest along both the southern and northeastern coasts of New Guinea. The southern part flows through Torres Strait, between New Guinea and Australia, into the Arafura Sea. Here, it gradually loses its identity, part of it flowing on toward the west as part of the South Equatorial Current of the Indian Ocean, and part of it following the coast of Australia and finally joining the easterly flowing West Wind Drift. The northern part of New Guinea Coastal Current both curves in a clockwise direction to help form the Pacific Equatorial Countercurrent and off Mindanao turns southward to form a southward flowing boundary current called the Mindanao Current. During the northern hemisphere winter, the New Guinea Coastal Current may reverse direction for a few months.

    3208. Indian Ocean Currents
    Indian Ocean currents follow generally the pattern of the Atlantic and Pacific but with differences caused principally by the monsoons, the more limited extent of water in the Northern Hemisphere, and by limited communication with the Pacific Ocean along the eastern boundary. During the northern hemisphere winter, the North Equatorial Current and South Equatorial Current flow toward the west, with the weaker, eastward Equatorial Countercurrent flowing between them, as in the Atlantic and Pacific (but somewhat south of the equator). But during the northern hemisphere summer, both the North Equatorial Current and the Equatorial Countercurrent are replaced by the Southwest Monsoon Current, which flows eastward and southeastward across the Arabian Sea and the Bay of Bengal. Near Sumatra, this current curves in a clockwise direction and flows westward, augmenting the South Equatorial Current, and setting up a clockwise circulation in the northern part of the Indian Ocean. Off the coast of Somalia, the Somali Current reverses direction during the northern hemisphere summer with northward currents reaching speeds of 5 knots or more. Twice a year, around May and November, westerly winds along the equator result in an eastward Equatorial Jet which feeds warm water towards Sumatra.

    As the South Equatorial Current approaches the coast of Africa, it curves toward the southwest, part of it flowing through the Mozambique Channel between Madagascar and the mainland, and part flowing along the east coast of Madagascar. At the southern end of this island the two join to form the strong Agulhas Current, which is analogous to the Gulf Stream. This current, when opposed by strong winds from Southern Ocean storms, creates dangerously large seas. South of South Africa, the Agulhas Current retroflects, and most of the flow curves sharply southward and then eastward to join the West Wind Drift; this junction is often marked by a broken and confused sea, made much worse by westerly storms. A small part of the Agulhas Current rounds the southern end of Africa and helps form the Benguela Current; occasionally, strong eddies are formed in the retroflection region and these too move into the Southeastern Atlantic. The eastern boundary currents in the Indian Ocean are quite different from those found in the Atlantic and Pacific. The seasonally reversing South Java Current has strongest westward flow during August when monsoon winds are easterly and the Equatorial jet is inactive. Along the coast of Australia, a vigorous poleward flow, the Leeuwin Current, runs against the prevailing winds.

    3209. Arctic Currents
    The waters of the North Atlantic enter the Arctic Ocean between Norway and Svalbard. The currents flow easterly, north of Siberia, to the region of the Novosibirskiye Ostrova, where they turn northerly across the North Pole, and continue down the Greenland coast to form the East Greenland Current. On the American side of the Arctic basin, there is a weak, continuous clockwise flow centered in the vicinity of 80N, 150W. A current north through Bering Strait along the American coast is balanced by an outward southerly flow along the Siberian coast, which eventually becomes part of the Kamchatka Current. Each of the main islands or island groups in the Arctic, as far as is known, seems to have a clockwise nearshore circulation around it. The Barents Sea, Kara Sea, and Laptev Sea each have a weak counterclockwise circulation. A similar but weaker counterclockwise current system appears to exist in the East Siberian Sea.

    3210. Ocean Eddies And Rings
    Eddies with horizontal diameters varying from 50-150 km have their own pattern of surface currents. These features may have either a warm or a cold core and currents flow around this core, either cyclonically for cold cores or anticyclonically for warm cores. The most intense of these features are called rings and are formed by the pinching off of meanders of western boundary currents such as the Gulf Stream. Maximum speed associated with these features is about 2 knots. Rings have also been observed to pinch off from the Agulhas retroflexion and to then drift to the northwest into the South Atlantic. Similarly, strong anticyclonic eddies are occasionally spawned by the loop current into the Western Gulf Mexico.

    In general, mesoscale variability is strongest in the region of western boundary currents and in the Circumpolar Current. The strength of mesoscale eddies is greatly reduced at distances of 200-400 km from these strong boundary currents, because mean currents are generally weaker in these regions. The eddies may be sufficiently strong to reverse the direction of the surface currents.

    3211. Undercurrents
    At the equator and along some ocean boundaries, shallow undercurrents exist, flowing in a direction counter to that at the surface. These currents may affect the operation of submarines or trawlers. The most intense of these flows, called the Pacific Equatorial Undercurrent, is found at the equator in the Pacific. It is centered at a depth of 150m to the west of the Galapagos, is about 4 km wide, and eastward speeds of up to 1.5 m/s have been observed. Equatorial Undercurrents are also observed in the Atlantic and Indian Ocean, but they are somewhat weaker. In the Atlantic, the Equatorial Undercurrent is found to the east of 24W and in the Indian Ocean, it appears to be seasonal. Undercurrents also exist along ocean boundaries. They seem to be most ubiquitous at the eastern boundary of oceans. Here they are found at depths of 100-200m, may be 100 km wide, and have maximum speeds of 0.5 m/s.

    3212. Ocean Currents And Climate
    Many of the ocean currents exert a marked influence upon the climate of the coastal regions along which they flow. Thus, warm water from the Gulf Stream, continuing as the North Atlantic, Northeast Drift, and Irminger Currents, arrives off the southwest coast of Iceland, warming it to the extent that Reykjavik has a higher average winter temperature than New York City, far to the south. Great Britain and Labrador are about the same latitude, but the climate of Great Britain is much milder because of the relatively warm currents. The west coast of the United States is cooled in the summer by the California Current, and warmed in the winter by the Davidson Current. Partly as a result of this circulation, the range of monthly average temperature is comparatively small.

    Currents exercise other influences besides those on temperature. The pressure pattern is affected materially, as air over a cold current contracts as it is cooled, and that over a warm current expands. As air cools above a cold ocean current, fog is likely to form. Frost smoke occurs over a warm current which flows into a colder region. Evaporation is greater from warm water than from cold water, adding to atmospheric moisture.

    3213. Ocean Current Observations
    Historically, our views of the surface circulation of the ocean have been shaped by reports of ocean currents provided by mariners. As mentioned at the start of this chapter, these observations consist of reports of the difference between the dead reckoning and the observed position of the vessel. These observations were routinely collected until the start of World War II.
    Two observation systems are generally used for surface current studies. The first utilizes autonomous free-drifting buoys which are tracked by satellite or relay their position via satellite. These buoys consist of either a spherical or cylindrical surface float which is about 0.5m in diameter with a drogue at a depth of about 35m. The second system utilizes acoustic Doppler current profilers. These profilers utilize hull mounted transducers, operate at a frequency of 150 kHz, and have pulse repetition rates of about 1 second. They can penetrate to about 300m, and, where water is shallower than this depth, track the bottom. Merchant and naval vessels are increasingly being outfitted with acoustic Doppler current profilers which, when operated with the Global Positioning System, provide accurate observations of currents.
    Last edited by a moderator: Apr 30, 2015
  5. Fishers of Men

    Fishers of Men Senior Member

    CHAPTER 33

    3300. Introduction
    Ocean Waves are the most widely observed phenomenon at sea, and possibly the least understood by the average seaman. More than any other single factor, ocean waves are likely to cause a navigator to change course or speed to avoid damage to ship and cargo. Wind-generated ocean waves have been measured at more than 100 feet high, and tsunamis, caused by earthquakes, far higher. A mariner with knowledge of basic facts concerning waves is able to use them to his advantage, avoid hazardous conditions, and operate with a minimum of danger if such conditions cannot be avoided. See Chapter 38, Weather Routing, for details on how to avoid areas of severe waves.

    3301. Causes Of Waves
    Waves on the surface of the sea are caused principally by wind, but other factors, such as submarine earthquakes, volcanic eruptions, and the tide, also cause waves. If a breeze of less than 2 knots starts to blow across smooth water, small wavelets called ripples form almost instantaneously. When the breeze dies, the ripples disappear as suddenly as they formed, the level surface being restored by surface tension of the water. If the wind speed exceeds 2 knots, more stable gravity waves gradually form, and progress with the wind.
    While the generating wind blows, the resulting waves may be referred to as sea. When the wind stops or changes direction, waves that continue on without relation to local winds are called swell.

    Unlike wind and current, waves are not deflected appreciably by the rotation of the earth, but move in the direction in which the generating wind blows. When this wind ceases, friction and spreading cause the waves to be reduced in height, or attenuated, as they move. However, the reduction takes place so slowly that swell often continues until it reaches some obstruction, such as a shore. The Fleet Numerical Meteorology and Oceanography Center produces synoptic analyses and predictions of ocean wave heights using a spectral numerical model. The wave information consists of heights and directions for different periods and wavelengths. Verification of projected data has proven the model to be very good. Information from the model is provided to the U.S. Navy on a routine basis and is a vital input to the Optimum Track Ship Routing program.

    3302. Wave Characteristics
    Ocean waves are very nearly in the shape of an inverted cycloid, the figure formed by a point inside the rim of a wheel rolling along a level surface. This shape is shown in Figure 3302a. The highest parts of waves are called crests, and the intervening lowest parts, troughs. Since the crests are steeper and narrower than the troughs, the mean or still water level is a little lower than halfway between the crests and troughs. The vertical distance between trough and crest is called wave height, labeled H in Figure 3302a. The horizontal distance between successive crests, measured in the direction of travel, is called wavelength, labeled L. The time interval between passage of successive crests at a stationary point is called wave period (P). Wave height, length, and period depend upon a number of factors, such as the wind speed, the length of time it has blown, and its fetch (the straight distance it has traveled over the surface).

    Table 3302 indicates the relationship between wind speed, fetch, length of time the wind blows, wave height, and wave period in deep water.
    If the water is deeper than one-half the wavelength (L), this length in feet is theoretically related to period (P) in seconds by the formula:


    The actual value has been found to be a little less than this for swell, and about two-thirds the length determined by this formula for sea. When the waves leave the generating area and continue as free waves, the wavelength and period continue to increase, while the height decreases. The rate of change gradually decreases.


    The speed (S) of a free wave in deep water is nearly independent of its height or steepness. For swell, its relationship in knots to the period (P) in seconds is given by the formula:
    S = 3.03P.
    The relationship for sea is not known.

    The theoretical relationship between speed, wavelength, and period is shown in Figure 3302b. As waves continue on beyond the generating area, the period, wavelength, and speed remain the same. Because the waves of each period have different speeds they tend to sort themselves by periods as they move away from the generating area. The longer period waves move at a greater speed and move ahead. At great enough distances from a storm area the waves will have sorted themselves into sets based on period.
    All waves are attenuated as they propagate but the short period waves attenuate faster, so that far from a storm only the longer waves remain.
    The time needed for a wave system to travel a given distance is double that which would be indicated by the speed of individual waves. This is because each leading wave in succession gradually disappears and transfers its energy to following wave. The process occurs such that the whole wave system advances at a speed which is just half that of each individual wave. This process can easily be seen in the bow wave of a vessel. The speed at which the wave system advances is called group velocity.



    Because of the existence of many independent wave systems at the same time, the sea surface acquires a complex and irregular pattern. Since the longer waves overrun the shorter ones, the resulting interference adds to the complexity of the pattern. The process of interference, illustrated in Figure 3302c, is duplicated many times in the sea; it is the principal reason that successive waves are not of the same height. The irregularity of the surface may be further accentuated by the presence of wave systems crossing at an angle to each other, producing peak-like rises. In reporting average wave heights, the mariner has a tenency to neglect the lower ones. It has been found that the reported value is about the average for the highest one third. This is sometimes called the “significant” wave height. The approximate relationship between this height and others, is as follows.

    3303. Path Of Water Particles In A Wave
    As shown in Figure 3303, a particle of water on the surface of the ocean follows a somewhat circular orbit as a wave passes, but moves very little in the direction of motion of the wave. The common wave producing this action is called an oscillatory wave. As the crest passes, the particle moves forward, giving the water the appearance of moving with the wave. As the trough passes, the motion is in the opposite direction. The radius of the circular orbit decreases with depth, approaching zero at a depth equal to about half the wavelength. In shallower water the orbits become more elliptical, and in very shallow water the vertical motion disappears almost completely. Since the speed is greater at the top of the orbit than at the bottom, the particle is not at exactly its original point following passage of a wave, but has moved slightly in the wave’s direction of motion. However, since this advance is small in relation to the vertical displacement, a floating object is raised and lowered by passage of a wave, but moved little from its original position. If this were not so, a slow moving vessel might experience considerable difficulty in making way against a wave train. In Figure 3303 the forward displacement is greatly exaggerated.


    3304. Effects Of Currents On Waves
    A following current increases wavelengths and decreases wave heights. An opposing current has the opposite effect, decreasing the length and increasing the height. This effect can be dangerous in certain areas of the world where a stream current opposes waves generated by severe weather.
    An example of this effect is off the Coast of South
    Africa, where the Agulhas current is often opposed by westerly storms, creating steep, dangerous seas. A strong opposing current may cause the waves to break, as in the case of overfalls in tidal currents. The extent of wave alteration is dependent upon the ratio of the still-water wave speed to the speed of the current.
    Moderate ocean currents running at oblique angles to wave directions appear to have little effect, but strong tidal currents perpendicular to a system of waves have been observed to completely destroy them in a short period of time.

    3305. The Effect Of Ice On Waves
    When ice crystals form in seawater, internal friction is greatly increased. This results in smoothing of the sea surface. The effect of pack ice is even more pronounced. A vessel following a lead through such ice may be in smooth water even when a gale is blowing and heavy seas are beating against the outer edge of the pack. Hail or torrential rain is also effective in flattening the sea, even in a high wind.

    3306. Waves And Shallow Water
    When a wave encounters shallow water, the movement of the water is restricted by the bottom, resulting in reduced wave speed. In deep water wave speed is a function of period. In shallow water, the wave speed becomes a function of depth. The shallower the water, the slower the wave speed. As the wave speed slows, the period remains the same, so the wavelength becomes shorter. Since the energy in the waves remains the same, the shortening of wavelengths results in increased heights. This process is called shoaling. If the wave approaches a shallow area at an angle, each part is slowed successively as the depth decreases. This causes a change in direction of motion, or refraction, the wave tending to change direction parallel to the depth curves. The effect is similar to the refraction of light and other forms of radiant energy.
    As each wave slows, the next wave behind it, in deeper water, tends to catch up. As the wavelength decreases, the height generally becomes greater. The lower part of a wave, being nearest the bottom, is slowed more than the top. This may cause the wave to become unstable, the faster-moving top falling forward or breaking. Such a wave is called a breaker, and a series of breakers is surf. Swell passing over a shoal but not breaking undergoes a decrease in wavelength and speed, and an increase in height, which may be sudden and dramatic, depending on the steepness of the seafloor’s slope. This ground swell may cause heavy rolling if it is on the beam and its period is the same as the period of roll of a vessel, even though the sea may appear relatively calm. It may also cause a rage sea, when the swell waves encounter water shoal enough to make them break. Rage seas are dangerous to small craft, particularly approaching from seaward, as the vessel can be overwhelmed by enormous breakers in perfectly calm weather. The swell waves, of course, may have been generated hundreds of miles away. In the open ocean they are almost unnoticed due to their very long period and wavelength. Figure 3306 illustrates the approximate alteration of the characteristics of waves as they cross a shoal.


    3307 Energy Of Waves
    The potential energy of a wave is related to the vertical distance of each particle from its still-water position. Therefore potential energy moves with the wave. In contrast, the kinetic energy of a wave is related to the speed of the particles, distributed evenly along the entire wave.

    The amount of kinetic energy in a wave is tremendous. A 4-foot, 10-second wave striking a coast expends more than 35,000 horsepower per mile of beach. For each 56 miles of
    coast, the energy expended equals the power generated at Hoover Dam.

    An increase in temperature of the water in the relatively narrow surf zone in which this energy is expended would seem to be indicated, but no pronounced increase has been measured.Apparently, any heat that may be generated is dissipated to the deeper water beyond the surf zone.

    3308 Wave Measurement Aboard Ship
    With suitable equipment and adequate training, reliable measurements of the height, length, period, and speed of waves can be made. However, the mariner’s estimates of height and length often contain relatively large errors. There is a tendency to underestimate the heights of low waves, and overestimate the heights of high ones. There are numerous accounts of waves 75 to 80 feet high, or even higher, although waves more than 55 feet high are very rare. Wavelength is usually underestimated. The motions of the vessel from which measurements are made contribute to such errors.
    Height. Measurement of wave height is particularly difficult. A microbarograph can be used if the wave is long enough or the vessel small enough to permit the vessel to ride from crest to trough. If the waves are approaching from dead ahead or dead astern, this requires a wavelength at least twice the length of the vessel. For most accurate results the instrument should be placed at the center of roll and pitch, to minimize the effects of these motions. Wave height can often be estimated with reasonable accuracy by comparing it with freeboard of the vessel. This is less accurate as wave height and vessel motion increase. If a point of observation can be found at which the top of a wave is in line with the horizon when the observer is in the trough, the wave height is equal to height of eye. However, if the vessel is rolling or pitching, this height at the moment of observation may be difficult to determine. The highest wave ever reliably reported was 112 feet observed from the USS Ramapo in 1933.
    Length. The dimensions of the vessel can be used to determine wavelength. Errors are introduced by perspective and disturbance of the wave pattern by the vessel. These errors are minimized if observations are made from maximum height. Best results are obtained if the sea is from dead ahead or dead astern.
    Period. If allowance is made for the motion of the vessel, wave period can be determined by measuring the interval between passages of wave crests past the observer. The relative
    motion of the vessel can be eliminated by timing the passage of successive wave crests past a patch of foam or a floating object at some distance from the vessel. Accuracy of results can be improved by averaging several observations.
    Speed. Speed can be determined by timing the passage of the wave between measured points along the side of the ship, if corrections are applied for the direction of travel for the wave and the speed of the ship.
    The length, period, and speed of waves are interrelated by the relationships indicated previously. There is no definite mathematical relationship between wave height and length, period, or speed.

    3309. Tsunamis
    Tsunamis are ocean waves produced by sudden, largescale motion of a portion of the ocean floor or the shore, such as a volcanic eruption, earthquake (sometimes called seaquake if it occurs at sea), or landslide. If they are caused by a submarine earthquake, they are usually called seismic sea waves. The point directly above the disturbance, at which the waves originate, is called the epicenter. Either a tsunami or a storm tide that overflows the land is popularly called a tidal wave, although it bears no relation to the tide.
    If a volcanic eruption occurs below the surface of the sea, the escaping gases cause a quantity of water to be pushed upward in the shape of a dome. The same effect is caused by the sudden rising of a portion of the bottom. As this water settles back, it creates a wave which travels at high speed across the surface of the ocean.

    Tsunamis are a series of waves. Near the epicenter, the first wave may be the highest. At greater distances, the highest wave usually occurs later in the series, commonly between the third and the eighth wave. Following the maximum, they again become smaller, but the tsunami may be detectable for several days.

    In deep water the wave height of a tsunami is probably never greater than 2 or 3 feet. Since the wavelength is usually considerably more than 100 miles, the wave is not conspicuous at sea. In the Pacific, where most tsunamis occur, the wave period varies between about 15 and 60 minutes, and the speed in deep water is more than 400 knots. The approximate speed can be computed by the formula:
    where S is the speed in knots, g is the acceleration due to gravity (32.2 feet per second per second), and d is the depth of water in feet. This formula is applicable to any wave in water having a depth of less than half the wavelength. For most ocean waves it applies only in shallow water, because of the relatively short wavelength.

    When a tsunami enters shoal water, it undergoes the same changes as other waves. The formula indicates that speed is proportional to depth of water. Because of the great speed of a tsunami when it is in relatively deep water, the slowing is relatively much greater than that of an ordinary wave crested by wind. Therefore, the increase in height is also much greater. The size of the wave depends upon the nature and intensity of the disturbance.
    The height and destructiveness of the wave arriving at any place depends upon its distance from the epicenter, topography of the ocean floor, and the coastline. The angle at which the wave arrives, the shape of the coastline, and the topography along the coast and offshore, all have an effect. The position of the shore is also a factor, as it may be sheltered by intervening land, or be in a position where waves have a tendency to converge, either because of refraction or reflection, or both.

    Tsunamis 50 feet in height or higher have reached the shore, inflicting widespread damage. On April 1, 1946, seismic sea waves originating at an epicenter near the Aleutians, spread over the entire Pacific. Scotch Cap Light on Unimak Island, 57 feet above sea level, was completely destroyed. Traveling at an average speed of 490 miles per hour, the waves reached the Hawaiian Islands in 4 hours and 34 minutes, where they arrived as waves 50 feet above the high water level, and flooded a strip of coast more than 1,000 feet wide at some places. They left a death toll of 173 and property damage of $25 million. Less destructive waves reached the shores of North and South America, as well as Australia, 6,700 miles from the epicenter.

    After this disaster, a tsunami warning system was set up in the Pacific, even though destructive waves are relatively rare (averaging about one in 20 years in the Hawaiian Islands). This system monitors seismic disturbances throughout the Pacific basin and predicts times and heights of tsunamis.
    Warnings are immediately sent out if a disturbance is detected.
    In addition to seismic sea waves, earthquakes below the surface of the sea may produce a longitudinal wave that travels upward at the speed of sound. When a ship encounters such a wave, it is felt as a sudden shock which may be so severe that the crew thinks the vessel has struck bottom.

    3310. Storm Tides
    In relatively tideless seas like the Baltic and Mediterranean, winds cause the chief fluctuations in sea level. Elsewhere, the astronomical tide usually masks these variations. However, under exceptional conditions, either severe extra-tropical storms or tropical cyclones can produce changes in sea level that exceed the normal range of tide. Low sea level is of little concern except to shipping, but a rise above ordinary high-water mark, particularly when it is accompanied by high waves, can result in a catastrophe.
    Although, like tsunamis, these storm tides or storm surges are popularly called tidal waves, they are not associated with the tide. They consist of a single wave crest and hence have no period or wavelength.
    Three effects in a storm induce a rise in sea level. The first is wind stress on the sea surface, which results in a piling-up of water (sometimes called “wind set-up”). The second effect is the convergence of wind-driven currents, which elevates the sea surface along the convergence line. In shallow water, bottom friction and the effects of local topography cause this elevation to persist and may even intensify it. The low atmospheric pressure that accompanies severe storms causes the third effect, which is sometimes referred to as the “inverted barometer.” An inch of mercury is equivalent to about 13.6 inches of water, and the adjustment of the sea surface to the reduced pressure can amount to several feet at equilibrium.
    All three of these causes act independently, and if they happen to occur simultaneously, their effects are additive. In addition, the wave can be intensified or amplified by the effects of local topography. Storm tides may reach heights of 20 feet or more, and it is estimated that they cause three-fourths of the deaths attributed to hurricanes.

    To be cont.
  6. Fishers of Men

    Fishers of Men Senior Member

    Chapter 33 cont.

    3311. Standing Waves And Seiches
    Previous articles in this chapter have dealt with progressive waves which appear to move regularly with time. When two systems of progressive waves having the same period travel in opposite directions across the same area, a series of standing waves may form. These appear to remain stationary. Another type of standing wave, called a seiche, sometimes occurs in a confined body of water. It is a long wave, usually having its crest at one end of the confined space, and its trough at the other. Its period may be anything from a few minutes to an hour or more, but somewhat less than the tidal period. Seiches are usually attributed to strong winds or differences in atmospheric pressure.

    3312. Tide Waves
    There are, in general, two regions of high tide separated by two regions of low tide, and these regions move progressively westward around the earth as the moon revolves in its orbit. The high tides are the crests of these tide waves, and the low tides are the troughs. The wave is not noticeable at sea, but becomes apparent along the coasts, particularly in funnelshaped estuaries. In certain river mouths, or estuaries of particular configuration, the incoming wave of high water overtakes the preceding low tide, resulting in a high-crested, roaring wave which progresses upstream in a surge called a bore.

    3313. Internal Waves
    Thus far, the discussion has been confined to waves on the surface of the sea, the boundary between air and water. Internal waves, or boundary waves, are created below the surface, at the boundaries between water strata of different densities. The density differences between adjacent water strata in the sea are considerably less than that between sea and air. Consequently, internal waves are much more easily formed than surface waves, and they are often much larger. The maximum height of wind waves on the surface is about 60 feet, but internal wave heights as great as 300 feet have been encountered. Internal waves are detected by a number of observations of the vertical temperature distribution, using recording devices such as the bathythermograph.

    They have periods as short as a few minutes, and as long as 12 or 24 hours, these greater periods being associated with the tides. A slow-moving ship, operating in a freshwater layer having a depth approximating the draft of the vessel, may produce short-period internal waves. This may occur off rivers emptying into the sea, or in polar regions in the vicinity of melting ice. Under suitable conditions, the normal propulsion energy of the ship is expended in generating and maintaining these internal waves and the ship appears to “stick” in the water, becoming sluggish and making little headway. The phenomenon, known as dead water, disappears when speed is increased by a few knots. The full significance of internal waves has not yet been determined, but it is known that they may cause submarines to rise and fall like a ship at the surface, and they may also affect sound transmission in the sea.

    3314. Waves And Ships
    The effects of waves on a ship vary considerably with the type of ship, its course and speed, and the condition of the sea. A short vessel has a tendency to ride up one side of a wave and down the other side, while a larger vessel may tend to ride through the waves on an even keel. If the waves are of such length that the bow and stern of a vessel are alternately riding in successive crests and troughs, the vessel is subject to heavy sagging and hogging stresses, and under extreme conditions may break in two. (Edmund Fitzgerald)A change of heading may reduce the danger. Because of the danger from sagging and hogging, a small vessel is sometimes better able to ride out a storm than a large one. If successive waves strike the side of a vessel at the same phase of successive rolls, relatively small waves can cause heavy rolling.

    The same effect, if applied to the bow or stern in time with the natural period of pitch, can cause heavy pitching. A change of either heading or speed can quickly reduce the effect.

    A wave having a length twice that of a ship places that ship in danger of falling off into the trough of the sea, particularly if it is a slow-moving vessel. (Or under powered)The effect is especially pronounced if the sea is broad on the bow or broad on the quarter. An increase of speed reduces the hazard.

    3315. Using Oil To Calm Breaking Waves
    Historically oil was effective in modifying the effects of breaking waves, and was useful to vessels when lowering or hoisting boats in rough weather. Its effect was greatest in deep water, where a small quantity sufficed if the oil were made to spread to windward of the vessel.
    Environmental concerns have led to this procedure being discontinued.

    3316. Refraction
    As explained previously, waves are slowed in shallow water, causing refraction if the waves approach the beach at an angle. Along a perfectly straight beach, with uniform shoaling, the wave fronts tend to become parallel to the shore. Any irregularities in the coastline or bottom contours, however, affect the refraction, causing irregularities. In the case of a ridge perpendicular to the beach, for instance, the shoaling is more rapid, causing greater refraction towards the ridge. The waves tend to align themselves with the bottom contours. Waves on both sides of the ridge have a component of motion toward the ridge. This convergence of wave energy toward the ridge causes an increase in wave or breaker height. A submarine canyon or valley perpendicular to the beach, on the other hand, produces divergence, with a decrease in wave or breaker height. These effects are illustrated in Figure 3316. Bends in the coast line have a similar effect, convergence occurring at a point, and divergence if the coast is concave to the sea. Points act as focal areas for wave energy and experience large breakers. Concave bays have small breakers because the energy is spread out as the waves approach the beach.


    Under suitable conditions, currents also cause refraction. This is of particular importance at entrances of tidal estuaries. When waves encounter a current running in the opposite direction, they become higher and shorter. This results in a choppy sea, often with breakers. When waves move in the same direction as current, they decrease in height, and become longer. Refraction occurs when waves encounter a current at an angle.
    Refraction diagrams, useful in planning amphibious operations, can be prepared with the aid of nautical charts or aerial photographs. When computer facilities are available, computer programs can be used to produce refraction diagrams quickly and accurately.


    3317. Classes Of Breakers
    In deep water, swell generally moves across the surface as somewhat regular, smooth undulations. When shoal water is reached, the wave period remains the same, but the
    speed decreases. The amount of decrease is negligible until the depth of water becomes about one-half the wavelength, when the waves begin to “feel” bottom. There is a slight decrease in wave height, followed by a rapid increase, if the waves are traveling perpendicular to a straight coast with a uniformly sloping bottom. As the waves become higher and shorter, they also become steeper, and the crest narrows. When the speed of the crest becomes greater than that of the wave, the front face of the wave becomes steeper than the rear face.

    This process continues at an accelerating rate as the depth of water decreases. If the wave becomes too unstable, it topples forward to form a breaker.

    There are three general classes of breakers.
    A spilling breaker breaks gradually over a considerable distance.
    A plunging breaker tends to curl over and break with a single crash.
    A surging breaker peaks up, but surges up the beach without spilling or plunging. It is classed as a breaker even though it does not actually break. The type of breaker which forms is determined by the steepness of the beach and the steepness of the wave before it reaches shallow water, as illustrated in Figure 3317.

    Long waves break in deeper water, and have a greater breaker height. A steep beach also increases breaker height. The height of breakers is less if the waves approach the beach at an acute angle. With a steeper beach slope there is greater tendency of the breakers to plunge or surge.

    Following the uprush of water onto a beach after the breaking of a wave, the seaward backrush occurs. The returning water is called backwash. It tends to further slow the bottom of a wave, thus increasing its tendency to break.

    This effect is greater as either the speed or depth of the backwash increases. The still water depth at the point of breaking is approximately 1.3 times the average breaker height. Surf varies with both position along the beach and time. A change in position often means a change in bottom contour, with the refraction effects discussed before. At the same point, the height and period of waves vary considerably from wave to wave. A group of high waves is usually followed by several lower ones. Therefore, passage through surf can usually be made most easily immediately following a series of higher waves.
    Since surf conditions are directly related to height of the waves approaching a beach, and to the configuration of the bottom, the state of the surf at any time can be predicted if one has the necessary information and knowledge of the principles involved. Height of the sea and swell can be predicted from wind data, and information on bottom configuration can sometimes be obtained from the largest scale nautical chart. In addition, the area of lightest surf along a beach can be predicted if details of the bottom configuration are available. Surf predictions may, however, be significantly in error due to the presence of swell from unknown storms hundreds of miles away.

    3318. Currents In The Surf Zone
    In and adjacent to the surf zone, currents are generated by waves approaching the bottom contours at an angle, and by irregularities in the bottom.
    Waves approaching at an angle produce a longshore current parallel to the beach, inside of the surf zone. Longshore currents are most common along straight beaches. Their speeds increase with increasing breaker height, decreasing wave period, increasing angle of breaker line with the beach, and increasing beach slope. Speed seldom exceeds 1 knot, but sustained speeds as high as 3 knots have been recorded. Longshore currents are usually constant in direction.

    They increase the danger of landing craft broaching to. Where the bottom is sandy a good distance offshore, one or more sand bars typically form. The innermost bar will break in even small waves, and will isolate the longshore current. The second bar, if one forms, will break only in heavier weather, and the third, if present, only in storms. It is possible to move parallel to the coast in small craft in relatively deep water in the area between these bars, between the lines of breakers.

    3319. Rip Currents
    As explained previously, wave fronts advancing over nonparallel bottom contours are refracted to cause convergence or divergence of the energy of the waves. Energy concentrations in areas of convergence form barriers to the returning backwash, which is deflected along the beach to areas of less resistance. Backwash accumulates at weak points, and returns seaward in concentrations, forming rip currents through the surf. At these points the large volume of returning water has a retarding effect upon the incoming waves, thus adding to the condition causing the rip current. The waves on one or both sides of the rip, having greater energy and not being retarded by the concentration of backwash, advance faster and farther up the beach. From here, they move along the beach as feeder currents. At some point of low resistance, the water flows seaward through the surf, forming the neck of the rip current. Outside the breaker line the current widens and slackens, forming the head. The various parts of a rip current are shown in Figure 3319.


    Rip currents may also be caused by irregularities in the beach face. If a beach indentation causes an uprush to advance farther than the average, the backrush is delayed and this in turn retards the next incoming foam line (the front of a wave as it advances shoreward after breaking) at that point. The foam line on each side of the retarded point continues in its advance, however, and tends to fill in the retarded area, producing a rip current.
    Rip currents are dangerous for swimmers, but may provide a clear path to the beach for small craft, as they tend to scour out the bottom and break through any sand bars that have formed. Rip currents also change location over time as conditions change.

    3320. Beach Sediments
    In the surf zone, large amounts of sediment are suspended in the water. When the water’s motion decreases, the sediments settle to the bottom. The water motion can be either waves or currents. Promontories or points are rocky because the large breakers scour the points and small sediments are suspended in the water and carried away. Bays tend to have sandy beaches because of the smaller waves. In the winter when storms create large breakers and surf, the waves erode beaches and carry the particles offshore where offshore sand bars form; sandy beaches tend to be narrower in stormy seasons. In the summer the waves gradually move the sand back to the beaches and the offshore sand bars decrease; then sandy beaches tend to be wider. Longshore currents move large amounts of sand along the coast. These currents deposit sand on the upcurrent side of a jetty or pier, and erode the beach on the downcurrent side. Groins are sometime built to impede the longshore flow of sediments and preserve beaches for recreational use. As with jetties, the downcurrent side of each groin will have the best water for approaching the beach.
    Last edited by a moderator: Apr 30, 2015
  7. Fishers of Men

    Fishers of Men Senior Member


    3400. Ice And The Navigator
    Sea ice has posed a problem to the polar navigator since antiquity. During a voyage from the Mediterranean to England and Norway sometime between 350 BC and 300 BC, Pytheas of Massalia sighted a strange substance which he described as “neither land nor air nor water” floating upon and covering the northern sea over which the summer sun barely set. Pytheas named this lonely region Thule, hence Ultima Thule (farthest north or land’s end). Thus began over 20 centuries of polar exploration.

    Ice is of direct concern to the navigator because it restricts and sometimes controls his movements; it affects his dead reckoning by forcing frequent and sometimes inaccurately determined changes of course and speed; it affects his piloting by altering the appearance or obliterating the features of landmarks; it hinders the establishment and maintenance of aids to navigation; it affects his use of electronics by affecting propagation of radio waves; it produces changes in surface features and in radar returns from these features; it affects celestial navigation by altering the refraction and obscuring the horizon and celestial bodies either directly or by the weather it influences, and it affects charts by introducing several plotting problems. Because of his direct concern with ice, the prospective polar navigator must acquaint himself with its nature and extent in the area he expects to navigate. In addition to this volume, books, articles, and reports of previous polar operations and expeditions will help acquaint the polar navigator with the unique conditions at the ends of the earth.


    3401. Formation Of Ice
    As it cools, water contracts until the temperature of maximum density is reached. Further cooling results in expansion.
    The maximum density of fresh water occurs at a temperature of 4.0°C, and freezing takes place at 0°C. The addition of salt lowers both the temperature of maximum density and, to a lesser extent, that of freezing. These relationships are shown in Figure 3401. The two lines meet at a salinity of 24.7 parts per thousand, at which maximum density occurs at the freezing temperature of –1.3°C. At this and greater salinities, the temperature of maximum density of sea water is coincident with the freezing point temperature, i.e., the density increases as the temperature gets colder. At a salinity of 35 parts per thousand, the approximate average for the oceans, the freezing point is –1.88°C. As the density of surface seawater increases with decreasing temperature, convective density-driven currents are induced bringing warmer, less dense water to the surface. If the polar seas consisted of water with constant salinity, the entire water column would have to be cooled to the freezing point in this manner before ice would begin to form. This is not the case, however, in the polar regions where the vertical salinity distribution is such that the surface waters are underlain at shallow depth by waters of higher salinity. In this instance density currents form a shallow mixed layer which subsequently cannot mix with the deep layer of warmer but saltier water. Ice will then begin forming at the water surface when density currents cease and the surface water reaches its freezing point. In shoal water, however, the mixing process can be sufficient to extend the freezing temperature from the surface to the bottom. Ice crystals can, therefore, form at any depth in this case. Because of their decreased density, they tend to rise to the surface, unless they form at the bottom and attach themselves there. This ice, called anchor ice, may continue to grow as additional ice freezes to that already formed.

    3402. Land Ice
    Ice of land origin is formed on land by the freezing of freshwater or the compacting of snow as layer upon layer adds to the pressure on that beneath.
    Under great pressure, ice becomes slightly plastic, and is forced downward along an inclined surface. If a large area is relatively flat, as on the Antarctic plateau, or if the outward flow is obstructed, as on Greenland, an ice cap forms and remains throughout the year. The thickness of these ice caps ranges from nearly 1 kilometer on Greenland to as much as 4.5 kilometers on the Antarctic Continent. Where ravines or mountain passes permit flow of the ice, a glacier is formed. This is a mass of snow and ice which continuously flows to lower levels, exhibiting many of the characteristics of rivers of water. The flow may be more than 30 meters per day, but is generally much less. When a glacier reaches a comparatively level area, it spreads out. When a glacier flows into the sea, the buoyant force of the water breaks off pieces from time to time, and these float away as icebergs. Icebergs may be described as dome shaped, sloping or pinnacled, glacier, or weathered.

    A floating iceberg seldom melts uniformly because of lack of uniformity in the ice itself, differences in the temperature above and below the waterline, exposure of one side to the sun, strains, cracks, mechanical erosion, etc. The inclusion of rocks, silt, and other foreign matter further accentuates the differences. As a result, changes in equilibrium take place, which may cause the berg to periodically tilt or capsize. Parts of it may break off or calve, forming separate smaller bergs. A relatively large piece of floating ice, generally extending 1 to 5 meters above the sea surface and normally about 100 to 300 square meters in area, is called a bergy bit. A smaller piece of ice large enough to inflict serious damage to a vessel is called a growler because of the noise it sometimes makes as it bobs up and down in the sea. Growlers extend less than 1 meter above the sea surface and normally occupy an area of about 20 square meters. Bergy bits and growlers are usually pieces calved from icebergs, but they may be the remains of a mostly melted iceberg.

    The principal danger from icebergs is their tendency to break or capsize. Soon after a berg is calved, while remaining in far northern waters, 60–80% of its bulk is submerged. But as the berg drifts into warmer waters, the underside can sometimes melt faster than the exposed portion, especially in very cold weather. As the mass of the submerged portion deteriorates, the berg becomes increasingly unstable, and it will eventually roll over. Icebergs that have not yet capsized have a jagged and possibly dirty appearance. A recently capsized berg will be smooth, clean, and curved in appearance. Previous waterlines at odd angles can sometimes be seen after one or more capsizings.

    The stability of a berg can sometimes be noted by its reaction to ocean swells. The livelier the berg, the more unstable it is. It is extremely dangerous for a vessel to approach an iceberg closely, even one which appears stable, because in addition to the danger from capsizing, unseen cracks can cause icebergs to split in two or calve off large chunks.

    Another danger is from underwater extensions, called rams, which are usually formed due to melting or erosion above the waterline at a faster rate than below. Rams may also extend from a vertical ice cliff, also known as an ice front, which forms the seaward face of a massive ice sheet or floating glacier; or from an ice wall, which is the ice cliff forming the seaward margin of a glacier which is aground. In addition to rams, large portions of an iceberg may extend well beyond the waterline at greater depths.
    Strangely, icebergs may be helpful to the mariner in some ways. The melt water found on the surface of icebergs is a source of freshwater, and in the past some daring seamen have made their vessels fast to icebergs which, because they are affected more by currents than the wind, have proceeded to tow them out of the ice pack.

    Icebergs can be used as a navigational aid in extreme latitudes where charted depths may be in doubt or non-existent. Since an iceberg (except a large tabular berg) must be at least as deep in the water as it is high to remain upright, a grounded berg can provide an estimate of the minimum water depth at its location. Water depth will be at least equal to the exposed height of the grounded iceberg. Grounded bergs remain stationary while current and wind move sea ice past them. Drifting ice may pile up against the upcurrent side of a grounded berg.



    3403. Sea Ice
    Sea ice forms by the freezing of seawater and accounts for 95 percent of all ice encountered. The first indication of the formation of new sea ice (up to 10 centimeters in thickness) is the development of small individual, needle-like crystals of ice, called spicules, which become suspended in the top few centimeters of seawater. These spicules, also known as frazil ice, give the sea surface an oily appearance. Grease ice is formed when the spicules coagulate to form a soupy layer on the surface, giving the sea a matte appearance.

    The next stage in sea ice formation occurs when shuga, an accumulation of spongy white ice lumps a few centimeters across, develops from grease ice. Upon further freezing, and depending upon wind exposure, seas, and salinity, shuga and grease ice develop into nilas, an elastic crust of high salinity, up to 10 centimeters in thickness, with a matte surface, or into ice rind, a brittle, shiny crust of low salinity with a thickness up to approximately 5 centimeters. A layer of 5 centimeters of freshwater ice is brittle but strong enough to support the weight of a heavy man. In contrast, the same thickness of newly formed sea ice will support not more than about 10 percent of this weight, although its strength varies with the temperatures at which it is formed; very cold ice supports a greater weight than warmer ice. As it ages, sea ice becomes harder and more brittle.

    New ice may also develop from slush which is formed when snow falls into seawater which is near its freezing point, but colder than the melting point of snow. The snow does not melt, but floats on the surface, drifting with the wind into beds. If the temperature then drops below the freezing point of the seawater, the slush freezes quickly into a soft ice similar to shuga. Sea ice is exposed to several forces, including currents, waves, tides, wind, and temperature variations. In its early stages, its plasticity permits it to conform readily to virtually any shape required by the forces acting upon it. As it becomes older, thicker, more brittle, and exposed to the influence of wind and wave action, new ice usually separates into circular pieces from 30 centimeters to 3 meters in diameter and up to approximately 10 centimeters in thickness with raised edges due to individual pieces striking against each other.
    These circular pieces of ice are called pancake ice (Figure 3403) and may break into smaller pieces with strong wave motion.


    Any single piece of relatively flat sea ice less than 20 meters across is called an ice cake. With continued low temperatures, individual ice cakes and pancake ice will, depending on wind or wave motion, either freeze together to form a continuous sheet or unite into pieces of ice 20 meters or more across. These larger pieces are then called ice floes, which may further freeze together to form an ice covered area greater than 10 kilometers across known as an ice field . In wind sheltered areas thickening ice usually forms a continuous sheet before it can develop into the characteristic ice cake form. When sea ice reaches a thickness of between 10 to 30 centimeters it is referred to as gray and gray-white ice, or collectively as young ice, and is the transition stage between nilas and first-year ice. First-year ice usually attains a thickness of between 30 centimeters and 2 meters in its first winter’s growth.
    Sea ice may grow to a thickness of 10 to 13 centimeters within 48 hours, after which it acts as an insulator between the ocean and the atmosphere progressively slowing its further growth. However, sea ice may grow to a thickness of between 2 to 3 meters in its first winter. Ice which has survived at least one summer’s melt is classified as old ice. If it has survived only one summer’s melt it may be referred to as second-year ice, but this term is seldom used today. Old ice which has attained a thickness of 3 meters or more and has survived at least two summers’ melt is known as multiyear ice and is almost salt free. This term is increasingly used to refer to any ice more than one season old. Old ice can be recognized by a bluish tone to its surface color in contrast to the greenish tint of first-year ice, but it is often covered with snow. Another sign of old ice is a smoother, more rounded appearance due to melting/refreezing and weathering.

    Greater thicknesses in both first and multiyear ice are attained through the deformation of the ice resulting from the movement and interaction of individual floes. Deformation
    processes occur after the development of new and young ice and are the direct consequence of the effects of winds, tides, and currents. These processes transform a relatively flat sheet of ice into pressure ice which has a rough surface. Bending, which is the first stage in the formation of pressure ice, is the upward or downward motion of thin and very plastic ice. Rarely, tenting occurs when bending produces an upward displacement of ice forming a flat sided arch with a cavity beneath. More frequently, however, rafting takes place as one piece of ice overrides another. When pieces of first-year ice are piled haphazardly over one another forming a wall or line of broken ice, referred to as a ridge, the process is known as ridging.

    Pressure ice with topography consisting of numerous mounds or hillocks is called hummocked ice, each mound being called a hummock. The motion of adjacent floes is seldom equal. The rougher the surface, the greater is the effect of wind, since each piece extending above the surface acts as a sail. Some ice floes are in rotary motion as they tend to trim themselves into the wind. Since ridges extend below as well as above the surface, the deeper ones are influenced more by deep water currents. When a strong wind blows in the same direction for a considerable period, each floe exerts pressure on the next one, and as the distance increases, the pressure becomes tremendous. Ridges on sea ice are generally about 1 meter high and 5 meters deep, but under considerable pressure may attain heights of 20 meters and depths of 50 meters in extreme cases.

    The alternate melting and growth of sea ice, combined with the continual motion of various floes that results in separation as well as consolidation, causes widely varying conditions within the ice cover itself. The mean areal density, or concentration, of pack ice in any given area is expressed in tenths. Concentrations range from: open water (total concentration of all ice is less than one tenth), very open pack (1 to 3 tenths concentration), open pack (4 to 6 tenths concentration), close pack (7 to 8 tenths concentration), very close pack (9 to 10 to less than 10 to 10 concentration), to compact or consolidated pack (10 to 10 or complete coverage). The extent to which an ice cover of varying concentrations can be penetrated by a vessel varies from place to place and with changing weather conditions. With a concentration of 1 to 3 tenths in a given area, an unreinforced vessel can generally navigate safely, but the danger of receiving heavy damage is always present. When the concentration increases to between 3 and 5 tenths, the area becomes only occasionally accessible to an unreinforced vessel, depending upon the wind and current. With concentrations of 5 to 7 tenths, the area becomes accessible only to ice strengthened vessels, which on occasion will require icebreaker assistance. Navigation in areas with concentrations of 7 tenths or more should only be attempted by icebreakers.

    Within the ice cover, openings may develop resulting from a number of deformation processes. Long, jagged cracks may appear first in the ice cover or through a single floe. When these cracks part and reach lengths of a few meters to many kilometers, they are referred to as fractures. If they widen further to permit passage of a ship, they are called leads. In winter, a thin coating of new ice may cover the water within a lead, but in summer the water usually remains ice-free until a shift in the movement forces the two sides together again. A lead ending in a pressure ridge or other impenetrable barrier is a blind lead.
    A lead between pack ice and shore is a shore lead, and one between pack and fast ice is a flaw lead. Navigation in these two types of leads is dangerous, because if the pack ice closes with the fast ice, the ship can be caught between the two, and driven aground or caught in the shear zone between. Before a lead refreezes, lateral motion generally occurs between the floes, so that they no longer fit and unless the pressure is extreme, numerous large patches of open water remain. These nonlinear shaped openings enclosed in ice are called polynyas.
    Polynyas may contain small fragments of floating ice and may be covered with miles of new and young ice. Recurring polynyas occur in areas where upwelling of relatively warmer water occurs periodically. These areas are often the site of historical native settlements, where the polynyas permit fishing and hunting at times before regular seasonal ice breakup. Thule, Greenland, is an example. Sea ice which is formed in situ from seawater or by the freezing of pack ice of any age to the shore and which remains attached to the coast, to an ice wall, to an ice front, or between shoals is called fast ice.

    The width of this fast ice varies considerably and may extend for a few meters or several hundred kilometers. In bays and other sheltered areas, fast ice, often augmented by annual snow accumulations and the seaward extension of land ice, may attain a thickness of over 2 meters above the sea surface. When a floating sheet of ice grows to this or a greater thickness and extends over a great horizontal distance, it is called an ice shelf. Massive ice shelves, where the ice thickness reaches several hundred meters, are found in both the Arctic and Antarctic.

    The majority of the icebergs found in the Antarctic do not originate from glaciers, as do those found in the Arctic, but are calved from the outer edges of broad expanses of shelf ice. Icebergs formed in this manner are called tabular icebergs, having a box like shape with horizontal dimensions measured in kilometers, and heights above the sea surface approaching 60 meters. See Figure 3402b. The largest Antarctic ice shelves are found in the Ross and Weddell Seas. The expression “tabular iceberg” is not applied to bergs which break off from Arctic ice shelves; similar formations there are called ice islands. These originate when shelf ice, such as that found on the northern coast of Greenland and in the bays of Ellesmere Island, breaks up. As a rule, Arctic ice islands are not as large as the tabular icebergs found in the Antarctic. They attain a thickness of up to 55 meters and on the average extend 5 to 7 meters above the sea surface. Both tabular icebergs and ice islands possess a gently rolling surface. Because of their deep draft, they are influenced much more by current than wind. Arctic ice islands have been used as floating scientific platforms from which polar research has been conducted.

    3404. Thickness Of Sea Ice
    Sea ice has been observed to grow to a thickness of almost 3 meters during its first year. However, the thickness of firstyear ice that has not undergone deformation does not generally exceed 2 meters. In coastal areas where the melting rate is less than the freezing rate, the thickness may increase during succeeding winters, being augmented by compacted and frozen snow, until a maximum thickness of about 3.5 to 4.5 meters may eventually be reached. Old sea ice may also attain a thickness of over 4 meters in this manner, or when summer melt water from its surface or from snow cover runs off into the sea and refreezes under the ice where the seawater temperature is below the freezing point of the fresher melt water.

    The growth of sea ice is dependent upon a number of meteorological and oceanographic parameters. Such parameters include air temperature, initial ice thickness, snow depth, wind speed, seawater salinity and density, and the specific heats of sea ice and seawater. Investigations, however, have shown that the most influential parameters affecting sea ice growth are air temperature, wind speed, snow depth and initial ice thickness. Many complex equations have been formulated to predict ice growth using these four parameters.
    However, except for the first two, these parameters are not routinely observed for remote polar locations.
    Field measurements suggest that reasonable growth estimates can be obtained from air temperature data alone.



    Various empirical formulae have been developed based on this premise. All appear to perform better under thin ice conditions when the temperature gradient through the ice is linear, generally true for ice less than 100 centimeters thick. Differences in predicted thicknesses between models generally reflect differences in environmental parameters (snowfall, heat content of the underlying water column, etc.) at the measurement site. As a result, such equations must be considered partially site specific and their general use approached with caution. For example, applying an equation derived from central Arctic data to coastal conditions or to Antarctic conditions could lead to substantial errors. For this reason Zubov’s formula is widely cited as it represents an average of many years of observations from the Russian Arctic:


    where h is the ice thickness in centimeters for a given day and is the cumulative number of frost degree days in degrees Celsius since the beginning of the freezing season. A frost degree day is defined as a day with a mean temperature of 1below an arbitrary base. The base most commonly used is the freezing point of freshwater (0C). If, for example, the mean temperature on a given day is 5below freezing, then five frost degree days are noted for that day. These frost degree days are then added to those noted the next day to obtain an accumulated value, which is then added to those noted the following day.
    This process is repeated daily throughout the ice growing season. Temperatures usually fluctuate above and below freezing for several days before remaining below freezing. Therefore, frost degree day accumulations are initiated on the first day of the period when temperatures remain below freezing. The relationship between frost degree day accumulations and theoretical ice growth curves at Point Barrow, Alaska is shown in Figure 3404a. Similar curves for other Arctic stations are contained in publications available from the U.S. Naval Oceanographic Office and the National Ice Center. Figure 3404b graphically depicts the relationship between accumulated frost degree days (C) and ice thickness in centimeters. During winter, the ice usually becomes covered with snow, which insulates the ice beneath and tends to slow down its rate of growth. This thickness of snow cover varies considerably from region to region as a result of differing climatic conditions. Its depth may also vary widely within very short distances in response to variable winds and ice topography. While this snow cover persists, about 80 to 85 percent of the incoming radiation is reflected back to space. Eventually, however, the snow begins to melt, as the air temperature rises above 0C in early summer and the resulting freshwater forms puddles on the surface. These puddles absorb about 90 percent of the incoming radiation and rapidly enlarge as they melt the surrounding snow or ice.
    Eventually the puddles penetrate to the bottom surface of the floes and as thawholes. This slow process is characteristic of ice in the Arctic Ocean and seas where movement is restricted by the coastline or islands. Where ice is free to drift into warmer waters (e.g., the Antarctic, East Greenland, and the Labrador Sea), decay is accelerated in response to wave erosion as well as warmer air and sea temperatures.

    To be cont.
  8. Fishers of Men

    Fishers of Men Senior Member

    ch 34 cont.
    When you see a "?" mark by a number it is supposed to be degrees, sometimes it works other times not...I don't know why!

    3405. Salinity Of Sea Ice
    Sea ice forms first as salt-free crystals near the surface of the sea. As the process continues, these crystals are joined together and, as they do so, small quantities of brine are trapped within the ice. On the average, new ice 15 centimeters thick contains 5 to 10 parts of salt per thousand. With lower temperatures, freezing takes place faster. With faster freezing, a greater amount of salt is trapped in the ice.

    Depending upon the temperature, the trapped brine may either freeze or remain liquid, but because its density is greater than that of the pure ice, it tends to settle down through the pure ice. As it does so, the ice gradually freshens, becoming clearer, stronger, and more brittle. At an age of 1 year, sea ice is sufficiently fresh that its melt water, if found in puddles of sufficient size, and not contaminated by spray from the sea, can be used to replenish the freshwater supply of a ship. However, ponds of sufficient size to water ships are seldom found except in ice of great age, and then much of the meltwater is from snow which has accumulated on the surface of the ice. When sea ice reaches an age of about 2 years, virtually all of the salt has been eliminated. Icebergs, having formed from precipitation, contain no salt, and uncontaminated melt water obtained from them is fresh.

    The settling out of the brine gives sea ice a honeycomb structure which greatly hastens its disintegration when the temperature rises above freezing. In this state, when it is called rotten ice, much more surface is exposed to warm air and water, and the rate of melting is increased. In a day’s time, a floe of apparently solid ice several inches thick may disappear completely.

    3406. Density Of Ice
    The density of freshwater ice at its freezing point is 0.917gm/cm3. Newly formed sea ice, due to its salt content, is more dense, 0.925 gm/cm3 being a representative value. The density decreases as the ice freshens. By the time it has shed most of its salt, sea ice is less dense than freshwater ice, because ice formed in the sea contains more air bubbles. Ice having no salt but containing air to the extent of 8 percent by volume (an approximately maximum value for sea ice) has a density of 0.845 gm/cm3.
    The density of land ice varies over even wider limits. That formed by freezing of freshwater has a density of 0.917gm/cm3, as stated above. Much of the land ice, however, is formed by compacting of snow. This results in the entrapping of relatively large quantities of air. Névé, a snow which has become coarse grained and compact through temperature change, forming the transition stage to glacier ice, may have an air content of as much as 50 percent by volume. By the time the ice of a glacier reaches the sea, its density approaches that of freshwater ice. A sample taken from an iceberg on the Grand Banks had a density of 0.899gm/cm3.

    When ice floats, part of it is above water and part is below the surface. The percentage of the mass below the surface can be found by dividing the average density of the ice by the density of the water in which it floats. Thus, if an iceberg of density 0.920 floats in water of density 1.028 (corresponding to a salinity of 35 parts per thousand and a temperature of 1°C), 89.5 percent of its mass will be below the surface.

    The height to draft ratio for a blocky or tabular iceberg probably varies fairly closely about 1:5. This average ratio was computed for icebergs south of Newfoundland by considering density values and a few actual measurements, and by seismic means at a number of locations along the edge of the Ross Ice Shelf near Little America Station. It was also substantiated by density measurements taken in a nearby hole drilled through the 256-meter thick ice shelf. The height to draft ratios of icebergs become significant when determining their drift.

    3407. Drift Of Sea Ice
    Although surface currents have some affect upon the drift of pack ice, the principal factor is wind. Due to Coriolis force, ice does not drift in the direction of the wind, but varies from approximately 18° to as much as 90° from this direction, depending upon the force of the surface wind and the ice thickness. In the Northern Hemisphere, this drift is to the right of the direction toward which the wind blows, and in the Southern Hemisphere it is toward the left. Although early investigators computed average angles of approximately 28° or 29° for the drift of close multiyear pack ice, large drift angles were usually observed with low, rather than high, wind speeds. The relationship between surface wind speed, ice thickness, and drift angle was derived theoretically for the drift of consolidated pack under equilibrium (a balance of forces acting on the ice) conditions, and shows that the drift angle increases with increasing ice thickness and decreasing surface wind speed. A slight increase also occurs with higher latitude. Since the cross-isobar deflection of the surface wind over the oceans is approximately 20°, the deflection of the ice varies, from approximately along the isobars to as much as 70° to the right of the isobars, with low pressure on the left and high pressure on the right in the Northern Hemisphere. The positions of the low and high pressure areas are, of course, reversed in the Southern Hemisphere.

    The rate of drift depends upon the roughness of the surface and the concentration of the ice. Percentages vary from approximately 0.25 percent to almost 8 percent
    of the surface wind speed as measured approximately 6 meters above the ice surface. Low concentrations of heavily ridged or hummocked floes drift faster than high concentrations of lightly ridged or hummocked floes with the same wind speed. Sea ice of 8 to 9 tenths concentrations and six tenths hummocking or close multiyear ice will drift at approximately 2 percent of the surface wind speed. Additionally, the response factors of 1 and 5 tenths ice concentrations, respectively, are approximately three times and twice the magnitude of the response factor for 9 tenths ice concentrations with
    the same extent of surface roughness. Isolated ice floes have been observed to drift as fast as 10 percent to 12 percent of strong surface winds.


    The rates at which sea ice drifts have been quantified through empirical observation. The drift angle, however, has been determined theoretically for 10 tenths ice concentration. This relationship presently is extended to the drift of all ice concentrations, due to the lack of basic knowledge of the dynamic forces that act upon, and result in redistribution of sea ice, in the polar regions.

    3408. Iceberg Drift
    Icebergs extend a considerable distance below the surface and have relatively small “sail areas” compared to their subsurface mass. Therefore, the near-surface current is thought to be primarily responsible for drift; however, observations have shown that wind can be the dominant force that governs iceberg drift at a particular location or time. Also, the current and wind may contribute nearly equally to the resultant drift.

    Two other major forces which act on a drifting iceberg are the Coriolis force and, to a lesser extent, the pressure gradient force which is caused by gravity owing to a tilt of the sea surface, and is important only for iceberg drift in a major current. Near-surface currents are generated by a variety of factors such as horizontal pressure gradients owing to density variations in the water, rotation of the earth, gravitational attraction of the moon, and slope of the sea surface. Wind not only acts directly on an iceberg, but also indirectly by generating waves and a surface current in about the same direction as the wind.

    Because of inertia, an iceberg may continue to move from the influence of wind for some time after the wind stops or changes direction. The relative influence of currents and winds on the drift of an iceberg varies according to the direction and magnitude of the forces acting on its sail area and subsurface cross-sectional area. The resultant force therefore involves the proportions of the iceberg above and below the sea surface in relation to the velocity and depth of the current, and the velocity and duration of the wind. Studies tend to show that, generally, where strong currents prevail, the current is dominant.

    In regions of weak currents, however, winds that blow for a number of hours in a steady direction materially affect the drift of icebergs. Generally, it can be stated that currents tend to have a greater effect on deep-draft icebergs, while winds tend to have a greater effect on shallow-draft icebergs. As icebergs waste through melting, erosion, and calving, observations indicate the height to draft ratio may approach 1:1 during their last stage of decay, when they are referred to as valley, winged, horned, or spired icebergs. The height to draft ratios found for icebergs in their various stages are presented in Table 3408a. Since wind tends to have a greater effect on shallow than on deep-draft icebergs, the wind can be expected to exert increasing influence on iceberg drift as wastage increases.
    Simple equations which precisely define iceberg drift cannot be formulated at present because of the uncertainty in the water and air drag coefficients associated with iceberg motion. Values for these parameters not only vary from iceberg to iceberg, but they probably change for the same iceberg over its period of wastage. Present investigations utilize an analytical approach, facilitated by computer calculations, in which the air and water drag coefficients are varied within reasonable limits. Combinations of these drag values are then used in several increasingly complex water models that try to duplicate observed iceberg trajectories. The results indicate that with a wind-generated current, Coriolis force, and a uniform wind, but without a gradient current, small and medium icebergs will drift with the percentages of the wind as given in Table 3408b.


    The drift will be to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. When gradient currents are introduced, trajectories vary considerably depending on the magnitude of the wind and current, and whether they are in the same or opposite direction. When a 1-knot current and wind are in the same direction, drift is to the right of both wind and current with drift angles increasing linearly from approximately 5at 10 knots to 22at 60 knots.

    When the wind and a 1-knot current are in opposite directions, drift is to the left of the current, with the angle increasing from approximately 3at 10 knots, to 20at 30 knots, and to 73at 60 knots. As a limiting case for increasing wind speeds, drift may be approximately normal (to the right) to the wind direction. This indicates that the wind generated current is clearly dominating the drift. In general, the various models used demonstrated that a combination of the wind and current was responsible for the drift of icebergs.

    3409. Extent Of Ice In The Sea
    When an area of sea ice, no matter what form it takes or how it is disposed, is described, it is referred to as pack ice. In both polar regions the pack ice is a very dynamic feature, with wide deviations in its extent dependent upon changing oceanographic and meteorological phenomena. In winter the Arctic pack extends over the entire Arctic Ocean, and for a varying distance outward from it; the limits recede considerably during the warmer summer months.

    The average positions of the seasonal absolute and mean maximum and minimum extents of sea ice in the Arctic region are plotted in Figure 3409a.


    Each year a large portion of the ice from the Arctic Ocean moves outward between Greenland and Spitsbergen (Fram Strait) into the North Atlantic Ocean and is replaced by new ice. Because of this constant annual removal and replacement of sea ice, relatively little of the Arctic pack ice is more than 10 years old. Ice covers a large portion of the Antarctic waters and is probably the greatest single factor contributing to the isolation of the Antarctic Continent. During the austral winter (June through September), ice completely surrounds the continent, forming an almost impassable barrier that extends northward on the average to about 54S in the Atlantic and to about 62S in the Pacific.

    Disintegration of the pack ice during the austral summer months of December through March allows the limits of the ice edge to recede considerably, opening some coastal areas of the Antarctic to navigation. The seasonal absolute and mean maximum and minimum positions of the Antarctic ice limit are shown in Figure 3409b.


    Historical information on sea conditions for specific localities and time periods can be found in publications of the Naval Ice Center/National Ice Center (formerly Naval Polar Oceanography Center/U.S. Navy/NOAA Joint Ice Center) and the Defense Mapping Agency Hydrographic/Topographic Center (DMAHTC). National Ice Center (NIC) publications include sea ice annual atlases (1972 to present for Eastern Arctic, Western Arctic and Antarctica), sea ice climatologies, and forecasting guides. NIC sea ice annual atlases include years 1972 to the present for all Arctic and Antarctic seas. NIC ice climatologies describe multiyear statistics for ice extent and coverage. NIC forecasting guides cover procedures for the production of short-term (daily, weekly), monthly, and seasonal predictions. DMAHTC publications include sailing directions which describe localized ice conditions and the effect of ice on Arctic navigation.

    3410. Icebergs In The North Atlantic
    Sea level glaciers exist on a number of landmasses bordering the northern seas, including Alaska, Greenland, Svalbard (Spitsbergen), Zemlya Frantsa-Iosifa (Franz Josef Land), Novaya Zemlya, and Severnaya Zemlya (Nicholas II Land). Except in Greenland and Franz Josef Land, the rate of calving is relatively slow, and the few icebergs produced
    melt near their points of formation. Many of those produced along the western coast of Greenland, however, are eventually carried into the shipping lanes of the North Atlantic, where they constitute a major menace to ships.
    Those calved from Franz Josef Land glaciers drift southwest in the Barents Sea to the vicinity of Bear Island.

    Generally the majority of icebergs produced along the east coast of Greenland remain near their source. However, a small number of bergy bits, growlers, and small icebergs are transported south from this region by the East Greenland Current around Kap Farvel at the southern tip of Greenland and then northward by the West Greenland Current into Davis Strait to the vicinity of 67N. Relatively few of these icebergs menace shipping, but some are carried to the south and southeast of Kap Farvel by a counterclockwise current gyre centered near 57N and 43W.

    The main source of the icebergs encountered in the North Atlantic is the west coast of Greenland between 67N and 76N, where approximately 10,000–15,000 icebergs are calved each year. In this area there are about 100 lowlying coastal glaciers, 20 of them being the principal producers of icebergs. Of these 20 major glaciers, 2 located in Disko Bugt between 69N and 70N are estimated to contribute 28 percent of all icebergs appearing in Baffin Bay and the Labrador Sea. The West Greenland Current carries icebergs from this area northward and then westward until they encounter the south flowing Labrador Current. West Greenland icebergs generally spend their first winter locked in the Baffin Bay pack ice; however, a large number can also be found within the sea ice extending along the entire Labrador coast by late winter. During the next spring and summer, when they are freed by the break up of the pack ice, they are transported farther southward by the Labrador Current.

    The general drift patterns of icebergs that are prevalent in the eastern portion of the North American Arctic are shown in Figure 3410a.
    Observations over a 79-year period show that an average of 427 icebergs per year reach latitudes south of 48N, with approximately 10 percent of this total carried south of the Grand Banks (43N) before they melt. Icebergs may be encountered during any part of the year, but in the Grand Banks area they are most numerous during spring. The maximum monthly average of iceberg sightings below 48N occurs during April, May and June, with May having the highest average of 129.

    The variation from average conditions is considerable. More than 2,202 icebergs have been sighted south of latitude 48N in a single year (1984), while in 1966 not a single iceberg was encountered in this area. In the years of 1940 and 1958, only one iceberg was observed south of 48N.

    The length of the iceberg “season” as defined by the International Ice Patrol also varies considerably, from 97 days in 1965 to 203 days in 1992, with an average length of 132 days. Although this variation has not been fully explained, it is apparently related to wind conditions, the distribution of pack ice in Davis Strait, and to the amount of pack ice off Labrador. It has been suggested that the distribution of the Davis Strait-Labrador Sea pack ice influences the melt rate of the icebergs as they drift south. Sea ice will decrease iceberg erosion by damping waves and holding surface water temperatures below 0C, so as the areal extent of the sea ice increases the icebergs will tend to survive longer.


    Stronger than average northerly or northeasterly winds during late winter and spring will enhance sea ice drift to the south, which also may lengthen iceberg lifetimes. There are also large interannual variations in the number of icebergs calved from Greenland’s glaciers, so the problem of forecasting the length and severity of an iceberg season is exceedingly complex.

    Average iceberg and pack ice limits in this area during May are shown in Figure 3410b. Icebergs have been observed in the vicinity of Bermuda, the Azores, and within 400 to 500 kilometers of Great Britain. Pack ice may also be found in the North Atlantic, some having been brought south by the Labrador Current and some coming through Cabot Strait after having formed in the Gulf of St. Lawrence.


    3411. The International Ice Patrol
    The International Ice Patrol was established in 1914 by the International Convention for the Safety of Life at Sea (SOLAS), held in 1913 as a result of the sinking of the RMS Titanic in 1912. The Titanic struck an iceberg on its maiden voyage and sank with the loss of 1,513 lives. In accordance with the agreement reached at the SOLAS conventions of 1960 and 1974, the International Ice Patrol is conducted by the U.S. Coast Guard, which is responsible for the observation and dissemination of information concerning ice conditions in the North Atlantic. Information on ice conditions for the Gulf of St. Lawrence and the coastal waters of Newfoundland and Labrador, including the Strait of Belle Isle, is provided by ECAREG Canada (Eastern Canada Traffic System), through any Coast Guard Radio Station, from the month of December through late June. Sea ice data for these areas can also be obtained from the Ice Operations Officer, located at Dartmouth, Nova Scotia, via Sydney, Halifax, or St. John’s marine radio.

    During the war years of 1916-18 and 1941-45, the Ice Patrol was suspended. Aircraft were added to the patrol force following World War II, and today perform the majority of
    the reconnaissance work. During each ice season, aerial reconnaissance surveys are made in the vicinity of the Grand Banks off Newfoundland to determine the southeastern, southern, and southwestern limit of the seaward extent of icebergs.

    The U.S. Coast Guard aircraft use Side-Looking Airborne Radar (SLAR) as well as Forward-Looking Airborne Radar (FLAR) to help detect and identify icebergs in this notoriously fog-ridden area. Reports of ice sightings are also requested and collected from ships transiting the Grand Banks area. When reporting ice, vessels are requested to detail the concentration and stage of development of sea ice, number of icebergs, the bearing of the principal sea ice edge, and the present ice situation and trend over the preceding three hours. These five parameters are part of the ICE group of the ship synoptic code which is addressed in more detail in Section 3416 on ice observation. In addition to ice reports, masters who do not issue routine weather reports are urged to make sea surface temperature and weather reports to the Ice Patrol every six hours when within latitudes 40° to 52°N and longitudes 38° to 58°W (the Ice Patrol Operations Area). Ice reports may be sent at no charge using INMARSAT Code 42.

    The Ice Patrol activities are directed from an Operations Center at Avery Point, Groton, Connecticut. The Ice Patrol gathers all sightings and puts them into a computer model which analyzes and predicts iceberg drift and deterioration. Due to the large size of the Ice Patrol’s operations area, icebergs are infrequently resighted. The model predictions are crucial to setting the limits of all known ice. The fundamental model force balance is between iceberg acceleration and accelerations due to air and water drag, the Coriolis force, and a sea surface slope term. The model is primarily driven by a water current that combines a depthand time-independent geostrophic (mean) current with a depth- and time-dependent current driven by the wind (Ekman flow).

    Environmental parameters for the model, including sea surface temperature, wave height and period, and wind, are obtained from the U.S. Navy’s Fleet Numerical Meteorology and Oceanography Center (FNMOC) in Monterey, California every 12 hours. The International Ice Patrol also deploys from 12–15 World Ocean Circulation Experiment (WOCE) drifting buoys per year, and uses the buoy drifts to alter the climatological mean (geostrophic) currents used by the model in the immediate area of the buoys. The buoy drift data have been archived at the National Oceanographic Data Center (NODC) and are available for use by researchers outside the Coast Guard. Sea surface temperature, wave height and wave period are the main forces determining the rate of iceberg deterioration. Ship observations of these variables are extremely important in making model inputs more accurately reflect actual situations.
    The results from the iceberg drift and deterioration model are used to compile bulletins that are issued twice daily during the ice season by radio communications from Boston, Massachusetts; St. John’s, Newfoundland; and other radio stations. Bulletins are also available over INMARSAT. When icebergs are sighted outside the known limits of ice, special safety broadcasts are issued in between the regularly scheduled bulletins.

    Iceberg positions in the ice bulletins are updated for drift and deterioration at 12-
    hour intervals. A radio-facsimile chart is also broadcast twice a day throughout the ice season. A summary of broadcast times and frequencies is found in Pub. 117, Radio Navigational Aids.

    The Ice Patrol, in addition to patrolling possible iceberg areas, conducts oceanographic surveys, maintains upto-date records of the currents in its area of operation to aid in predicting the drift of icebergs, and studies iceberg conditions in general.

    3412. Ice Detection
    Safe navigation in the polar seas depends on a number of factors, not the least of which is accurate knowledge of the location and amount of sea ice that lies between the mariner and his destination. Sophisticated electronic equipment, such as radar, sonar, and the visible, infrared, and microwave radiation sensors on board satellites, have added to our ability to detect and thus avoid ice.

    As a ship proceeds into higher latitudes, the first ice encountered is likely to be in the form of icebergs, because such large pieces require a longer time to disintegrate. Icebergs can easily be avoided if detected soon enough. The distance at which an iceberg can be seen visually depends upon meteorological visibility, height of the iceberg, source and condition of lighting, and the observer. On a clear day with excellent visibility, a large iceberg might be sighted at a distance of 20 miles. With a low-lying haze around the horizon, this distance will be reduced. In light fog or drizzle this distance is further reduced, down to near zero in heavy fog.
    In a dense fog an iceberg may not be perceptible until it is close aboard where it will appear in the form of a luminous, white object if the sun is shining; or as a dark, somber mass with a narrow streak of blackness at the waterline if the sun is not shining. If the layer of fog is not too thick, an iceberg may be sighted from aloft sooner than from a point lower on the vessel, but this does not justify omitting a bow lookout. The diffusion of light in a fog will produce a blink, or area of whiteness, above and at the sides of an iceberg which will appear to increase the apparent size of its mass.
    On dark, clear nights icebergs may be seen at a distance of from 1 to 3 miles, appearing either as white or black objects with occasional light spots where waves break against it. Under such conditions of visibility growlers are a greater menace to vessels; the vessel’s speed should be reduced and a sharp lookout maintained.

    The moon may either help or hinder, depending upon its phase and position relative to ship and iceberg. A full moon in the direction of the iceberg interferes with its detection, while moonlight from behind the observer may produce a blink which renders the iceberg visible for a greater distance, as much as 3 or more miles. A clouded sky at night, through which the moonlight is intermittent, also renders ice detection difficult. A night sky with heavy passing clouds may also dim or obscure any object which has been sighted, and fleecy cumulus and cumulonimbus clouds often may give the appearance of blink from icebergs.

    If an iceberg is in the process of disintegration, its presence may be detected by a cracking sound as a piece breaks off, or by a thunderous roar as a large piece falls into the water. These sounds are unlikely to be heard due to shipboard noise. The appearance of small pieces of ice in the water often indicates the presence of an iceberg nearby. In calm weather these pieces may form a curved line with the parent iceberg on the concave side. Some of the pieces broken from an iceberg are themselves large enough to be a menace to ships.
    As the ship moves closer towards areas known to contain sea ice, one of the most reliable signs that pack ice is being approached is the absence of swell or wave motion in
    a fresh breeze or a sudden flattening of the sea, especially from leeward. The observation of icebergs is not a good indication that pack ice will be encountered soon, since icebergs may be found at great distances from pack ice. If the sea ice is approached from windward, it is usually compacted and the edge will be sharply defined. However, if it is approached from leeward, the ice is likely to be loose and somewhat scattered, often in long narrow arms.

    Another reliable sign of the approach of pack ice not yet in sight is the appearance of a pattern, or sky map, on the horizon or on the underside of distant, extensive cloud areas, created by the varying amounts of light reflected from different materials on the sea or earth’s surface. A bright white glare, or snow blink, will be observed above a snow covered surface. When the reflection on the underside of clouds is caused by an accumulation of distant ice, the glare is a little less bright and is referred to as an ice blink.
    A relatively dark pattern is reflected on the underside of clouds when it is over land that is not snow covered. This is known as a land sky. The darkest pattern will occur when the clouds are above an open water area, and is called a water sky.

    A mariner experienced in recognizing these sky maps will find them useful in avoiding ice or searching out openings which may permit his vessel to make progress through an ice field. Another indication of the presence of sea ice is the formation of thick bands of fog over the ice edge, as moisture condenses from warm air when passing over the colder ice. An abrupt change in air or sea temperature or seawater salinity is not a reliable sign of the approach of icebergs or pack ice.

    The presence of certain species of animals and birds can also indicate that pack ice is in close proximity. The sighting of walruses, seals, or polar bears in the Arctic should warn the mariner that pack ice is close at hand. In the Antarctic, the usual precursors of sea ice are penguins, terns, fulmars, petrels, and skuas.

    When visibility becomes limited, radar can prove to be an invaluable tool for the polar mariner. Although many icebergs will be observed visually on clear days before there is a return on the radarscope, radar under bad weather conditions will detect the average iceberg at a range of about 8 to 10 miles. The intensity of the return is a function of the nature of the iceberg’s exposed surface (slope, surface roughness); however, it is unusual to find an iceberg which will not produce a detectable echo.

    Large, vertical-sided tabular icebergs of the Antarctic and Arctic ice islands are usually detected by radar at ranges of 15 to 30 miles; a range of 37 miles has been reported.
    Whereas a large iceberg is almost always detected by radar in time to be avoided, a growler large enough to be a serious menace to a vessel may be lost in the sea return and escape detection. If an iceberg or growler is detected by radar, tracking is sometimes necessary to distinguish it from a rock, islet, or another ship.

    Radar can be of great assistance to an experienced radar observer. Smooth sea ice, like smooth water, returns little or no echo, but small floes of rough, hummocky sea ice capable of inflicting damage to a ship can be detected in a smooth sea at a range of about 2 to 4 miles. The return may be similar to sea return, but the same echoes appear at each sweep. A lead in smooth ice is clearly visible on a radarscope, even though a thin coating of new ice may have formed in the opening. A light covering of snow obliterating many of the features to the eye has little effect upon a radar return. The ranges at which ice can be detected by radar are somewhat dependent upon refraction, which is sometimes quite abnormal in polar regions. Experience in interpretation is gained through comparing various radar returns with actual observations.

    Echoes from the ship’s whistle or horn may sometimes indicate the presence of icebergs and can give an indication of direction. If the time interval between the sound and its echo is measured, the distance in meters can be determined by multiplying the number of seconds by 168. However, echoes are very unreliable reliable because only ice with a large vertical area facing the ship returns enough echo to be heard. Once an echo is heard, a distinct pattern of horn blasts (not a Navigational Rules signal) should be made to confirm that the echo is not another vessel.
    At relatively short ranges, sonar is sometimes helpful in locating ice. The initial detection of icebergs may be made at a distance of about 3 miles or more, but usually considerably less. Growlers may be detected at a distance of ½ to 2 miles, and even smaller pieces may be detected in time to avoid them.

    Ice in the polar regions is best detected and observed from the air, either from aircraft or by satellite. Fixedwinged aircraft have been utilized extensively for obtaining
    detailed aerial ice reconnaissance information since the early 1930’s, and will no doubt continue to provide this invaluable service for many years to come. Some ships, particularly icebreakers, proceeding into high latitudes carry helicopters, which are invaluable in locating leads and determining the relative navigability of different portions of the ice pack. Ice reports from personnel at Arctic and Antarctic coastal shore stations can also prove valuable to the polar mariner.

    The enormous ice reconnaissance capabilities of meteorological satellites were confirmed within hours of the launch by the National Aeronautics and Space Administration (NASA) of the first experimental meteorological satellite, TIROS I, on April 1, 1960. With the advent of the polar-orbiting meteorological satellites during the mid and late 1960’s, the U.S. Navy initiated an operational satellite ice reconnaissance program which could observe ice and its movement in any region of the globe on a daily basis, depending upon solar illumination. Since then, improvements in satellite sensor technology have provided a capability to make detailed global observations of ice properties under all weather and lighting conditions. The current suite of airborne
    and satellite sensors employed by the National Ice Center include: aerial reconnaissance including visual and Side-Looking Airborne Radar (SCAR), TIROS AVHRR visual and infrared, Defense Meteorological Satellite Program (DMSP) Operational Linescan System (OLS) visual and infrared, all-weather passive microwave from the DMSP Special Sensor Microwave Imager (SSM/I) and the ERS-1 Synthetic Aperture Radar (SAR). Examples of satellite imagery of ice covered waters are shown in Figure 3412a and Figure 3412b.



    3413. Operations In Ice
    Operations in ice-prone regions necessarily require considerable advanced planning and many more precautionary measures than those taken prior to a typical open ocean voyage. The crew, large or small, of a polar-bound vessel should be thoroughly indoctrinated in the fundamentals of polar operations, utilizing the best information sources available. The subjects covered should include training in ship handling in ice, polar navigation, effects of low temperatures on materials and equipment, damage control procedures, communications problems inherent in polar regions, polar meteorology, sea ice terminology, ice observing and reporting procedures (including classification and codes) and polar survival.
    Training materials should consist of reports on previous Arctic and Antarctic voyages, sailing directions, ice atlases, training films on polar operations, and U.S. Navy service manuals detailing the recommended procedures to follow during high latitude missions. Various sources of information can be obtained from the Director, National Ice Center, 4251 Suitland Road, Washington, D.C., 20395 and from the Office of Polar Programs, National Science Foundation, Washington, D.C. The preparation of a vessel for polar operations is of extreme importance and the considerable experience gained from previous operations should be drawn upon to bring the ship to optimum operating condition. At the very least, operations conducted in ice-infested waters require that the vessel’s hull and propulsion system undergo certain modifications.

    The bow and waterline of the forward part of the vessel should be heavily reinforced. Similar reinforcement should also be considered for the propulsion spaces of the vessel. Cast iron propellers and those made of a bronze alloy do not possess the strength necessary to operate safely in ice.
    Therefore, it is strongly recommended that propellers made of these materials be replaced by steel. Other desirable features are the absence of vertical sides, deep placement of the propellers, a blunt bow, metal guards to protect propellers from ice damage, and lifeboats for 150 percent of personnel aboard.
    The complete list of desirable features depends upon the area of operations, types of ice to be encountered, length of stay in the vicinity of ice, anticipated assistance by icebreakers, and possibly other factors. Strength requirements and the minimum thicknesses deemed necessary for the vessel’s frames and additional plating to be used as reinforcement, as well as other procedures needed to outfit a vessel for ice operations, can be obtained from the American Bureau of Shipping. For a more definitive and complete guide to the ice strengthening of ships, the mariner may desire to consult the procedures outlined in Rules for Ice Strengthening of Ships, from the Board of Navigation, Helsinki, Finland. Equipment necessary to meet the basic needs of the crew and to insure the successful and safe completion of the polar voyage should not be overlooked. A minimum list of essential items should consist of polar clothing and footwear, 100% u/v protection sunglasses, food, vitamins, medical supplies, fuel, storage batteries, antifreeze, explosives, detonators, fuses, meteorological supplies, and survival kits containing sleeping bags, trail rations, firearms, ammunition, fishing gear, emergency medical supplies, and a repair kit.

    The vessel’s safety depends largely upon the thoroughness of advance preparations, the alertness and skill of its crew, and their ability to make repairs if damage is incurred. Spare propellers, rudder assemblies, and patch materials, together with the equipment necessary to effect emergency repairs of structural damage should be carried. Examples of repair materials needed include quick setting cement, oakum, canvas, timbers, planks, pieces of steel of varying shapes, welding equipment, clamps, and an assortment of nuts, bolts, washers, screws, and nails.

    Ice and snow accumulation on the vessel poses a definite capsize hazard. Mallets, baseball bats, ax handles, and scrapers to aid in the removal of heavy accumulations of ice, together with snow shovels and stiff brooms for snow removal should be provided. A live steam line may be useful in removing ice from superstructures.

    Navigation in polar waters is at best difficult and, during poor conditions, impossible. Environmental conditions encountered in high latitudes such as fog, storms, compass anomalies, atmospheric effects, and, of course, ice, hinder polar operations. Also, deficiencies in the reliability and detail of hydrographic and geographical information presented on polar navigation charts, coupled with a distinct lack of reliable bathymetry, current, and tidal data, add to the problems of polar navigation. Much work is being carried out in polar regions to improve the geodetic control, triangulation, and quality of hydrographic and topographic information necessary for accurate polar charts. However, until this massive task is completed, the only resource open to the polar navigator, especially during periods of poor environmental conditions, is to rely upon the basic principles of navigation and adapt them to unconventional methods when abnormal situations arise.

    Upon the approach to pack ice, a careful decision is needed to determine the best action. Often it is possible to go around the ice, rather than through it. Unless the pack is quite loose, this action usually gains rather than loses time. When skirting an ice field or an iceberg, do so to windward, if a choice is available, to avoid projecting tongues of ice or individual pieces that have been blown away from the main body of ice.
    When it becomes necessary to enter pack ice, a thorough examination of the distribution and extent of the ice conditions should be made beforehand from the highest possible location. Aircraft (particularly helicopters) and direct satellite readouts are of great value in determining the nature of the ice to be encountered. The most important features to be noted include the location of open water, such as leads and polynyas, which may be manifested by water sky; icebergs; and the presence or absence of both ice under pressure and rotten ice. Some protection may be offered the propeller and rudder assemblies by trimming the vessel down by the stern slightly (not more than 2–3 feet) prior to entering the ice; however, this precaution usually impairs the maneuvering characteristics of most vessels not specifically built for ice breaking.

    Selecting the point of entry into the pack should be done with great care; and if the ice boundary consists of closely packed ice or ice under pressure, it is advisable to skirt the edge until a more desirable point of entry is located. Seek areas with low ice concentrations, areas of rotten ice or those containing navigable leads, and if possible enter from leeward on a course perpendicular to the ice edge. It is also advisable to take into consideration the direction and force of the wind, and the set and drift of the prevailing currents when determining the point of entry and the course followed thereafter.
    Due to wind induced wave action, ice floes close to the periphery of the ice pack will take on a bouncing motion which can be quite hazardous to the hull of thin-skinned vessels. In addition, note that pack ice will drift slightly to the right of the true wind in the Northern Hemisphere and to the left in the Southern Hemisphere, and that leads opened by the force of the wind will appear perpendicular to the wind direction. If a suitable entry point cannot be located due to less than favorable conditions, patience may be called for. Unfavorable conditions generally improve over a short period of time by a change in the wind, tide, or sea state. Once in the pack, always try to work with the ice, not against it, and keep moving, but do not rush. Respect the ice but do not fear it. Proceed at slow speed at first, staying in open water or in areas of weak ice if possible. The vessel’s speed may be safely increased after it has been ascertained how well it handles under the varying ice conditions encountered. It is better to make good progress in the general direction desired than to fight large thick floes in the exact direction to be made good. However, avoid the temptation to proceed far to one side of the intended track; it is almost always better to back out and seek a more penetrable area.
    During those situations when it becomes necessary to back, always do so with extreme caution and with the rudder amidships. If the ship is stopped by ice, the first command should be “rudder amidships,” given while the screw is still turning. This will help protect the propeller when backing and prevent ice jamming between rudder and hull. If the rudder becomes ice-jammed, man after steering, establish communications, and do not give any helm commands until the rudder is clear. A quick full-ahead burst may clear it. If it does not, try going to “hard rudder” in the same direction slowly while turning full or flank speed ahead.

    Ice conditions may change rapidly while a vessel is working in pack ice, necessitating quick maneuvering. Conventional vessels, even though ice strengthened, are not built for ice breaking. The vessel should be conned to first attempt to place it in leads or polynyas, giving due consideration to wind conditions. The age, thickness, and size of ice which can be navigated depends upon the type, size, hull strength, and horsepower of the vessel employed. If contact with an ice floe is unavoidable, never strike it a glancing blow. This maneuver may cause the ship to veer off in a direction which will swing the stern into the ice. If possible, seek weak spots in the floe and hit it head-on at slow speed. Unless the ice is rotten or very young, do not attempt to break through the floe, but rather make an attempt to swing it aside as speed is slowly increased. Keep clear of corners and projecting points of ice, but do so without making sharp turns which may throw the stern against the ice, resulting in a damaged propeller, propeller shaft, or rudder.

    The use of full rudder in nonemergency situations is not recommended because it may swing either the stern or mid-section of the vessel into the ice. This does not preclude use of alternating full rudder (swinging the rudder) aboard ice-breakers as a technique for penetrating heavy ice.
    Offshore winds may open relatively ice free navigable coastal leads, but such leads should not be entered without benefit of icebreaker escort. If it becomes necessary to enter coastal leads, narrow straits, or bays, an alert watch should be maintained since a shift in the wind may force drifting ice down upon the vessel. An increase in wind on the windward side of a prominent point, grounded iceberg, or land ice tongue extending into the sea will also endanger a vessel. It is wiser to seek out leads toward the windward side of the main body of the ice pack. In the event that the vessel is under imminent danger of being trapped close to shore by pack ice, immediately attempt to orient the vessel’s bow seaward.

    This will help to take advantage of the little maneuvering room available in the open water areas found between ice floes. Work carefully through these areas, easing the ice floes aside while maintaining a close watch on the general movement of the ice pack. If the vessel is completely halted by pack ice, it is best to keep the rudder amidships, and the propellers turning at slow speed. The wash of the propellers will help to clear ice away from the stern, making it possible to back down safely. When the vessel is stuck fast, an attempt first should be made to free the vessel by going full speed astern. If this maneuver proves ineffective, it may be possible to get the vessel’s stern to move slightly, thereby causing the bow to shift, by quickly shifting the rudder from one side to the other while going full speed ahead. Another attempt at going astern might then free the vessel. The vessel may also be freed by either transferring water from ballast tanks, causing the vessel to list, or by alternately flooding and emptying the fore and aft tanks. A heavy weight swung out on the cargo boom might give the vessel enough list to break free. If all these methods fail, the utilization of deadmen (2– to 4–meter lengths of timber buried in holes out in the ice and to which a vessel is moored) and ice anchors (a stockless, singlefluked hook embedded in the ice) may be helpful. With a deadman or ice anchors attached to the ice astern, the vessel may be warped off the ice by winching while the engines are going full astern. If all the foregoing methods fail, explosives placed in holes cut nearly to the bottom of the ice approximately 10 to 12 meters off the beam of the vessel and detonated while the engines are working full astern might succeed in freeing the vessel. A vessel may also be sawed out of the ice if the air temperature is above the freezing point of seawater. When a vessel becomes so closely surrounded by ice that all steering control is lost and it is unable to move, it is beset. It may then be carried by the drifting pack into shallow water or areas containing thicker ice or icebergs with their accompanying dangerous underwater projections. If ice forcibly presses itself against the hull, the vessel is said to be nipped, whether or not damage is sustained. When this occurs, the gradually increasing pressure may be capable of holing the vessel’s bottom or crushing the sides. When a vessel is beset or nipped, freedom may be achieved through the careful maneuvering procedures, the physical efforts of the crew, or by the use of explosives similar to those previously detailed. Under severe conditions the mariner’s best ally may be patience since there will be many times when nothing can be done to improve the vessel’s plight until there is a change in meteorological conditions. It may be well to preserve fuel and perform any needed repairs to the vessel and its engines. Damage to the vessel while it is beset is usually attributable to collisions or pressure exerted between the vessel’s hull, propellers, or rudder assembly, and the sharp corners of ice floes. These collisions can be minimized greatly by attempting to align the vessel in such a manner as to insure that the pressure from the surrounding pack ice is distributed as evenly as possible over the hull. This is best accomplished when medium or large ice floes encircle the vessel.
    In the vicinity of icebergs, either in or outside of the pack ice, a sharp lookout should be kept and all icebergs given a wide berth. The commanding officers and masters of all vessels, irrespective of their size, should treat all icebergs with great respect. The best locations for lookouts are generally in a crow’s nest, rigged in the foremast or housed
    in a shelter built specifically for a bow lookout in the eyes of a vessel. Telephone communications between these sites and the navigation bridge on larger vessels will prove in valuable.

    It is dangerous to approach close to an iceberg of any size because of the possibility of encountering underwater extensions, and because icebergs that are disintegrating may suddenly capsize or readjust their masses to new positions of equilibrium. In periods of low visibility the utmost caution is needed at all times. Vessel speed should be reduced and the watch prepared for quick maneuvering. Radar becomes an effective tool in this case, but does not negate the need for trained lookouts.

    Since icebergs may have from eight to nine-tenths of their masses below the water surface, their drift is generally influenced more by currents than winds, particularly under light wind conditions. The drift of pack ice, on the other hand, is usually dependent upon the wind. Under these conditions, icebergs within the pack may be found moving at a different rate and in a different direction from that of the pack ice. In regions of strong currents, icebergs should always be given a wide berth because they may travel upwind under the influence of contrary currents, breaking heavy pack in their paths and endangering vessels unable to work clear. In these situations, open water will generally be found to leeward of the iceberg, with piled up pack ice to windward. Where currents are weak and a strong wind predominates, similar conditions will be observed as the wind driven ice pack overtakes an iceberg and piles up to windward with an open water area lying to leeward.
    Under ice, submarine operations require knowledge of prevailing and expected sea ice conditions to ensure maximum operational efficiency and safety. The most important ice features are the frequency and extent of downward projections (bummocks and ice keels) from the underside of the ice canopy (pack ice and enclosed water areas from the point of view of the submariner), the distribution of thin ice areas through which submarines can attempt to surface, and the probable location of the outer pack edge where submarines can remain surfaced during emergencies to rendezvous with surface ship or helicopter units.
    Bummocks are the subsurface counterpart of hummocks, and ice keels are similarly related to ridges.
    When the physical nature of these ice features is considered, it is apparent that ice keels may have considerable horizontal extent, whereas individual bummocks can be expected to have little horizontal extent. In shallow water lanes to the Arctic Basin, such as the Bering Strait and the adjoining portions of the Bering Sea and Chukchi Sea, deep bummocks and ice keels may leave little vertical room for submarine passage. Widely separated bummocks may be circumnavigated but make for a hazardous passage. Extensive ice areas, with numerous bummocks or ice keels which cross the lane may effectively block both surface and submarine passage into the Arctic Basin.
    Bummocks and ice keels may extend downward approximately five times their vertical extent above the ice surface. Therefore, observed ridges of approximately 10 meters may extend as much as 50 meters below sea level. Because of the direct relation of the frequency and vertical extent between these surface features and their subsurface counterparts, aircraft ice reconnaissance should be conducted over a planned submarine cruise track before under ice operations commence.

    Skylights are thin places (usually less than 1 meter thick) in the ice canopy, and appear from below as relatively light translucent patches in dark surroundings. The undersurface of a skylight is usually flat; not having been subjected to great pressure. Skylights are called large if big enough for a submarine to attempt to surface through them; that is, have a linear extent of at least 120 meters. Skylights smaller than 120 meters are referred to as small. An ice canopy along a submarine’s track that contains a number of large skylights or other features such as leads and polynyas which permit a submarine to surface more frequently than 10 times in 30 miles, is called friendly ice. An ice canopy containing no large skylights or other features which permit a submarine to surface is called hostile ice.

    to be cont.
    Last edited by a moderator: Apr 30, 2015
  9. Fishers of Men

    Fishers of Men Senior Member

    Chapter 34 cont

    3414. Great Lakes Ice
    Large vessels have been navigating the Great Lakes since the early 1760’s. This large expanse of navigable water has since become one of the world’s busiest waterways. Due to the northern geographical location of the Great Lakes Basin and its susceptibility to Arctic outbreaks of polar air during winter, the formation of ice plays a major disruptive role in the region’s economically vital marine industry. Because of the relatively large size of the five Great Lakes, the ice cover which forms on them is affected by the wind and currents to a greater degree than on smaller lakes. The Great Lakes’ northern location results in a long ice growth season, which in combination with the effect of wind and current, imparts to their ice covers some of the characteristics and behavior of an Arctic ice pack.

    Since the five Great Lakes extend over a distance of approximately 800 kilometers in a north-south direction, each lake is influenced differently by various meteorological phenomena. These, in combination with the fact that each lake also possesses different geographical characteristics, affect the extent and distribution of their ice covers.
    The largest, deepest, and most northern of the Great Lakes is Lake Superior. Initial ice formation normally begins at the end of November or early December in harbors and bays along the north shore, in the western portion of the lake and over the shallow waters of Whitefish Bay. As the season progresses, ice forms and thickens in all coastal areas of the lake perimeter prior to extending offshore. This formation pattern can be attributed to a maximum depth in excess of 400 meters and an associated large heat storage capacity that hinders early ice formation in the center of the lake. During a normal winter, ice not under pressure ranges in thickness from 45–85 centimeters. During severe winters, maximum thicknesses are reported to approach 100 centimeters. Winds and currents acting upon the ice have been known to cause ridging with heights approaching 10 meters. During normal years, maximum ice cover extends over approximately 75% of the lake surface with heaviest ice conditions occurring by early March. This value increases to 95% coverage during severe winters and decreases to less than 20% coverage during a mild winter.

    Winter navigation is most difficult in the southeastern portion of the lake due to heavy ridging and compression of the ice under the influence of prevailing westerly winds. Breakup normally starts near the end of March with ice in a state of advanced deterioration by the middle of April. Under normal conditions, most of the lake is ice-free by the first week of May.

    Lake Michigan extends in a north-south direction over 490 kilometers and possesses the third largest surface area of the five Great Lakes. Depths range from 280 meters in the center of the lake to 40 meters in the shipping lanes through the Straits of Mackinac, and less in passages between island groups.

    During average years, ice formation first occurs in the shallows of Green Bay and extends eastward along the northern coastal areas into the Straits of Mackinac during the second half of December and early January. Ice formation and accumulation proceeds southward with coastal ice found throughout the southern perimeter of the lake by late January. Normal ice thicknesses
    range from 10–20 centimeters in the south to 40–60 centimeters in the north. During normal years, maximum ice cover extends over approximately 40% of the lake surface with heaviest conditions occurring in late February and early March. Ice coverage increases to 85–90% during a severe winter and decreases to only 10–15% during a mild year. Coverage of 100% occurs, but rarely.

    Throughout the winter, ice formed in mid-lake areas tends to drift eastward because of prevailing westerly winds. This movement of ice causes an area in the southern central portion of the lake to remain ice-free throughout a normal winter. Extensive
    ridging of ice around the island areas adjacent to the Straits of Mackinac presents the greatest hazard to year-round navigation on this lake. Due to an extensive length and northsouth orientation, ice formation and deterioration often occur simultaneously in separate regions of this lake. Ice break-up normally begins by early March in southern areas and progresses to the north by early April. Under normal conditions, only 5–10% of the lake surface is ice covered by mid-April with lingering ice in Green Bay and the Straits of Mackinac completely melting by the end of April.


    Lake Huron, the second largest of the Great Lakes, has maximum depths of 230 and 170 meters in the central basin west of the Bruce peninsula and in Georgian Bay, respectively.
    The pattern of ice formation in Lake Huron is similar to the north-south progression described in Lake Michigan. Initial ice formation normally begins in the North Channel and along the eastern coast of Saginaw and Georgian Bays by mid-December.

    Ice rapidly expands into the western and southern coastal areas before extending out into the deeper portions of the lake by late January. Normal ice thicknesses are 45–75 centimeters. During severe winters, maximum ice thicknesses often exceed 100 centimeters with windrows of ridged ice achieving thicknesses of up to 10 meters.

    During normal years, maximum ice cover occurs in late February with 60% coverage in Lake Huron and nearly 95% coverage in Georgian Bay. These values increase to 85–90% in Lake Huron and nearly 100% in Georgian Bay during severe winters. The percent of lake surface area covered by ice decreases to 20–25% for both bodies of water during mild years. During the winter, ice as a hazard to navigation is of greatest concern in the St. Mary’s River/North Channel area and the Straits of Mackinac. Ice break-up normally begins in mid-March in southern coastal areas with melting conditions rapidly spreading northward by early April. A recurring threat to navigation is the southward drift and accumulation of melting ice at the entrance of the St. Clair river. Under normal conditions, the lake becomes ice free by the first week of May.


    The shallowest and most southern of the Great Lakes is Lake Erie. Although the maximum depth nears 65 meters in the eastern portion of the lake, an overall mean depth of only 20 meters results in the rapid accumulation of ice over a short period of time with the onset of winter. Initial ice formation begins in the very shallow western portion of the lake in mid-December with ice rapidly extending eastward by early January.

    The eastern portion of the lake does not normally become ice covered until late January. During a normal winter, ice thicknesses range from 25–45 centimeters in Lake Erie. During the period of rapid ice growth, prevailing winds and currents routinely move existing ice to the northeastern end of the lake. This accumulation of ice under pressure is often characterized by ridging with maximum heights of 8–10 meters. During a severe winter, initial ice formation may begin in late November with maximum seasonal ice
    thicknesses exceeding 70 centimeters. Since this lake reacts rapidly to changes in air temperature, the variability of percent ice cover is the greatest of the five Great Lakes.

    During normal years, ice cover extends over approximately 90–95% of the lake surface by mid to late February. This value increases to nearly 100% during a severe winter and decreases to 30% ice coverage during a mild year.

    Lake St. Clair, on the connecting waterway to Lake Huron, is normally consolidated from the middle of January until early March. Ice break-up normally begins in the western portion of Lake Erie in early March with the lake becoming mostly ice-free by the middle of the month. The exception to this rapid deterioration is the extreme eastern end of the lake where ice often lingers until early May.

    Lake Ontario has the smallest surface area and second greatest mean depth of the Great Lakes. Depths range from 245 meters in the southeastern portion of the lake to 55 meters in the approaches to the St. Lawrence River.

    Like Lake Superior, a large mean depth gives Lake Ontario a large heat storage capacity which, in combination with a small surface area, causes Lake Ontario to respond slowly to changing meteorological conditions. As a result, this lake produces the smallest amount of ice cover found on any of the Great Lakes. Initial ice formation normally begins from the middle to late December in the Bay of Quinte and extends to the western coastal shallows near the mouth of the St. Lawrence River by early January. By the first half of February, Lake Ontario is almost 20% ice covered with shore ice lining the perimeter of the lake.

    During normal years, ice cover extends over approximately 25% of the lake surface by the second half of February.
    During this period of maximum ice coverage, ice is typically concentrated in the northeastern portion of the lake by prevailing westerly winds and currents. Ice coverage can extend over 50–60% of the lake surface during a severe winter and less than 10% during a mild year. Level lake ice thicknesses normally fall within the 20–60 centimeter range with occasional reports exceeding 70 centimeters during severe years. Ice break-up normally begins in early March with the lake generally becoming ice free by mid-April.


    The maximum ice cover distribution attained by each of the Great lakes for mild, normal and severe winters is shown in Figure 3414a, Figure 3414b and Figure 3414c.

    It should be noted that although the average maximum ice cover for each lake appears on the same chart, the actual occurrence of each distribution takes place during the time periods described within the preceding narratives. Information concerning ice analyses and forecasts for the Great Lakes can be obtained from the Director, National Ice Center, 4251 Suitland Road, Washington D.C. 20395 and the National Weather Service Forecast Office located in Cleveland, Ohio. Ice climatological information can be obtained from the Great Lakes Environmental Research Laboratory, Ann Arbor, Michigan.

    3415. Importance Of Ice Information
    Advance knowledge of ice conditions to be encountered and how these conditions will change over specified time periods are invaluable for both the planning and operational phases of a voyage to the polar regions. Branches of the United States Federal Government responsible for providing operational ice products and services for safety of navigation include the Departments of Defense (U.S. Navy), Commerce (NOAA), and Transportation (U.S. Coast Guard). Manpower and resources from these agencies comprise the National Ice Center (NIC), which replaced the Navy/NOAA Joint Ice Center. The NIC provides ice products and services to U.S. Government military and civilian interests. Routine and tailored ice products of the NIC shown in Table 3417 can be separated into two categories:
    a) analyses which describe current ice conditions and
    b) forecasts which define the expected changes in the existing ice cover over a specified time period.

    The content of sea ice analyses is directly dependent upon the planned use of the product, the required level of detail, and the availability of on-site ice observations and/or remotely-sensed data. Ice analyses are produced by blending relatively small numbers of visual ice observations from ships, shore stations and fixed wing aircraft with increasing amounts of remotely sensed data. These data include aircraft and satellite imagery in the visual, infrared, passive microwave and radar bands. The efficient receipt and accurate interpretation of these data are critical to producing a near real-time (24–48 hour old) analysis or “picture” of the ice cover. In general, global and regional scale ice analyses depict ice edge location, ice concentrations within the pack and the ice stages of development or thickness.
    Local scale ice analyses emphasize the location of thin ice covered or open water leads/polynyas, areas of heavy compression, frequency of ridging, and the presence or absence of dangerous multiyear ice and/or icebergs. The parameters defined in this tactical scale analysis are considered critical to both safety of navigation and the efficient routing of ships through the sea ice cover.

    3416. Ice Forecasts And Observations
    Sea ice forecasts are routinely separated into four temporal classes: short-term (24–72 hour), weekly (5–7 days), monthly (15–30 days) and seasonal (60–90 days) forecasts. Short-term forecasts are generally paired with local-scale ice analyses and focus on changes in the ice cover based on ice drift, ice formation and ablation, and divergent/convergent processes. Of particular importance are the predicted location of the ice edge and the presence or absence of open water polynyas and coastal/flaw leads. The accurate prediction of the location of these ice features are important for both ice avoidance and ice exploitation purposes.

    Similar but with less detail, weekly ice forecasts also emphasize the change in ice edge location and concentration areas within the pack. The National Ice Center presently employs several prediction models to produce both short-term and weekly forecasts. These include empirical models which relate ice drift with geostrophic winds and a coupled dynamic/thermodynamic model called the Polar Ice Prediction System (PIPS). Unlike earlier models, the latter accounts for the effects of ice thickness, concentration, and growth on ice drift.

    Monthly ice forecasts predict changes in overall ice extent and are based upon the predicted trends in air temperatures, projected paths of transiting low pressure systems, and continuity of ice conditions. Seasonal or 90 day ice forecasts predict seasonal ice severity and the projected impact on annual shipping operations. Of particular interest to the National Ice Center are seasonal forecasts for the Alaskan North Slope, Baffin Bay for the annual resupply of Thule, Greenland, and Ross Sea/McMurdo Sound in Antarctica. Seasonal forecasts are also important to Great Lakes and St. Lawrence Seaway shipping interests.

    Ice services provided to U.S. Government agencies upon request include aerial reconnaissance for polar shipping operations, ship visits for operational briefing and training, and optimum track ship routing (OTSR) recommendations through ice-infested seas. Commercial operations interested in ice products may obtain routinely produced ice products from the National Ice Center as well as ice analyses and forecasts for Alaskan waters from the National Weather Service Forecast Office in Anchorage, Alaska.

    Specific information on request procedures, types of ice products, ice services, methods of product dissemination and ship weather support is contained in the publication “Environmental Services for Polar Operations” prepared and distributed by the Director, National Ice Center, 4251 Suitland Road, Washington, D.C., 20395.

    The U.S. Coast Guard has an additional responsibility, separate from the National Ice Center, for providing icebreaker support for polar operations and the administration and operations of the International Ice Patrol (IIP). Inquiries for further information on these subjects should be sent to Commandant (G–N10–3), 2100 Second Street S.W., Washington D.C. 20593. Other countries which provide sea ice information services are as follows: Arctic – Canada, Denmark (Greenland), Japan (Seas of Okhotsk, Japan and Bo Hai), Iceland, Norway, Russia and the United Kingdom; Antarctic – Argentina, Australia, Chile, Germany, Japan, and Russia; and Baltic – Finland, Germany, Sweden and Russia. Except for the United States, the ice information services of all countries place specific focus upon ice conditions in territorial seas or waters adjacent to claims on the Antarctic continent.
    The National Ice Center of the United States is the only organization which provides global ice products and services. Names and locations of foreign sea ice service organizations can be found in “Sea Ice Information Services in the World,” WMO Publication No. 574.


    The complete format and tables for the code are described in the WMO publication “Manual on Codes”, Volume 1, WMO No. 306. This publication is available from the Secretariat of the World Meteorological Organization, Geneva, Switzerland.

    A more complete and detailed reporting code (ICEOB) has been in use since 1972 by vessels reporting to the U.S. National Ice Center. 1993 revisions to this code and the procedures for use are described in the “Ice Observation Handbook” prepared and distributed by the Director, National Ice Center, 4251 Suitland Road, Washington D.C., 20395. All ice observation codes make use of special nomenclature which is precisely defined in several languages by the WMO publication “Sea Ice Nomenclature”, WMO No. 259, TP 145. This publication, available from the Secretariat of the WMO, contains descriptive definitions along with photography of most ice features. This publication is very useful for vessels planning to submit ice observations.

    3417. Distribution Of Ice Products And Services
    The following is intended as a brief overview of the distribution methods for NIC products and services. For detailed information the user should consult the publications discussed in section 3416 or refer specific inquiries to Director, National Ice Center, 4251 Suitland Road, FOB #4, Room 2301, Washington, D.C. 20395 or call (301) 763–1111 or –2000. Facsimile inquiries can be phoned to (301) 763–1366 and will generally be answered by mail, therefore addresses must be included.
    NIC ice product distribution methods are as follows:
    1. Autopolling: Customer originated menu-driven facsimile product distribution system. Call (301) 763–3190/3191 for menu directions or (301) 763–5972 for assignment of Personal Identification Number (PIN).
    2. Autodin: Alphanumeric message transmission to
    U.S. Government organizations or vessels. Address is NAVICECEN Suitland MD.
    3. OMNET/SCIENCENET: electronic mail and bulletin board run by OMNET, Inc. (617) 265–9230. Product request messages may be sent to mailbox NATIONAL.ICE.CTR. Ice products are routinely posted on bulletin board SEA.ICE.
    4. INTERNET: Product requests may be forwarded to electronic mail address which is available by request from the NIC at (301) 763–5972.
    5. Mail Subscription: For weekly Arctic and Antarctic sea ice analysis charts from the National Climatic Data Center, NESDIS, NOAA, 37 Battery Park Ave., Asheville, NC, 28801–2733. Call (704) 271–4800 with requests for ice products.
    6. Mail: Annual ice atlases and multiyear ice climatologies are available either from the National Ice Center (if in stock) or from the National Technical Information Service, 5285 Port Royal Road, Springfield, VA, 22161. Call (703) 487–4600 for sales service desk. Digital files (in SIGRID format) of weekly NIC ice analyses may be obtained from the National Snow and Ice Data Center, CIRES, Box 449, University of Colorado, Boulder, Colorado 80309. Call (303)
    492–5171 for information.

  10. Fishers of Men

    Fishers of Men Senior Member

    CHAPTER 35
    And you thought we were done with the weather portion? Naw! Remember the "?" mark means degrees.
    3500. Introduction
    Weather is the state of the earth’s atmosphere with respect to temperature, humidity, precipitation, visibility, cloudiness, and other factors. Climate refers to the average long-term meteorological conditions of a place or region. All weather may be traced to the effect of the sun on the earth. Most changes in weather involve large-scale horizontal motion of air. Air in motion is called wind. This motion is produced by differences of atmospheric pressure, which are attributable both to differences of temperature and the nature of the motion itself.
    Weather is of vital importance to the mariner. The wind and state of the sea affect dead reckoning. Reduced visibility limits piloting. The state of the atmosphere affects electronic navigation and radio communication. If the skies are overcast, celestial observations are not available; and under certain conditions refraction and dip are disturbed. When wind was the primary motive power, knowledge of the areas of favorable winds was of great importance. Modern vessels are still affected considerably by wind and sea.

    3501. The Atmosphere
    The atmosphere is a relatively thin shell of air, water vapor, and suspended particulates surrounding the earth. Air is a mixture gases and, like any gas, is elastic and highly compressible. Although extremely light, it has a definite weight which can be measured. A cubic foot of air at standard sea-level temperature and pressure weighs 1.22 ounces, or about 1/817th the weight of an equal volume of water. Because of this weight, the atmosphere exerts a pressure upon the surface of the earth of about 15 pounds per square inch.
    As altitude increases, air pressure decreases due to the decreased weight of air above. With less pressure, the density decreases. More than three-fourths of the air is concentrated within a layer averaging about 7 statute miles thick, called the troposphere. This is the region of most “weather,” as the term is commonly understood. The top of the troposphere is marked by a thin transition zone called the tropopause, immediately above which is the stratosphere. Beyond this lie several other layers having distinctive characteristics. The average height of the tropopause ranges from about 5 miles or less at high latitudes to about 10 miles at low latitudes.
    The standard atmosphere is a conventional vertical structure of the atmosphere characterized by a standard sealevel pressure of 1013.25 millibars of mercury (29.92 inches) and a sea-level air temperature of 15C (59F). The temperature decreases with height (i.e., standard lapse rate) being a uniform 2C (3.6F) per thousand feet to 11 kilometers (36,089 feet) and thereafter remains constant at –56.5C (69.7F).
    Research has indicated that the jet stream is important in relation to the sequence of weather. The jet stream refers to relatively strong (60 knots) quasi-horizontal winds, usually concentrated within a restricted layer of the atmosphere. There are two commonly known jet streams. The sub-tropical jet stream (STJ) occurs in the region of 30N during the northern hemisphere winter, decreasing in summer. The core of highest winds in the STJ is found at about 12km altitude (40,000 feet) an in the region of 70W, 40E, and 150E, although considerable variability is common.

    The polar frontal jet stream (PFJ) is found in middle to upper-middle latitudes and is discontinuous and variable. Maximum jet stream winds have been measured by weather balloons at 291 knots.
    3502. General Circulation Of The Atmosphere
    The heat required to warm the air is supplied originally by the sun. As radiant energy from the sun arrives at the earth, about 29 percent is reflected back into space by the earth and its atmosphere, 19 percent is absorbed by the atmosphere, and the remaining 52 percent is absorbed by the surface of the earth. Much of the earth’s absorbed heat is radiated back into space. Earth’s radiation is in comparatively long waves relative to the short-wave radiation from the sun because it emanates from a cooler body. Long-wave radiation, readily absorbed by the water vapor in the air, is primarily responsible for the warmth of the atmosphere near the earth’s surface. Thus, the atmosphere acts much like the glass on the roof of a greenhouse. It allows part of the incoming solar radiation to reach the surface of the earth but is heated by the terrestrial radiation passing outward. Over the entire earth and for long periods of time, the total outgoing energy must be equivalent to the incoming energy (minus any converted to another form and retained), or the temperature of the earth and its atmosphere would steadily increase or decrease. In local areas, or over relatively short periods of time, such a balance is not required, and in fact does not exist, resulting in changes such as those occurring from one year to another, in different seasons and in different parts of the day.

    The more nearly perpendicular the rays of the sun strike the surface of the earth, the more heat energy per unit area is received at that place. Physical measurements show that in the tropics, more heat per unit area is received than is radiated away, and that in polar regions, the opposite is true. Unless there were some process to transfer heat from the tropics to polar regions, the tropics would be much warmer than they are, and the polar regions would be much colder. Atmospheric motions bring about the required transfer of heat. The oceans also participate in the process, but to a lesser degree.
    If the earth had a uniform surface and did not rotate on its axis, with the sun following its normal path across the sky (solar heating increasing with decreasing latitude), a simple circulation would result, as shown in Figure 3502a. However, the surface of the earth is far from uniform, being covered with an irregular distribution of land and water. Additionally, the earth rotates about its axis so that the portion heated by the sun continually changes. In addition, the axis of rotation is tilted so that as the earth moves along its orbit about the sun, seasonal changes occur in the exposure of specific areas to the sun’s rays, resulting in variations in the heat balance of these areas. These factors, coupled with others, result in constantly changing large-scale movements of air. For example, the rotation of the earth exerts an apparent force, known as Coriolis force, which diverts the air from a direct path between high and low pressure areas. The diversion of the air is toward the right in the Northern Hemisphere and toward the left in the Southern Hemisphere. At some distance above the surface of the earth, the wind tends to blow along lines connecting points of equal pressure called isobars. The wind is called a geostrophic wind if the isobars are straight (great circles) and a gradient wind if they are curved. Near the surface of the earth, friction tends to divert the wind from the isobars toward the center of low pressure.

    At sea, where friction is less than on land, the wind follows the isobars more closely. A simplified diagram of the general circulation pattern is shown in Figure 3502b. Figure 3502c and Figure 3502d give a generalized picture of the world’s pressure distribution and wind systems as actually observed. A change in pressure with horizontal distance is called a pressure gradient. It is maximum along a normal (perpendicular) to the isobars. A force results which is called pressure gradient force and is always directed from high to low pressure. Speed of the wind is approximately proportional to this pressure gradient.


    3503. The Doldrums
    A belt of low pressure at the earth’s surface near the equator known as the doldrums occupies a position approximately midway between high pressure belts at about latitude 30to 35on each side. Except for significant intradiurnal changes, the atmospheric pressure along the equatorial low is almost uniform. With minimal pressure gradient, wind speeds are light and directions are variable. Hot, sultry days are common. The sky is often overcast, and showers and thundershowers are relatively frequent; in these atmospherically unstable areas, brief periods of strong wind occur.

    The doldrums occupy a thin belt near the equator, the eastern part in both the Atlantic and Pacific being wider than the western part. However, both the position and extent
    of the belt vary with longitude and season. During all seasons in the Northern Hemisphere, the belt is centered in the eastern Atlantic and Pacific; however, there are wide excursions of the doldrum regions at longitudes with considerable landmass. On the average, the position is at 5N, frequently called the meteorological equator.

    3504. The Trade Winds
    The trade winds at the surface blow from the belts of high pressure toward the equatorial belts of low pressure. Because of the rotation of the earth, the moving air is deflected toward the west. Therefore, the trade winds in the Northern Hemisphere are from the northeast and are called the northeast trades, while those in the Southern Hemisphere are from the southeast and are called the southeast trades. The trade-wind directions are best defined over eastern ocean areas.

    The trade winds are generally considered among the most constant of winds, blowing for days or even weeks with little change of direction or speed. However, at times they weaken or shift direction, and there are regions where the general pattern is disrupted. A notable example is found in the island groups of the South Pacific, where the trades are practically nonexistent during January and February. Their best development is attained in the South Atlantic and in the South Indian Ocean. In general, they are stronger during the winter than during the summer season. In July and August, when the belt of equatorial low pressure moves to a position some distance north of the equator, the southeast trades blow across the equator, into the Northern Hemisphere, where the earth’s rotation diverts them toward the right, causing them to be southerly and southwesterly winds. The “southwest monsoons” of the African and Central American coasts originate partly in these diverted southeast trades.

    Cyclones from the middle latitudes rarely enter the regions of the trade winds, although tropical cyclones originate within these areas.

    3505. The Horse Latitudes
    Along the poleward side of each trade-wind belt, and corresponding approximately with the belt of high pressure in each hemisphere, is another region with weak pressure gradients and correspondingly light, variable winds. These are called the horse latitudes, apparently so named because becalmed sailing ships threw horses overboard in this region when water supplies ran short. The weather is generally good although low clouds are common. Compared to the doldrums, periods of stagnation in the horse latitudes are less persistent. The difference is due primarily to the rising currents of warm air in the equatorial low, which carry large amounts of moisture. This moisture condenses as the air cools at higher levels, while in the horse latitudes the air is apparently descending and becoming less humid as it is warmed at lower heights.

    3506. The Prevailing Westerlies
    On the poleward side of the high pressure belt in each hemisphere, the atmospheric pressure again diminishes. The currents of air set in motion along these gradients toward the poles are diverted by the earth’s rotation toward the east, becoming southwesterly winds in the Northern Hemisphere and northwesterly in the Southern Hemisphere. These two wind systems are known as the prevailing westerlies of the temperate zones.

    In the Northern Hemisphere this relatively simple pattern is distorted considerably by secondary wind circulations, due primarily to the presence of large landmasses. In the North Atlantic, between latitudes 40and 50, winds blow from some direction between south and northwest during 74 percent of the time, being somewhat more persistent in winter than in summer. They are stronger in winter, too, averaging about 25 knots (Beaufort 6) as compared with 14 knots (Beaufort 4) in the summer. In the Southern Hemisphere the westerlies blow throughout the year with a steadiness approaching that of the trade winds. The speed, though variable, is generally between 17 and 27 knots (Beaufort 5 and 6). Latitudes 40S to 50S (or 55S) where these boisterous winds occur, are called the roaring forties. These winds are strongest at about latitude 50S.

    The greater speed and persistence of the westerlies in the Southern Hemisphere are due to the difference in the atmospheric pressure pattern, and its variations, from the Northern Hemisphere. In the comparatively landless Southern Hemisphere, the average yearly atmospheric pressure diminishes much more rapidly on the poleward side of the high pressure belt, and has fewer irregularities due to continental interference, than in the Northern Hemisphere.

    3507. Polar Winds
    Partly because of the low temperatures near the geographical poles of the earth, the surface pressure tends to remain higher than in surrounding regions, since cold air is more dense than warm air. Consequently, the winds blow outward from the poles, and are deflected westward by the rotation of the earth, to become northeasterlies in the Arctic, and southeasterlies in the Antarctic. Where the polar easterlies meet the prevailing westerlies, near 50N and 50S on the average, a discontinuity in temperature and wind exists. This discontinuity is called the polar front. Here the warmer low-latitude air ascends over the colder polar air creating a zone of cloudiness and precipitation. In the Arctic, the general circulation is greatly modified by surrounding landmasses. Winds over the Arctic Ocean are somewhat variable, and strong surface winds are rarely encountered.

    In the Antarctic, on the other hand, a high central landmass is surrounded by water, a condition which augments, rather than diminishes, the general circulation. The high pressure, although weaker than in the horse latitudes, is stronger than in the Arctic, and of great persistence especially in eastern Antarctica. The cold air from the plateau areas moves outward and downward toward the sea and is deflected toward the west by the earth’s rotation. The winds remain strong throughout the year, frequently attaining hurricane force near the base of the mountains. These are some of the strongest surface winds encountered anywhere in the world, with the possible exception of those in well-developed tropical cyclones.

    3508. Modifications Of The General Circulation
    The general circulation of the atmosphere is greatly modified by various conditions.
    The high pressure in the horse latitudes is not uniformly distributed around the belts, but tends to be accentuated at several points, as shown in Figure 3502c and Figure 3502d. These semi-permanent highs remain at about the same places with great persistence.

    Semi-permanent lows also occur in various places, the most prominent ones being west of Iceland, and over the Aleutians (winter only) in the Northern Hemisphere, and in the Ross Sea and Weddell Sea in the Antarctic areas. The regions occupied by these semi-permanent lows are sometimes called the graveyards of the lows, since many lows move directly into these areas and lose their identity as they merge with and reinforce the semi-permanent lows. The low pressure in these areas is maintained largely by the migratory lows which stall there, with topography also important, especially in Antarctica.
    Another modifying influence is land, which undergoes greater temperature changes than does the sea. During the summer, a continent is warmer than its adjacent oceans. Therefore, low pressures tend to prevail over the land. If a climatological belt of high pressure encounters a continent, its pattern is distorted or interrupted, whereas a belt of low pressure is intensified over the same area. In winter, the opposite effect takes place, belts of high pressure being intensified over land and those of low pressure being weakened. The most striking example of a wind system produced by the alternate heating and cooling of a landmass is the monsoon (seasonal wind) of the China Sea and Indian Ocean. A portion of this effect is shown in Figure 3508a and Figure 3508b.


    In the summer, low pressure prevails over the warm continent of Asia, and relatively higher pressure prevails over the adjacent sea. Between these two systems the wind blows in a nearly steady direction. The lower portion of the pattern is in the Southern Hemisphere, extending to about 10south latitude. Here the rotation of the earth causes a deflection to the left, resulting in southeasterly winds. As they cross the equator, the deflection is in the opposite direction, causing them to curve toward the right, becoming southwesterly winds. In the winter, the positions of high and low pressure areas are interchanged, and the direction of flow is reversed. In the China Sea, the summer monsoon blows from the southwest, usually from May to September. The strong winds are accompanied by heavy squalls and thunderstorms, the rainfall being much heavier than during the winter monsoon. As the season advances, squalls and rain become less frequent. In some places the wind becomes a light breeze which is unsteady in direction, or stops altogether, while in other places it continues almost undiminished, with changes in direction or calms being infrequent. The winter monsoon blows from the northeast, usually from October to April. It blows with a steadiness similar to that of the trade winds, often attaining the speed of a moderate gale (28–33 knots). Skies are generally clear during this season, and there is relatively little rain. The general circulation is further modified by winds of cyclonic origin and various local winds. Some common local winds are listed by local name below.

    Abroholos: A squall frequent from May through August between Cabo de Sao Tome and Cabo Frio on the coast of Brazil.

    Bali wind: A strong east wind at the eastern end of Java.

    Barat: A heavy northwest squall in Manado Bay on the north coast of the island of Celebes, prevalent from December to February.

    Barber: A strong wind carrying damp snow or sleet and spray that freezes upon contact with objects, especially the beard and hair.

    Bayamo: A violent wind blowing from the land on the south coast of Cuba, especially near the Bight of Bayamo.

    Bentu de Soli: An east wind on the coast of Sardinia.

    Bora: A cold, northerly wind blowing from the Hungarian basin into the Adriatic Sea. See also FALL WIND.

    Borasco: A thunderstorm or violent squall, especially in the Mediterranean.

    Brisa, Briza 1: A northeast wind which blows on the coast of South America or an east wind which blows on Puerto Rico during the trade wind season. 2. The northeast monsoon in the Philippines.

    Brisote: The northeast trade wind when it is blowing stronger than usual on Cuba.

    Brubu: A name for a squall in the East Indies.

    Bull’s Eye Squall: A squall forming in fair weather, characteristic of the ocean off the coast of South Africa. It is named for the peculiar appearance of the small isolated cloud marking the top of the invisible vortex of the storm.

    Cape Doctor: The strong southeast wind which blows on the South African coast. Also called the DOCTOR.

    Caver, Kaver: A gentle breeze in the Hebrides.

    Chubasco: A violent squall with thunder and lightning, encountered during the rainy season along the west coast of Central America.

    Churada: A severe rain squall in the Mariana Islands during the northeast monsoon. They occur from November to April or May, especially from January through March.

    Cierzo: See MISTRAL.

    Contrastes: Winds a short distance apart blowing from opposite quadrants, frequent in the spring and fall in the western Mediterranean.

    Cordonazo: The “Lash of St. Francis.” Name applied locally to southerly hurricane winds along the west coast of Mexico. It is associated with tropical cyclones in the southeastern North Pacific Ocean. These storms may occur from May to November, but ordinarily affect the coastal areas most severely near or after the Feast of St. Francis, October 4.

    Coromell: A night land breeze prevailing from November to May at La Paz, near the southern extremity of the Gulf of California.
    Doctor 1. A cooling sea breeze in the Tropics. 2. See HARMATTAN. 3. The strong SE wind which blows on the south African coast. Usually called CAPE DOCTOR.

    Elephanta: A strong southerly or southeasterly wind which blows on the Malabar coast of India during the months of September and October and marks the end of the southwest monsoon.

    Etesian: A refreshing northerly summer wind of the Mediterranean, especially over the Aegean Sea.

    Gregale: A strong northeast wind of the central Mediterranean.

    Harmattan: The dry, dusty trade wind blowing off the Sahara Desert across the Gulf of Guinea and the Cape Verde Islands. Sometimes called the DOCTOR, because of its supposed healthful properties.

    Knik: Wind A strong southeast wind in the vicinity of Palmer, Alaska, most frequent in the winter.

    Kona: Storm A storm over the Hawaiian Islands, characterized by strong southerly or southwesterly winds and heavy rains.

    Leste: A hot, dry, easterly wind of the Madeira and Canary Islands.

    Levanter: A strong easterly wind of the Mediterranean, especially in the Strait of Gibraltar, attended by cloudy, foggy, and sometimes rainy weather especially in winter.

    Levantera: A persistent east wind of the Adriatic, usually accompanied by cloudy weather.

    Levanto: A hot southeasterly wind which blows over the Canary Islands.

    Leveche: A warm wind in Spain, either a foehn or a hot southerly wind in advance of a low pressure area moving from the Sahara Desert. Called a SIROCCO in other parts of the Mediterranean area. Maestro A northwesterly wind with fine weather which blows, especially in summer, in the Adriatic. It is most frequent on the western shore. This wind is also found on the coasts of Corsica and Sardinia.

    Matanuska: Wind A strong, gusty, northeast wind which occasionally occurs during the winter in the vicinity of Palmer, Alaska. Mistral A cold, dry wind blowing from the north over the northwest coast of the Mediterranean Sea, particularly over the Gulf of Lions. Also called CIERZO. See also FALL WIND.

    Nashi, N’aschi: A northeast wind which occurs in winter on the Iranian coast of the Persian Gulf, especially near the entrance to the gulf, and also on the Makran coast. It is probably associated with an outflow from the central Asiatic anticyclone which extends over the high land of Iran. It is similar in character but less severe than the BORA.

    Norte: A strong cold northeasterly wind which blows in Mexico and on the shores of the Gulf of Mexico. It results from an outbreak of cold air from the north. It is the Mexican extension of a norther.

    Papagayo: A violet northeasterly fall wind on the Pacific coast of Nicaragua and Guatemala. It consists of the cold air mass of a norte which has overridden the mountains of Central America. See also TEHUANTEPECER.

    Santa Ana: A strong, hot, dry wind blowing out into San Pedro Channel from the southern California desert through Santa Ana Pass.

    Shamal: A summer northwesterly wind blowing over Iraq and the Persian Gulf, often strong during the day, but decreasing at night.

    Sharki: A southeasterly wind which sometimes blows in the Persian Gulf.
    Sirocco A warm wind of the Mediterranean area, either a foehn or a hot southerly wind in advance of a low pressure area moving from the Sahara or Arabian deserts. Called LEVECHE in Spain.

    Squamish: A strong and often violent wind occurring in many of the fjords of British Columbia. Squamishes occur in those fjords oriented in a northeast-southwest or east-west direction where cold polar air can be funneled westward. They are notable in Jervis, Toba, and Bute inlets and in Dean Channel and Portland Canal. Squamishes lose their strength when free of the confining fjords and are not noticeable 15 to 20 miles offshore.
    Suestado A storm with southeast gales, caused by intense cyclonic activity off the coasts of Argentina and Uruguay, which affects the southern part of the coast of Brazil in the winter.

    Sumatra: A squall with violent thunder, lightning, and rain, which blows at night in the Malacca Straits, especially during the southwest monsoon. It is intensified by strong mountain breezes.

    Taku: Wind A strong, gusty, east-northeast wind, occurring in the vicinity of Juneau, Alaska, between October and March. At the mouth of the Taku River, after which it is named, it sometimes attains hurricane force.

    Tehuantepecer: A violent squally wind from north or north-northeast in the Gulf of Tehuantepec (south of southern Mexico) in winter. It originates in the Gulf of Mexico as a norther which crosses the isthmus and blows through the gap between the Mexican and Guatamalan mountains. It may be felt up to 100 miles out to sea. See also PAPAGAYO.

    Tramontana: A northeasterly or northerly winter wind off the west coast of Italy. It is a fresh wind of the fine weather mistral type.

    Vardar: A cold fall wind blowing from the northwest down the Vardar valley in Greece to the Gulf of Salonica. It occurs when atmospheric pressure over eastern Europe is higher than over the Aegean Sea, as is often the case in winter. Also called VARDARAC.

    Warm Braw: A foehn wind in the Schouten Islands north of New Guinea.
    White Squall A sudden, strong gust of wind coming up without warning, noted by whitecaps or white, broken water; usually seen in whirlwind form in clear weather in the tropics.

    Williwaw: A sudden blast of wind descending from a mountainous coast to the sea, in the Strait of Magellan or the Aleutian Islands.

    3509. Types Of Air Masses
    Because of large differences in physical characteristics of the earth’s surface, particularly the oceanic and continental contrasts, the air overlying these surfaces acquires differing values of temperature and moisture. The processes of radiation and convection in the lower portions of the troposphere act in differing characteristic manners for a number of well-defined regions of the earth. The air overlying these regions acquires characteristics common to the particular area, but contrasting to those of other areas. Each distinctive part of the atmosphere, within which common characteristics prevail over a reasonably large area, is called an air mass.

    Air masses are named according to their source regions. Four regions are generally recognized: (1) equatorial (E), the doldrums area between the north and south trades;
    (2) tropical (T), the trade wind and lower temperate regions;
    (3) polar (P), the higher temperate latitudes; and (4) Arctic or Antarctic (A), the north or south polar regions of ice and snow. This classification is a general indication of relative temperature, as well as latitude of origin. Air masses are further classified as maritime (m) or continental ©, depending upon whether they form over water or land. This classification is an indication of the relative moisture content of the air mass. Tropical air might be designated maritime tropical (mT) or continental tropical (cT). Similarly, polar air may be either maritime polar (mP) or continental polar (cP). Arctic/Antarctic air, due to the predominance of landmasses and ice fields in the high latitudes, is rarely maritime Arctic (mA). Equatorial air is found exclusively over the ocean surface and is designated neither (cE) nor (mE), but simply (E).

    A third classification sometimes applied to tropical and polar air masses indicates whether the air mass is warm (w) or cold (k) relative to the underlying surface. Thus, the symbol mTw indicates maritime tropical air which is warmer than the underlying surface, and cPk indicates continental polar air which is colder than the underlying surface. The w and k classifications are primarily indications of stability (i.e., change of temperature with increasing height). If the air is cold relative to the surface, the lower portion of the air mass will be heated, resulting in instability (temperature markedly decreases with increasing height) as the warmer air tends to rise by convection. Conversely, if the air is warm relative to the surface, the lower portion of the air mass is cooled, tending to remain close to the surface. This is a stable condition (temperature increases with increasing height).
    Two other types of air masses are sometimes recognized. These are monsoon (M), a transitional form between cP and E; and superior (S), a special type formed in the free atmosphere by the sinking and consequent warming of air aloft.

    3510. Fronts
    As air masses move within the general circulation, they travel from their source regions to other areas dominated by air having different characteristics. This leads to a zone of separation between the two air masses, called a frontal zone or front, across which temperature, humidity, and wind speed and direction change rapidly. Fronts are represented on weather maps by lines; a cold front is shown with pointed barbs, a warm front with rounded barbs, and an occluded front with both, alternating. A stationary front is shown with pointed and rounded barbs alternating and on opposite sides of the line with the pointed barbs away from the colder air. The front may take on a wave-like character, becoming a “frontal wave.”

    Before the formation of frontal waves, the isobars (lines of equal atmospheric pressure) tend to run parallel to the fronts. As a wave is formed, the pattern is distorted somewhat, as shown in Figure 3510a. In this illustration, colder air is north of warmer air. In Figures 3510a–3510d isobars are drawn at 4-millibar intervals.

    The wave tends to travel in the direction of the general circulation, which in the temperate latitudes is usually in an easterly and slightly poleward direction.
    Along the leading edge of the wave, warmer air is replacing colder air. This is called the warm front. The trailing edge is the cold front, where colder air is underrunning and displacing warmer air.
    The warm air, being less dense, tends to ride up greatly over the colder air it is replacing. Partly because of the replacement of cold, dense air with warm, light air, the pressure decreases. Since the slope is gentle, the upper part of a warm frontal surface may be many hundreds of miles ahead of the surface portion. The decreasing pressure, indicated by a “falling barometer,” is often an indication of the approach of such a wave. In a slow-moving, well-developed wave, the barometer may begin to fall several days before the wave arrives. Thus, the amount and nature of the change of atmospheric pressure between observations, called pressure tendency, is of assistance in predicting the approach of such a system.

    The advancing cold air, being more dense, tends to ride under the warmer air at the cold front, lifting it to greater heights. The slope here is such that the upper-air portion of the cold front is behind the surface position relative to its motion. After a cold front has passed, the pressure increases, giving a rising barometer.

    In the first stages, these effects are not marked, but as the wave continues to grow, they become more pronounced, as shown in Figure 3510b. As the amplitude of the wave increases, pressure near the center usually decreases, and the low is said to “deepen.” As it deepens, its forward speed generally decreases.

    The approach of a well-developed warm front (i.e., when the warm air is mT) is usually heralded not only by falling pressure, but also by a more-or-less regular sequence of clouds. First, cirrus appear. These give way successively to cirrostratus, altostratus, altocumulus, and nimbostratus. Brief showers may precede the steady rain accompanying the nimbostratus.


    As the warm front passes, the temperature rises, the wind shifts clockwise (in the Northern Hemisphere), and the steady rain stops. Drizzle may fall from low-lying stratus clouds, or there may be fog for some time after the wind shift. During passage of the warm sector between the warm front and the cold front, there is little change in temperature or pressure. However, if the wave is still growing and the low deepening, the pressure might slowly decrease. In the warm sector the skies are generally clear or partly cloudy, with cumulus or stratocumulus clouds most frequent. The warm air is usually moist, and haze or fog may often be present. As the faster moving, steeper cold front passes, the wind veers (shifts clockwise in the Northern Hemisphere counterclockwise in the Southern Hemisphere), the temperature falls rapidly, and there are often brief and sometimes violent squalls with showers, frequently accompanied by thunder and lightning. Clouds are usually of the convective type. A cold front usually coincides with a well-defined wind-shift line (a line along which the wind shifts abruptly from southerly or southwesterly to northerly or northwesterly in the Northern Hemisphere, and from northerly or northwesterly to southerly or southwesterly in the Southern Hemisphere). At sea a series of brief showers accompanied by strong, shifting winds may occur along or some distance (up to 200 miles) ahead of a cold front.

    These are called squalls (in common nautical use, the term squall may be additionally applied to any severe local storm accompanied by gusty winds, precipitation, thunder, and lightning), and the line along which they occur is called a squall line.


    Because of its greater speed and steeper slope, which may approach or even exceed the vertical near the earth’s surface (due to friction), a cold front and its associated weather pass more quickly than a warm front. After a cold front passes, the pressure rises, often quite rapidly, the visibility usually improves, and the clouds tend to diminish. Clear, cool or cold air replaces the warm hazy air. As the wave progresses and the cold front approaches the slower moving warm front, the low becomes deeper and the warm sector becomes smaller, as shown in Figure 3510c. Finally, the faster moving cold front overtakes the warm front (Figure 3510d), resulting in an occluded front at the surface, and an upper front aloft (Figure 3510e). When the two parts of the cold air mass meet, the warmer portion tends to rise above the colder part. The warm air continues to rise until the entire frontal system dissipates. As the warmer air is replaced by colder air, the pressure gradually rises, a process called filling.

    This usually occurs within a few days after an occluded front forms. Finally, there results a cold low, or simply a low pressure system across which little or no gradient in temperature and moisture can be found. The sequence of weather associated with a low depends greatly upon the observer’s location with respect to the path of the center. That described above assumes that the low center passes poleward of the observer. If the low center passes south of the observer, between the observer and the equator, the abrupt weather changes associated with the passage of fronts are not experienced. Instead, the change from the weather characteristically found ahead of a warm front, to that behind a cold front, takes place gradually, the exact sequence dictated by distance from the center, and the severity and age of the low. Although each low generally follows this pattern, no two are ever exactly alike. Other centers of low pressure and high pressure, and the air masses associated with them, even though they may be 1,000 miles or more away, influence the formation and motion of individual low centers and their accompanying weather. Particularly, a high stalls or diverts a low. This is true of temporary highs as well as semi-permanent highs, but not to as great a degree.

    To be cont.
    Last edited by a moderator: Apr 30, 2015
  11. Fishers of Men

    Fishers of Men Senior Member

    ch 35 cont.

    3511. Cyclones And Anticyclones
    An area of relatively low pressure, generally circular, is called a cyclone. Its counterpart for high pressure is called an anticyclone. These terms are used particularly in connection with the winds associated with such centers. Wind tends to blow from an area of high pressure to one of low pressure, but due to rotation of the earth, wind is deflected toward the right in the Northern Hemisphere and toward the left in the Southern Hemisphere. Because of the rotation of the earth, therefore, the circulation tends to be counterclockwise around areas of low pressure and clockwise around areas of high pressure in the Northern Hemisphere, and the speed is proportional to the spacing of isobars. In the Southern Hemisphere, the direction of circulation is reversed. Based upon this condition, a general rule, known as Buys Ballot’s Law, or the Baric Wind Law, can be stated:
    If an observer in the Northern Hemisphere faces away from the surface wind, the low pressure is toward his left; the high pressure is toward his right.
    If an observer in the Southern Hemisphere faces away from the surface wind, the low pressure is toward his right; the high pressure is toward his left.
    In a general way, these relationships apply in the case of the general distribution of pressure, as well as to temporary local pressure systems.
    The reason for the wind shift along a front is that the isobars have an abrupt change of direction along these lines. Since the direction of the wind is directly related to the direction of isobars, any change in the latter results in a shift in the wind direction.

    In the Northern Hemisphere, the wind shifts toward the right (clockwise) when either a warm or cold front passes. In the Southern Hemisphere, the shift is toward the left (counterclockwise). When an observer is on the poleward side of the path of a frontal wave, wind shifts are reversed (i.e., to the left in the Northern Hemisphere and to the right in the Southern Hemisphere).
    In an anticyclone, successive isobars are relatively far apart, resulting in light winds. In a cyclone, the isobars are more closely spaced. With a steeper pressure gradient, the winds are stronger.

    Since an anticyclonic area is a region of outflowing winds, air is drawn into it from aloft. Descending air is warmed, and as air becomes warmer, its capacity for holding uncondensed moisture increases. Therefore, clouds tend to dissipate. Clear skies are characteristic of an anticyclone, although scattered clouds and showers are sometimes encountered. In contrast, a cyclonic area is one of converging winds. The resulting upward movement of air results in cooling, a condition favorable to the formation of clouds and precipitation. More or less continuous rain and generally stormy weather are usually associated with a cyclone. Between the two hemispheric belts of high pressure associated with the horse latitudes, called subtropical anticyclones, cyclones form only occasionally over certain areas at sea, generally in summer and fall. Tropical cyclones (hurricanes and typhoons) are usually quite violent. In the areas of the prevailing westerlies in temperate latitudes, migratory cyclones (lows) and anticyclones (highs) are a common occurrence. These are sometimes called extratropical cyclones and extratropical anticyclones to distinguish them from the more violent tropical cyclones. Formation occurs over sea and land. The lows intensify as they move poleward; the highs weaken as they move equatorward. In their early stages, cyclones are elongated, as shown in Figure 3510a, but as their life cycle proceeds, they become more nearly circular (Figure 3510b, Figure 3510c, and Figure 3510d).

    3512. Local Winds
    In addition to the winds of the general circulation and those associated with migratory cyclones and anticyclones, there are numerous local winds which influence the weather in various places.

    The most common are the land and sea breezes, caused by alternate heating and cooling of land adjacent to water. The effect is similar to that which causes the monsoons, but on a much smaller scale, and over shorter periods. By day the land is warmer than the water, and by night it is cooler. This effect occurs along many coasts during the summer. Between about 0900 and 1100 local time the temperature of the land becomes greater than that of the adjacent water. The lower levels of air over the land are warmed, and the air rises, drawing in cooler air from the sea.
    This is the sea breeze. Late in the afternoon, when the sun is low in the sky, the temperature of the two surfaces equalizes and the breeze stops. After sunset, as the land cools below the sea temperature, the air above it is also cooled. The contracting cool air becomes more dense, increasing the pressure near the surface. This results in an outflow of winds to the sea.
    This is the land breeze, which blows during the night and dies away near sunrise. Since the atmospheric pressure changes associated with this cycle are not great, the accompanying winds generally do not exceed gentle to moderate breezes. The circulation is usually of limited extent, reaching a distance of perhaps 20 miles inland, and not more than 5 or 6 miles offshore, and to a height of a few hundred feet. In the doldrums and subtropics, this process is repeated with great regularity throughout most of the year. As the latitude increases, it becomes less prominent, being masked by winds of migratory cyclones and anticyclones. However, the effect often may be present to reinforce, retard, or deflect stronger prevailing winds.
    Varying conditions of topography produce a large variety of local winds throughout the world. Winds tend to follow valleys, and to be deflected from high banks and shores. In mountain areas wind flows in response to temperature distribution and gravity.

    An anabolic wind is one that blows up an incline, usually as a result of surface heating. A katabatic wind is one which blows down an incline.

    There are two types, foehn and fall wind. The foehn (fãn) is a warm dry wind which initiates from horizontally moving air encountering a mountain barrier. As it blows upward to clear the mountains, it is cooled below the dew point, resulting in clouds and rain on the windward side. As the air continues to rise, its rate of cooling is reduced because the condensing water vapor gives off heat to the surrounding atmosphere. After crossing the mountain barrier, the air flows downward along the leeward slope, being warmed by compression as it descends to lower levels. Since it loses less heat on the ascent than it gains during descent, and since it has lost its moisture during ascent, it arrives at the bottom of the mountains as very warm, dry air. This accounts for the warm, arid regions along the eastern side of the Rocky Mountains and in similar areas. In the Rocky Mountain region this wind is known by the name chinook. It may occur at any season of the year, at any hour of the day or night, and have any speed from a gentle breeze to a gale. It may last for several days, or for a very short period. Its effect is most marked in winter, when it may cause the temperature to rise as much as 20°F to 30°F within 15 minutes, and cause snow and ice to melt within a few hours. On the west coast of the United States, a foehn wind, given the name Santa Ana, blows through a pass and down a valley of that name in Southern California. This wind is frequently very strong and may endanger small craft immediately off the coast.

    A cold wind blowing down an incline is called a fall wind. Although it is warmed somewhat during descent, as is the foehn, it remains cold relative to the surrounding air. It occurs when cold air is dammed up in great quantity on the windward side of a mountain and then spills over suddenly, usually as an overwhelming surge down the other side. It is usually quite violent, sometimes reaching hurricane force. A different name for this type wind is given at each place where it is common.

    The tehuantepecer of the Mexican and Central American coast, the pampero of the Argentine coast, the mistral of the western Mediterranean, and the bora of the eastern Mediterranean are examples of this wind.
    Many other local winds common to certain areas have been given distinctive names.

    A blizzard is a violent, intensely cold wind laden with snow mostly or entirely picked up from the ground, although the term is often used popularly to refer to any heavy snowfall accompanied by strong wind. A dust whirl is a rotating column of air about 100 to 300 feet in height, carrying dust, leaves, and other light material. This wind, which is similar to a waterspout at sea, is given various local names such as dust devil in southwestern United States and desert devil in South Africa. A gust is a sudden, brief increase in wind speed, followed by a slackening, or the violent wind or squall that accompanies a thunderstorm. A puff of wind or a light breeze affecting a small area, such as would cause patches of ripples on the surface of water, is called a cat’s paw.

    3513. Waterspouts
    A waterspout is a small, whirling storm over ocean or inland waters. Its chief characteristic is a funnel-shaped cloud; when fully developed it extends from the surface of the water to the base of a cumulus cloud. The water in a waterspout is mostly confined to its lower portion, and may be either salt spray drawn up by the sea surface, or freshwater resulting from condensation due to the lowered pressure in the center of the vortex creating the spout. The air in waterspouts may rotate clockwise or counterclockwise, depending on the manner of formation. They are found most frequently in tropical regions, but are not uncommon in higher latitudes.
    There are two types of waterspouts: those derived from violent convective storms over land moving seaward, called tornadoes, and those formed over the sea and which are associated with fair or foul weather. The latter type is most common, lasts a maximum of 1 hour, and has variable strength. Many waterspouts are no stronger than dust whirlwinds, which they resemble; at other times they are strong enough to destroy small craft or to cause damage to larger vessels, although modern ocean-going vessels have little to fear.
    Waterspouts vary in diameter from a few feet to several hundred feet, and in height from a few hundred feet to several thousand feet. Sometimes they assume fantastic shapes; in early stages of development an hour glass shape between cloud and sea is common. Since a waterspout is often inclined to the vertical, its actual length may be much greater than indicated by its height.

    3514. Deck Ice
    Ships traveling through regions where the air temperature is below freezing may acquire thick deposits of ice as a result of salt spray freezing on the rigging, deckhouses, and deck areas. This accumulation of ice is called ice accretion. Also, precipitation may freeze to the superstructure and exposed areas of the vessel, increasing the load of ice. On small vessels in heavy seas and freezing weather, deck ice may accumulate very rapidly and increase the topside weight enough to capsize the vessel. Fishing vessels with outriggers, Aframes, and other top hamper are particularly susceptible.

    3515. Fog
    Fog is a cloud whose base is at the surface of the earth. Fog is composed of droplets of water or ice crystals (ice fog) formed by condensation or crystallization of water vapor in the air.

    Radiation fog forms over low-lying land on clear, calm nights. As the land radiates heat and becomes cooler, it cools the air immediately above the surface. This causes a temperature inversion to form, the temperature increasing with height. If the air is cooled to its dew point, fog forms. Often, cooler and more dense air drains down surrounding slopes to heighten the effect. Radiation fog is often quite shallow, and is usually densest at the surface. After sunrise the fog may “lift” and gradually dissipate, usually being entirely gone by noon. At sea the temperature of the water undergoes little change between day and night, and so radiation fog is seldom encountered more than 10 miles from shore. Advection fog forms when warm, moist air blows over a colder surface and is cooled below its dew point. It is most commonly encountered at sea, may be quite dense, and often persists over relatively long periods. Advection fog is common over cold ocean currents. If the wind is strong enough to thoroughly mix the air, condensation may take place at some distance above the surface of the earth, forming low stratus clouds rather than fog.

    Off the coast of California, seasonal winds create an offshore current which displaces the warm surface water, causing an upwelling of colder water. Moist Pacific air is transported along the coast in the same wind system, and is cooled by the relatively cold water.

    Advection fog results. In the coastal valleys, fog is sometimes formed when moist air blown inland during the afternoon is cooled by radiation during the night.
    When very cold air moves over warmer water, wisps of visible water vapor may rise from the surface as the water “steams,” In extreme cases this frost smoke, or Arctic sea smoke, may rise to a height of several hundred feet, the portion near the surface constituting a dense fog which obscures the horizon and surface objects, but usually leaves the sky relatively clear.
    Haze consists of fine dust or salt particles in the air, too small to be individually apparent, but in sufficient number to reduce horizontal visibility and cast a bluish or yellowish veil over the landscape, subduing its colors and making objects appear indistinct. This is sometimes called dry haze to distinguish it from damp haze, which consists of small water droplets or moist particles in the air, smaller and more scattered than light fog. In international meteorological practice, the term “haze” is used to refer to a condition of atmospheric obscurity caused by dust and smoke.

    Mist is synonymous with drizzle in the United States but is often considered as intermediate between haze and fog in its properties. Heavy mist can reduce visibility to a mile or less. A mixture of smoke and fog is called smog. Normally it is not a problem in navigation except in severe cases accompanied by an offshore wind from the source, when it may reduce visibility to 2–4 miles.

    3516. Mirage
    Light is refracted as it passes through the atmosphere. When refraction is normal, objects appear slightly elevated, and the visible horizon is farther from the observer than it otherwise would be. Since the effects are uniformly progressive, they are not apparent to the observer. When refraction is not normal, some form of mirage may occur. A mirage is an optical phenomenon in which objects appear distorted, displaced (raised or lowered), magnified, multiplied, or inverted due to varying atmospheric refraction which occurs when a layer of air near the earth’s surface differs greatly in density from surrounding air. This may occur when there is a rapid and sometimes irregular change of temperature or humidity with height.
    If there is a temperature inversion (increase of temperature with height), particularly if accompanied by a rapid decrease in humidity, the refraction is greater than normal. Objects appear elevated, and the visible horizon is farther away. Objects which are normally below the horizon become visible. This is called looming.

    If the upper portion of an object is raised much more than the bottom part, the object appears taller than usual, an effect called towering.
    If the lower part of an object is raised more than the upper part, the object appears shorter, an effect called stooping.
    When the refraction is greater than normal, a superior mirage may occur. An inverted image is seen above the object, and sometimes an erect image appears over the inverted one, with the bases of the two images touching. Greater than normal refraction usually occurs when the water is much colder than the air above it.

    If the temperature decrease with height is much greater than normal, refraction is less than normal, or may even cause bending in the opposite direction. Objects appear lower than normal, and the visible horizon is closer to the observer. This is called sinking.

    Towering or stooping may occur if conditions are suitable. When the refraction is reversed, an inferior mirage may occur. A ship or an island appears to be floating in the air above a shimmering horizon, possibly with an inverted image beneath it. Conditions suitable to the formation of an inferior mirage occur when the surface is much warmer than the air above it. This usually requires a heated landmass, and therefore is more common near the coast than at sea.
    When refraction is not uniformly progressive, objects may appear distorted, taking an almost endless variety of shapes. The sun when near the horizon is one of the objects most noticeably affected.

    A fata morgana is a complex mirage characterized by marked distortion, generally in the vertical. It may cause objects to appear towering, magnified, and at times even multiplied.

    3517. Sky Coloring
    White light is composed of light of all colors. Color is related to wavelength, the visible spectrum varying from about 0.000038 to 0.000076 centimeters. The characteristics of each color are related to its wavelength (or frequency). The shorter the wavelength, the greater the amount of bending when light is refracted. It is this principle that permits the separation of light from celestial bodies into a spectrum ranging from red, through orange, yellow, green, and blue, to violet, with long-wave infrared being slightly outside the visible range at one end and short-wave ultraviolet being slightly outside the visible range at the other end. Light of shorter wavelength is scattered and diffracted more than that of longer wavelength.
    Light from the sun and moon is white, containing all colors. As it enters the earth’s atmosphere, a certain amount of it is scattered. The blue and violet, being of shorter wavelength than other colors, are scattered most. Most of the violet light is absorbed in the atmosphere. Thus, the scattered blue light is most apparent, and the sky appears blue. At great heights, above most of the atmosphere, it appears black.

    When the sun is near the horizon, its light passes through more of the atmosphere than when higher in the sky, resulting in greater scattering and absorption of blue and green light, so that a larger percentage of the red and orange light penetrates to the observer. For this reason the sun and moon appear redder at this time, and when this light falls upon clouds, they appear colored.

    This accounts for the colors at sunset and sunrise. As the setting sun approaches the horizon, the sunset colors first appear as faint tints of yellow and orange. As the sun continues to set, the colors deepen. Contrasts occur, due principally to difference in height of clouds. As the sun sets, the clouds become a deeper red, first the lower clouds and then the higher ones, and finally they fade to a gray.

    When there is a large quantity of smoke, dust, or other material in the sky, unusual effects may be observed. If the material in the atmosphere is of suitable substance and quantity to absorb the longer wave red, orange, and yellow radiation, the sky may have a greenish tint, and even the sun or moon may appear green. If the green light, too, is absorbed, the sun or moon may appear blue. A green moon or blue moon is most likely to occur when the sun is slightly below the horizon and the longer wavelength light from the sun is absorbed, resulting in green or blue light being cast upon the atmosphere in front of the moon. The effect is most apparent if the moon is on the same side of the sky as the sun.

    3518. Rainbows
    The rainbow, that familiar arc of concentric colored bands seen when the sun shines on rain, mist, spray, etc., is caused by refraction, internal reflection, and diffraction of sunlight by the drops of water. The center of the arc is a point 180° from the sun, in the direction of a line from the sun, through the observer. The radius of the brightest rainbow is 42°. The colors are visible because of the difference in the amount of refraction of the different colors making up white light, the light being spread out to form a spectrum. Red is on the outer side and blue and violet on the inner side, with orange, yellow, and green between, in that order from red. Sometimes a secondary rainbow is seen outside the primary one, at a radius of about 50°. The order of colors of this rainbow is reversed. On rare occasions a faint rainbow is seen on the same side as the sun. The radius of this rainbow and the order of colors are the same as those of the primary rainbow.

    A similar arc formed by light from the moon (a lunar rainbow) is called a moonbow. The colors are usually very faint. A faint, white arc of about 39° radius is occasionally seen in fog opposite the sun. This is called a fogbow, although its origin is controversial, some considering it a halo.

    3519. Halos
    Refraction, or a combination of refraction and reflection, of light by ice crystals in the atmosphere may cause a halo to appear. The most common form is a ring of light of radius 22° or 46° with the sun or moon at the center. Cirrostratus clouds are a common source of atmospheric ice crystals. Occasionally a faint, white circle with a radius of 90° appears around the sun. This is called a Hevelian halo.

    It is probably caused by refraction and internal reflection of the sun’s light by bipyramidal ice crystals. A halo formed by refraction is usually faintly colored like a rainbow, with red nearest the celestial body, and blue farthest from it. A brilliant rainbow-colored arc of about a quarter of a circle with its center at the zenith, and the bottom of the arc about 46° above the sun, is called a circumzenithal arc.
    Red is on the outside of the arc, nearest the sun. It is produced by the refraction and dispersion of the sun’s light striking the top of prismatic ice crystals in the atmosphere. It usually lasts for only about 5 minutes, but may be so brilliant as to be mistaken for an unusually bright rainbow. A similar arc formed 46° below the sun, with red on the upper side, is called a circumhorizontal arc.

    Any arc tangent to a heliocentric halo (one surrounding the sun) is called a tangent arc. As the sun increases in elevation, such arcs tangent to the halo of 22° gradually bend their ends toward each other. If they meet, the elongated curve enclosing the circular halo is called a circumscribed halo. The inner edge is red.
    A halo consisting of a faint, white circle through the sun and parallel to the horizon is called a parhelic circle. A similar one through the moon is called a paraselenic circle. They are produced by reflection of sunlight or moonlight from vertical faces of ice crystals.

    A parhelion (plural: parhelia) is a form of halo consisting of an image of the sun at the same altitude and some distance from it, usually 22°, but occasionally 46°. A similar phenomenon occurring at an angular distance of 120° (sometimes 90° or 140°) from the sun is called a paranthelion.
    One at an angular distance of 180°, a rare occurrence, is called an anthelion, although this term is also used to refer to a luminous, colored ring or glory sometimes seen around the shadow of one’s head on a cloud or fog bank. A parhelion is popularly called a mock sun or sun dog. Similar phenomena in relation to the moon are called paraselene (popularly a mock moon or moon dog), parantiselene, and antiselene. The term parhelion should not be confused with perihelion, the orbital point nearest the sun when the sun is the center of attraction.

    A sun pillar is a glittering shaft of white or reddish light occasionally seen extending above and below the sun, usually when the sun is near the horizon. A phenomenon similar to a sun pillar, but observed in connection with the moon, is called a moon pillar. A rare form of halo in which horizontal and vertical shafts of light intersect at the sun is called a sun cross. It is probably due to the simultaneous occurrence of a sun pillar and a parhelic circle.

    3520. Corona
    When the sun or moon is seen through altostratus clouds, its outline is indistinct, and it appears surrounded by a glow of light called a corona. This is somewhat similar in appearance to the corona seen around the sun during a solar eclipse. When the effect is due to clouds, however, the glow may be accompanied by one or more rainbow-colored rings of small radii, with the celestial body at the center.

    These can be distinguished from a halo by their much smaller radii and also by the fact that the order of the colors is reversed, red being on the inside, nearest the body, in the case of the halo, and on the outside, away from the body, in the case of the corona. A corona is caused by diffraction of light by tiny droplets of water. The radius of a corona is inversely proportional to the size of the water droplets. A large corona indicates small droplets. If a corona decreases in size, the water droplets are becoming larger and the air more humid. This may be an indication of an approaching rainstorm. The glow portion of a corona is called an aureole.

    3521. The Green Flash
    As light from the sun passes through the atmosphere, it is refracted. Since the amount of bending is slightly different for each color, separate images of the sun are formed in each color of the spectrum. The effect is similar to that of imperfect color printing, in which the various colors are slightly out of register. However, the difference is so slight that the effect is not usually noticeable. At the horizon, where refraction is maximum, the greatest difference, which occurs between violet at one end of the spectrum and red at the other, is about 10 seconds of arc.

    At latitudes of the United States, about 0.7 second of time is needed for the sun to change altitude by this amount when it is near the horizon. The red image, being bent least by refraction, is first to set and last to rise. The shorter wave blue and violet colors are scattered most by the atmosphere, giving it its characteristic blue color. Thus, as the sun sets, the green image may be the last of the colored images to drop out of sight. If the red, orange, and yellow images are below the horizon, and the blue and violet light is scattered and absorbed, the upper rim of the green image is the only part seen, and the sun appears green. This is the green flash. The shade of green varies, and occasionally the blue image is seen, either separately or following the green flash (at sunset). On rare occasions the violet image is also seen. These colors may also be seen at sunrise, but in reverse order. They are occasionally seen when the sun disappears behind a cloud or other obstruction.

    The phenomenon is not observed at each sunrise or sunset, but under suitable conditions is far more common than generally supposed. Conditions favorable to observation of the green flash are a sharp horizon, clear atmosphere, a temperature inversion, and a very attentive observer. Since these conditions are more frequently met when the horizon is formed by the sea than by land, the phenomenon is more common at sea. With a sharp sea horizon and clear atmosphere, an attentive observer may see the green flash at as many as 50 percent of sunsets and sunrises, although a telescope may be needed for some of the observations.

    Duration of the green flash (including the time of blue and violet flashes) of as long as 10 seconds has been reported, but such length is rare. Usually it lasts for a period of about ½ to 2 ½ seconds, with about 1 ¼ seconds being average. This variability is probably due primarily to changes in the index of refraction of the air near the horizon.

    Under favorable conditions, a momentary green flash has been observed at the setting of Venus and Jupiter. A telescope improves the chances of seeing such a flash from a planet, but is not a necessity.

    3522. Crepuscular Rays
    Crepuscular rays are beams of light from the sun passing through openings in the clouds, and made visible by illumination of dust in the atmosphere along their paths. Actually, the rays are virtually parallel, but because of perspective, appear to diverge. Those appearing to extend downward are popularly called backstays of the sun, or the sun drawing water. Those extending upward and across the sky, appearing to converge toward a point 180° from the sun, are called anticrepuscular rays.

    3523. Atmospheric Electricity
    Radio waves traveling through the atmosphere exhibit many of the properties of light, being refracted, reflected, diffracted, and scattered. These effects are discussed in greater detail in Chapter 10, Radio Waves in Navigation. Various conditions induce the formation of electrical charges in the atmosphere. When this occurs, there is often a difference of electron charge between various parts of the atmosphere, and between the atmosphere and earth or terrestrial objects. When this difference exceeds a certain minimum value, depending upon the conditions, the static electricity is discharged, resulting in phenomena such as lightning or St. Elmo’s fire.
    Lightning is the discharge of electricity from one part of a thundercloud to another, between different clouds, or between a cloud and the earth or a terrestrial object. Enormous electrical stresses build up within thunderclouds, and between such clouds and the earth. At some point the resistance of the intervening air is overcome. At first the process is a progressive one, probably starting as a brush discharge (St. Elmo’s fire), and growing by ionization. The breakdown follows an irregular path along the line of least resistance. A hundred or more individual discharges may be necessary to complete the path between points of opposite polarity. When this “leader stroke” reaches its destination, a heavy “main stroke” immediately follows in the opposite direction.

    This main stroke is the visible lightning, which may be tinted any color, depending upon the nature of the gases through which it passes. The illumination is due to the high degree of ionization of the air, which causes many of the atoms to become excited and emit radiation.
    Thunder, the noise that accompanies lightning, is caused by the heating and ionizing of the air by lightning, which results in rapid expansion of the air along its path and the sending out of a compression wave. Thunder may be heard at a distance of as much as 15 miles, but generally does not carry that far.
    The elapsed time between the flash of lightning and reception of the accompanying sound of thunder is an indication of the distance, because of the difference in travel time of light and sound. Since the former is comparatively instantaneous, and the speed of sound is about 1,117 feet per second, the approximate distance in nautical miles is equal to the elapsed time in seconds, divided by 5.5. If the thunder accompanying lightning cannot be heard due to its distance, the lightning is called heat lightning. St. Elmo’s fire is a luminous discharge of electricity from pointed objects such as the masts and antennas of ships, lightning rods, steeples, mountain tops, blades of grass, human hair, arms, etc., when there is a considerable difference in the electrical charge between the object and the air. It appears most frequently during a storm. An object from which St. Elmo’s fire emanates is in danger of being struck by lightning, since this discharge may be the initial phase of the leader stroke. Throughout history those who have not understood St. Elmo’s fire have regarded it with superstitious awe, considering it a supernatural manifestation. This view is reflected in the name corposant (from “corpo santo,” meaning “body of a saint”) sometimes given this phenomenon.

    The aurora is a luminous glow appearing in varied forms in the thin atmosphere high above the earth in high latitudes. It closely follows solar flare activity, and is believed caused by the excitation of atoms of oxygen and hydrogen, and molecules of nitrogen (N2). Auroras extend across hundreds of kilometers of sky, in colored sheets, folds, and rays, constantly changing in form and color. On occasion they are seen in temperate or even more southern latitudes. The maximum occurrence is at about 64–70of geomagnetic latitude. These are called the auroral zones in both northern and southern regions.

    The aurora of the northern regions is the Aurora Borealis or northern lights, and that of the southern region the Aurora Australis, or southern lights. The term polar lights is occasionally used to refer to either. In the northern zone, there is an apparent horizontal motion to the westward in the evening and eastward in the morning; a general southward motion occurs during the course of the night.
    Variation in auroral activity occurs in sequence with the 11-year sunspot cycle, and also with the 27-day period of the sun’s synodical rotation. Daily occurrence is greatest near midnight.

    3524. Forecasting Weather
    The prediction of weather at some future time is based upon an understanding of weather processes, and observations of present conditions. Thus, when there is a certain sequence of cloud types, rain usually can be expected to follow. If the sky is cloudless, more heat will be received from the sun by day, and more heat will be radiated outward from the warm earth by night than if the sky is overcast. If the wind is from a direction that transports warm, moist air over a colder surface, fog can be expected. A falling barometer indicates the approach of a “low,” probably accompanied by stormy weather. Thus, before meteorology passed from an “art” to “science,” many individuals learned to interpret certain atmospheric phenomena in terms of future weather, and to make reasonably accurate forecasts for short periods into the future.
    With the establishment of weather observation stations, continuous and accurate weather information became available. As observations expanded and communication techniques improved, knowledge of simultaneous conditions over wider areas became available. This made possible the collection of “synoptic” reports at civilian and military forecast centers.
    Individual observations are made at stations on shore and aboard vessels at sea. Observations aboard merchant ships at sea are made and transmitted on a voluntary and cooperative basis. The various national meteorological services supply shipmasters with blank forms, printed instructions, and other materials essential to the making, recording, and interpreting of observations. Any shipmaster can render a particularly valuable service by reporting all unusual or non-normal weather occurrences. Symbols and numbers are used to indicate on a synoptic chart, popularly called a weather map, the conditions at each observation station. Isobars are drawn through lines of equal atmospheric pressure, fronts are located and symbolically marked (See Figure 3525), areas of precipitation and fog are indicated, etc.
    Ordinarily, weather maps for surface observations are prepared every 6 (sometimes 3) hours. In addition, synoptic charts for selected heights are prepared every 12 (sometimes 6) hours. Knowledge of conditions aloft is of value in establishing the three-dimensional structure and motion of the atmosphere as input to the forecast.

    With the advent of the digital computer, highly sophisticated numerical models have been developed to analyze and forecast weather patterns. The civil and military weather centers prepare and disseminate vast numbers of weather charts (analyses and prognoses) daily to assist local forecasters in their efforts to provide users with accurate weather forecasts.

    The accuracy of forecast decreases with the length of the forecast period. A 12-hour forecast is likely to be more reliable than a 24-hour forecast. Long term forecasts for 2 weeks or a month in advance are limited to general statements. For example, a prediction may be made about which areas will have temperatures above or below normal, and how precipitation will compare with normal, but no attempt is made to state that rainfall will occur at a certain time and place.
    Forecasts are issued for various areas. The national meteorological services of most maritime nations, including the United States, issue forecasts for ocean areas and warnings of approaching storms. The efforts of the various nations are coordinated through the World Meteorological Organization.

    3525. Weather Forecast Dissemination
    Dissemination of weather information is carried out in a number of ways. Forecasts are widely broadcast by commercial and government radio stations and printed in newspapers. Shipping authorities on land are kept informed by telegraph and telephone. Visual storm warnings are displayed in various ports, and storm warnings are broadcast by radio.
    Through the use of codes, a simplified version of synoptic weather charts is transmitted to various stations ashore and afloat. Rapid transmission of completed maps is accomplished by facsimile. This system is based upon detailed scanning, by a photoelectric detector, of illuminated black and white copy. The varying degrees of light intensity are converted to electric energy, which is transmitted to the receiver and converted back to a black and white presentation. The proliferation of both commercial and restricted computer bulletin board systems having weather information has also greatly increased the accessibility of environmental data.
    Complete information on dissemination of weather information by radio is provided in Selected Worldwide Marine Weather Broadcasts, published jointly by the National Weather Service and the Naval Meteorology and Oceanography Command.

    This publication lists broadcast schedules and weather codes. Information on day and night visual storm warnings is given in the various volumes of Sailing Directions (Enroute), and (Planning Guide).


    3526. Interpreting Weather
    The factors which determine weather are numerous and varied. Ever-increasing knowledge regarding them makes possible a continually improving weather service. However, the ability to forecast is acquired through study and long practice, and therefore the services of a trained meteorologist should be utilized whenever available. The value of a forecast is increased if one has access to the information upon which it is based, and understands the principles and processes involved. It is sometimes as important to know the various types of weather which may be experienced as it is to know which of several possibilities is most likely to occur.
    At sea, reporting stations are unevenly distributed, sometimes leaving relatively large areas with incomplete reports, or none at all. Under these conditions, the locations of highs, lows, fronts, etc., are imperfectly known, and their very existence may even be in doubt. At such times the mariner who can interpret the observations made from his own vessel may be able to predict weather for the next several hours more reliably than a trained meteorologist ashore.
  12. What is the correlation of the above information with the increased activity of LM bass in a pond prior to the approach of a low pressure front with the wind rotating to sw direction and increasing in velocity.
  13. Fishers of Men

    Fishers of Men Senior Member

    Thats something that there is a lot of controversy over. Different opinions from different people over barometer change and such. I personally seem to find fish get very active at the first part of the fall. Some say it doesn't matter. I believe it does, and have thoroughly seen it in the salt water. I also when in Canada used to go out when everyone was comming in and have caught muskie that turned on at that time. lake erie seems to have a walleye feed like that and then they just shut down. Could be that when that rotation occurs, that every thing comes in "tune" with the magnetic influences in the earth as described in part 1. There is a noticeable similarity with the air/current/and magnetic directions. A small pond? Probably moves the bait to the other side and activates the predators...Give us your take on it.
    Thanks for the question, these are the kind of things we are all trying to figure out!
  14. Fishers of Men

    Fishers of Men Senior Member

    Any one want to carry this thread further?
    Kind of ran out of topics since part one and two about covered a little bit of everything. Some parts or things I might of missed? Or go into further detail on any certain sections?