《航海学》课程参考文献（地文资料）CHAPTER 34 ICE IN THE SEA

CHAPTER 34ICEINTHESEAINTRODUCTION3400.IceAndTheNavigatorpiloting by altering the appearance or obliterating the fea-tures of landmarks;it hinders the establishment andSea ice has posed a problem to the polar navigatormaintenanceof aids tonavigation; it affects his use of elec-sinceantiquity.During a voyage from the Mediterraneantotronicsby affecting propagation ofradio waves; itproducesEnglandandNorwaysometimebetween350BCand300changes in surfacefeatures and in radar returns from theseBC,Pytheasof Massalia sighted a strange substancewhichfeatures; it affects celestial navigation by altering the re-fraction and obscuring the horizon and celestial bodieshe described as“"neither land nor air nor water"floatingupon and covering thenorthern sea over which the summereither directly or by the weather it influences, and it affectschartsby introducing several plottingproblems.sun barely set.Pytheas named this lonely region Thule,hence Ultima Thule(farthestnorth orland's end).Thus be-Becauseof his directconcernwith ice,theprospectiveganover20centuries ofpolarexploration.polarnavigatormustacquainthimselfwithitsnatureandex-Ice is of direct concern to the navigator because it re-tent in the area he expects to navigate. In addition to thisstricts and sometimes controls his movements,it affects hisvolume, books, articles,and reports of previous polar oper-dead reckoningbyforcingfrequent and sometimes inaccu-ationsandexpeditionswillhelpacquaintthepolarnavigatorratelydetermined changesofcourseand speed, itaffects hiswith the unique conditions at the ends of the earth.403920OE1o09FREEZING POINT--20-30--5010203040SALINITYPARTSPERTHOUSANDFigure3401.Relationshipbetweentemperatureofmaximumdensityandfreezingpointforwaterofvaryingsalinity455

455 CHAPTER 34 ICE IN THE SEA INTRODUCTION 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 be￾gan over 20 centuries of polar exploration. Ice is of direct concern to the navigator because it re￾stricts and sometimes controls his movements; it affects his dead reckoning by forcing frequent and sometimes inaccu￾rately determined changes of course and speed; it affects his piloting by altering the appearance or obliterating the fea￾tures of landmarks; it hinders the establishment and maintenance of aids to navigation; it affects his use of elec￾tronics 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 re￾fraction 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 ex￾tent in the area he expects to navigate. In addition to this volume, books, articles, and reports of previous polar oper￾ations and expeditions will help acquaint the polar navigator with the unique conditions at the ends of the earth. Figure 3401. Relationship between temperature of maximum density and freezing point for water of varying salinity

456ICEINTHESEA3401.FormationOfIcebuoyantforce ofthe waterbreaks off piecesfromtimeto time,andthesefloatawayas icebergs.Icebergs maybedescribed asdome shaped, sloping or pinnacled (Figure 3402a), tabularAs it cools,water contractsuntil the temperature of(Figure3402b),glacier,orweatheredmaximum density is reached. Further cooling results in ex-A floating iceberg seldom melts uniformly because ofpansion.Themaximum density of fresh wateroccurs atatemperature of4.0°C,and freezing takes place at 0°C.Thelack ofuniformityinthe ice itself,differences in thetempera-additionofsaltlowersboththetemperatureofmaximumtureaboveandbelowthewaterline,exposureofone sidetothedensity and, to a lesser extent, that of freezing.These rela-sun, strains,cracks,mechanical erosion,etc.Theinclusion oftionships areshown inFigure3401.The two linesmeet at arocks, silt, and other foreign matterfurther accentuates the dif-salinityof24.7partsperthousand.atwhichmaximumden-ferences.Asaresultchangesinequilibriumtakeplace,whichsity occurs at the freezing temperature of -1.3°C.At thismay cause theberg to periodically tilt or capsize.Parts of itand greater salinities,thetemperature of maximum densitymay break offor calve,forming separate smaller bergs.A relof sea water is coincidentwith thefreezingpointtempera-ativelylargepiece offloatingice,generallyextending1to5ture, i. e., the density increases as the temperature getsmeters abovethesea surfaceand normallyabout100to300colder.Atasalinityof35partsperthousand.theapproxi-square meters in area, is called a bergy bit.A smaller piece ofmate averagefor the oceans,the freezing point is-1.88C.ice large enoughto inflict serious damage to a vessel is calledagrowler because of the noise it sometimes makes as itbobsAs thedensity of surface seawater increaseswithde-upanddownintheseaGrowlersextendlessthanImetercreasingtemperature,convective density-driven currentsabovethe sea surfaceandnormallyoccupyan area of about 20are induced bringing warmer, less dense water to the sur-square meters.Bergy bits and growlers are usually piecesface.If the polar seas consisted of water with constantcalved from icebergs,but they may be theremains ofamostlysalinity,theentirewater column wouldhaveto becooledtomelted icebergthe freezing point in this manner before ice would begin toform.This is not the case, however, in the polar regionsThe principal dangerfrom icebergs is theirtendencytobreakorcapsize.Soon after a berg is calved,whileremainingwherethevertical salinitydistributionissuchthatthesur-face waters are underlain at shallow depth by waters ofin far northern waters,60-80% of its bulk is submerged.Buthigher salinity.In this instance density currents form a shal-as the berg drifts into warmer waters, the underside can some-lowmixed layer which subsequently cannot mixwiththetimesmeltfasterthan theexposed portion,especiallyin verydeeplaverofwarmerbutsaltierwater.Icewillthenbegincold weather.As the mass of the submerged portion deterio-forming at thewater surface when density currents ceaserates,the berg becomes increasingly unstable,and itwilland the surface water reaches its freezing point. In shoaleventually roll over.Icebergs thathavenotyetcapsized haveajagged and possibly dirty appearance. A recently capsized bergwater, however, the mixing process can be sufficient to ex-tend the freezing temperature from the surface to thewill besmooth, clean,and curved in appearance.Previous wa-bottom.Icecrvstalscan,therefore.formatanydepthinthisterlinesatoddanglescansometimesbeseenafteroneormorecase.Because oftheirdecreased density,theytendtorisetocapsizings.thesurface,unlesstheyformatthebottomandattachthem-The stability of a berg can sometimes be noted by itsselves there.This ice, called anchor ice, may continue toreaction to ocean swells.The livelier theberg, the more ungrow as additional icefreezes to that already formedstable it is. It is extremely dangerous for a vessel toapproachan iceberg closelyevenone which appears stable,3402.Land Icebecause in addition to the dangerfrom capsizing,unseencracks can cause icebergs to split in two or calve off largechunks.Iceof land origin isformed on land by thefreezing ofAnother danger is from underwater extensions, calledfreshwater orthecompactingof snowas layerupon layeraddstothepressureonthatbeneath.ramswhichareusuallyformedduetomeltingorerosionabovethe waterlineat afaster ratethan below.Rams may also extendUnder great pressure, ice becomes slightly plastic, and isfrom avertical icecliff,alsoknownasanicefront,whichformsforced downward along an inclined surface.Ifa large area isthe seaward faceofa massive ice sheetor floatingglacier,orrelativelyflat, as onthe Antarctic plateau,or if theoutwardfrom an icewall,which isthe icecliffformingtheseaward mar-flow is obstructed, as on Greenland, an ice cap forms and re-gin of a glacier which is aground. In addition to rams, largemains throughout the year.The thickness of these ice capsportions of an iceberg may extend well beyond the waterlineatranges fromnearly1kilometer on Greenlandto as muchas 4.5greater depths.kilometers on the Antarctic Continent.Where ravines ormountain passes permit flow of the ice, a glacier is formedStrangely,icebergs maybe helpful to themariner in someThis is a mass of snowand icewhich continuouslyflowstoways.The melt water found on the surface of icebergs is alowerlevels,exhibitingmanyofthecharacteristicsofriversofsourceoffreshwater,and in thepast somedaring seamenhavewater.The flow may be more than 30 meters per day,but ismade their vessels fast to icebergs which, because they are af-generally much less. When a glacier reaches a comparativelyfected moreby currents thanthe wind,haveproceeded totowlevel area, itspreadsout.Whenaglacierflows intothesea,thethemoutoftheicepack

456 ICE IN THE SEA 3401. Formation Of Ice As it cools, water contracts until the temperature of maximum density is reached. Further cooling results in ex￾pansion. 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 rela￾tionships are shown in Figure 3401. The two lines meet at a salinity of 24.7 parts per thousand, at which maximum den￾sity 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 tempera￾ture, i. e., the density increases as the temperature gets colder. At a salinity of 35 parts per thousand, the approxi￾mate average for the oceans, the freezing point is –1.88°C. As the density of surface seawater increases with de￾creasing temperature, convective density-driven currents are induced bringing warmer, less dense water to the sur￾face. 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 sur￾face waters are underlain at shallow depth by waters of higher salinity. In this instance density currents form a shal￾low 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 ex￾tend 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 them￾selves 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 re￾mains 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 (Figure 3402a), tabular (Figure 3402b), glacier, or weathered. A floating iceberg seldom melts uniformly because of lack of uniformity in the ice itself, differences in the tempera￾ture 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 dif￾ferences. 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 rel￾atively 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 some￾times melt faster than the exposed portion, especially in very cold weather. As the mass of the submerged portion deterio￾rates, 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 wa￾terlines 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 un￾stable 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 mar￾gin 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 af￾fected more by currents than the wind, have proceeded to tow them out of the ice pack

457ICEINTHESEAIcebergs can be used as a navigational aid in extrememinimum water depthat its location.Waterdepth will beatlatitudes wherecharteddepths may be in doubtornon-ex-least equal to theexposed heightof thegrounded icebergistent.Sincean iceberg(excepta largetabularberg)mustGrounded bergs remain stationary whilecurrent and windbe at least as deep in the water as it is high to remain up-move sea icepastthem.Drifting icemaypile upagainsttheright, a grounded berg can provide an estimate of theupcurrent side ofa grounded berg.Figure3402a.Pinnaclediceberg.Figure3402b.Atabulariceberg

ICE IN THE SEA 457 Icebergs can be used as a navigational aid in extreme latitudes where charted depths may be in doubt or non-ex￾istent. Since an iceberg (except a large tabular berg) must be at least as deep in the water as it is high to remain up￾right, 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. Figure 3402a. Pinnacled iceberg. Figure 3402b. A tabular iceberg

458ICEINTHESEA3403.SeaIceSea iceforms bythefreezing of seawater andaccountsfor95percentof all iceencountered.Thefirst indication ofthe formation of new sea ice (upto 10 centimeters inthick-ness)isthedevelopmentof small individual,needle-likecrystals ofice,called spicules,which become suspended inthe top few centimeters of seawater.These spicules, alsoknown as frazil ice,give the sea surface an oily appearance.Grease ice is formed when the spicules coagulate to form asoupylayer on the surface,giving the sea a matte appear-ance.The next stagein sea iceformationoccurs whenshuga,an accumulation of spongy white ice lumps a fewcentimeters across,develops fromgrease ice.Uponfurtherfreezing,and depending upon wind exposure, seas, and sa-linity.shugaandgreaseicedevelopintonilas.anelasticcrustofhigh salinity,upto10centimeters inthickness,witha matte surface, or into ice rind, a brittle, shiny crust oflowsalinity with a thickness up to approximately5 centimeters.Figure3403.Pancakeice,withaniceberg inthebackgroundAlayerof5centimetersoffreshwaterice is brittlebut strongenoughto supporttheweightofa heavyman.In contrast,thesamethickness of newlyformed sea icewill supportnotmore than about 10 percent of this weight, although itsgray-white ice, or collectively as young ice,and is the tran-strength varies with the temperatures at which it is formed;sition stage between nilas and first-year ice.First-year iceverycoldicesupportsagreaterweightthanwarmerice.Asusually attains a thicknessof between30centimeters and2itages, sea ice becomes harderand more brittle.meters in itsfirst winter's growth.Newicemayalsodevelop fromslushwhichis formedSea ice may grow to a thickness of 10 to 13 centimeterswhen snow falls into seawater which is near its freezing point,within 48hours,afterwhich itacts as an insulator betweenbut colderthanthemeltingpointof snow.Thesnowdoesnotthe ocean and the atmosphere progressively slowing its fur-melt,butfloatsonthesurface,drifting withthewind intobedsther growth.However, sea icemay growtoa thickness ofIfthe temperaturethen drops below thefreezingpoint ofthesea-between 2 to 3meters in its first winter.Ice which has sur-water,theslushfreezesquicklyinto asoft ice similarto shugavived at least one summer's melt is classified as old ice.If itSea ice is exposed to several forces, including currents,hassurvivedonlyonesummersmeltitmaybereferredtoaswaves,tides, wind,and temperature variations. In its earlysecond-year ice, but this term is seldom used today.Old icewhich has attained a thickness of 3 meters or more and hasstages, its plasticity permits it to conform readily to virtuallyany shape required by the forces acting upon it. As it be-survived atleast two summers'melt isknown as multiyearcomes older, thicker, more brittle, and exposed to theice and is almost salt free. This term is increasingly used toinfluenceof wind and waveaction,newiceusuallysepa-refertoanyicemorethanoneseasonold.Oldicecanberec-ratesintocircularpiecesfrom30centimetersto3meters inognized by abluishtoneto its surfacecolor incontrasttothediameter and up to approximately 10 centimeters in thick-greenish tint of first-year ice, but it is often covered withness with raised edges dueto individual pieces strikingsnow.Another sign of old ice is a smoother, more roundedagainst each other.These circular pieces of ice are calledappearance duetomelting/refreezing and weatheringpancake ice (Figure 3403) and may break into smaller piec-Greater thicknesses in both first and multiyear ice arees with strong wave motion.Any singlepiece of relativelyattained through the deformation of the ice resulting fromflat sea ice less than20 meters across is called an ice cakethe movement and interaction of individual floes.Deforma-With continued lowtemperatures,individual ice cakes andtion processes occurafterthe development of new andpancake ice will,depending on wind or wavemotion,eitheryoung ice and are the direct consequence of the effects offreezetogether toformacontinuous sheetorunite intopiec-winds,tides, and currents.These processes transform arela-es of ice20meters or more across.These larger piecesaretivelyflat sheet of ice into pressure ice which has a roughthen called icefloes,which may furtherfreezetogether tosurface.Bending,whichis thefirststage intheformation ofform an ice covered areagreater than 10 kilometers acrosspressure ice, is theupward or downward motion of thin andknownasanicefieldvery plastic ice. Rarely, tenting occurs when bending pro-.In wind sheltered areas thickening ice usuallyforms aduces an upward displacementof iceforming a flat sidedcontinuous sheet before it candevelop into thecharacteris-arch with a cavity beneath.More frequently,however,raft-tic ice cake form. When sea ice reaches a thickness ofingtakesplaceasonepieceoficeoverridesanother.Whenbetween10to30centimetersitisreferredtoasgrayandpiecesoffirst-year icearepiled haphazardly over one anoth-

458 ICE IN THE SEA 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 thick￾ness) 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 appear￾ance. 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 sa￾linity, 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 sea￾water, 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 be￾comes older, thicker, more brittle, and exposed to the influence of wind and wave action, new ice usually sepa￾rates into circular pieces from 30 centimeters to 3 meters in diameter and up to approximately 10 centimeters in thick￾ness 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 piec￾es 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 piec￾es 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 characteris￾tic 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 tran￾sition 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 fur￾ther growth. However, sea ice may grow to a thickness of between 2 to 3 meters in its first winter. Ice which has sur￾vived 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 rec￾ognized 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. Deforma￾tion 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 rela￾tively 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 pro￾duces an upward displacement of ice forming a flat sided arch with a cavity beneath. More frequently, however, raft￾ing takes place as one piece of ice overrides another. When pieces of first-year ice are piled haphazardly over one anoth￾Figure 3403. Pancake ice, with an iceberg in the background

459ICEINTHESEAer forminga wallor lineofbroken ice,referred to as aridge,two, and driven aground or caught in the shear zone between.the process is known as ridging. Pressure ice with topogra-Before a lead refreezes,lateral motiongenerally occursphy consisting of numerous mounds or hillocks is calledbetween the floes, so that they no longer fit and unless thehummocked ice,eachmoundbeing called a hummockpressure is extreme, numerous largepatches ofopen waterThe motion of adiacentfloes is seldom equal.Theremain.These nonlinearshaped openings enclosed in icearecalled polynyas.Polynyas maycontain small fragments ofrougher the surface,thegreater is the effect of wind, sincefloating iceandmaybecoveredwithmiles ofnewand youngeachpieceextendingabovethesurfaceactsasasail.Someice floes are in rotary motion as they tend to trim them-ice. Recurring polynyas occur in areas where upwelling ofselves into the wind. Since ridges extend below as well asrelatively warmer water occurs periodically.These areas areoften the site of historical native settlements, where theabove the surface,the deeper ones are influenced morebypolynyas permit fishing and hunting at times before regulardeepwater currents.Whena strong wind blows inthe samedirectionfora considerableperiod,each floeexerts pres-seasonal ice breakup.Thule, Greenland, is an examplesure on the next one,and as thedistance increases,theSea ice which is formed in situfrom seawater or bythepressure becomes tremendous.Ridges on sea ice are gener-freezing of pack ice of anyagetothe shore and which re-ally about 1 meter high and 5 meters deep, but undermains attached to the coast, to an ice wall,to an icefront, orconsiderable pressure may attain heights of 20meters andbetween shoals is called fast ice.The width of this fast icedepthsof 50meters in extremecases.variesconsiderablyandmayextendforafewmetersorsevThe alternate melting and growth of sea ice, combinederal hundred kilometers,Inbays and othersheltered areas,withthecontinual motionofvariousfloesthatresults insep-fastice.oftenaugmentedbyannualsnowaccumulationsandtheseaward extensionof land ice,mayattain athickness ofaration aswell as consolidation, causeswidelyvaryingover 2meters above the sea surface.When a floating sheetconditions within the ice cover itself.The mean areal density,ofice grows to this or a greater thickness and extends overaorconcentration,ofpackiceinanygivenareaisexpressedingreat horizontal distance, it is called an ice shelf. Massivetenths.Concentrationsrangefrom:openwater(totalconcenice shelves, where the ice thickness reaches several hundredtration ofall ice is less than onetenth),very openpack (1 tometers, are found in both the Arctic and Antarctic3tenthsconcentration),openpack(4to6tenthsconcentra-tion),close pack (7to 8 tenths concentration),verycloseThe majority ofthe icebergs found in the Antarctic do notpack(9to10tolessthan 10to10concentration),tocompactoriginatefromglaciers,asdothosefoundintheArctic,butareor consolidated pack(10to10orcomplete coverage).Thecalvedfromtheouteredgesofbroad expanses ofshelfice.Ice-extenttowhichan icecoverofvaryingconcentrationscanbebergs formed in this manner are called tabular icebergspenetrated bya vessel variesfrom placeto placeand withhavingabox likeshapewith horizontal dimensions measuredchangingweather conditions.With a concentration of 1to3in kilometers, and heights above the sea surface approachingtenths in a given area, an unreinforced vessel can generally60meters.SeeFigure3402b.The largest Antarctic iceshelvesnavigate safely,but thedanger ofreceiving heavy damage isare found in the Ross and Weddell Seas.The expression tab-alwavspresent.Whentheconcentrationincreasestobetweenular iceberg"is not applied to bergs which break off from3and5tenths,theareabecomesonlyoccasionallyaccessibleArctic ice shelves:similarformations there are called ice is-to an unreinforced vessel, depending upon the wind and cur-lands.Theseoriginatewhenshelfice.suchasthatfoundontherent.With concentrations of 5to7tenths,the area becomesnortherncoastofGreenlandandinthebavsofEllesmereIsaccessible only to ice strengthened vessels, which on occa-land, breaks up. As a rule, Arctic ice islands are not as large assion will require icebreaker assistance.Navigation in areasthetabularicebergsfoundintheAntarctic.Theyattainathick-with concentrations of 7tenths or more should onlybeat-nessofupto55metersandontheaverageextend5to7meterstempted byicebreakers.above the sea surface.Bothtabular icebergs and ice islandspossess a gently rolling surface.Because of their deep draftWithin the icecover,openingsmay developresultingfromanumberofdeformationprocesses.Long,jaggedthey are influenced much more by current than wind. Arcticice islandshavebeen used asfloating scientificplatformsfromcracks may appearfirst in the ice cover or through a singlewhich polar research hasbeen conducted.floe.Whenthesecrackspartand reachlengths of afewmeterstomanykilometers.theyarereferredtoasfracturesIf theywiden furthertopermitpassageof a ship,theyare3404.ThicknessOfSea Icecalled leads.Inwinter,a thincoating ofnewicemay coverthewaterwithinaleadbutinsummerthewaterusuallyre-Seaice has beenobservedtogrowtoa thickness ofalmostmains ice-free until a shift in the movement forces the two3 meters during its first year.However, the thickness of first-sides together again. A lead ending in a pressure ridge or oth-year icethat has not undergonedeformation does not generallyer impenetrable barrier is a blind lead.exceed 2meters.In coastal areas where the melting rate is lessA lead between pack ice and shore is a shore lead, andthan the freezing rate, the thickness may increase during suc-onebetween pack and fast ice is a flawlead.Navigation inceeding winters, being augmented by compacted and frozenthesetwotypes ofleads is dangerous,because ifthepack icesnow.untilamaximumthicknessofabout3.5to4.5meterscloses with thefast ice,the ship canbe caught between themayeventuallybe reached.Oldseaicemay also attain athick-

ICE IN THE SEA 459 er forming a wall or line of broken ice, referred to as a ridge, the process is known as ridging. Pressure ice with topogra￾phy 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 them￾selves 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 pres￾sure on the next one, and as the distance increases, the pressure becomes tremendous. Ridges on sea ice are gener￾ally 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 sep￾aration 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 concen￾tration of all ice is less than one tenth), very open pack (1 to 3 tenths concentration), open pack (4 to 6 tenths concentra￾tion), 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 cur￾rent. With concentrations of 5 to 7 tenths, the area becomes accessible only to ice strengthened vessels, which on occa￾sion will require icebreaker assistance. Navigation in areas with concentrations of 7 tenths or more should only be at￾tempted 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 re￾mains ice-free until a shift in the movement forces the two sides together again. A lead ending in a pressure ridge or oth￾er 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 re￾mains 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 sev￾eral 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. Ice￾bergs 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 “tab￾ular iceberg” is not applied to bergs which break off from Arctic ice shelves; similar formations there are called ice is￾lands. These originate when shelf ice, such as that found on the northern coast of Greenland and in the bays of Ellesmere Is￾land, breaks up. As a rule, Arctic ice islands are not as large as the tabular icebergs found in the Antarctic. They attain a thick￾ness 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 first￾year 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 suc￾ceeding 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 thick-

460ICEINTHESEAness of over 4 meters in this manner, or when summer melthaveshownthatthemost influential parametersaffectingseawaterfromitssurfaceorfromsnowcoverrunsoffintotheseaice growth are air temperature, wind speed, snowdepth andand refreezes under the ice where the seawater temperature isinitial ice thickness. Many complex equations have been for-belowthefreezingpointofthefreshermelt water.mulated to predict icegrowth using thesefour parametersThe growth of sea ice is dependent upon a number ofHowever,except for thefirst two, these parameters are notmeteorologicalandoceanographicparameters.Suchparam-routinely observed for remotepolar locations.eters include air temperature,initial ice thickness, snowField measurements suggest that reasonable growthdepth, wind speed, seawater salinity and density,and the spe-cific heats of sea ice and seawater.Investigations,however,estimatescanbeobtainedfromairtemperaturedataaloneLAAUNEJULYOaEAYeanE宇啤帘-25SteWS04500SANC4000320-30002500a200015001000500AUGSEPTOCTCECNOVFEB190S会181504013020(ao)116SEt0090301aR60503020Figure34o4a.RelationshipbetweenaccumulatedfrostdegreedaysandtheoreticalicethicknessatPointBarrow,Alaska

460 ICE IN THE SEA ness 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 param￾eters include air temperature, initial ice thickness, snow depth, wind speed, seawater salinity and density, and the spe￾cific 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 for￾mulated 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. Figure 3404a. Relationship between accumulated frost degree days and theoretical ice thickness at Point Barrow, Alaska

461ICEINTHESEA30025015型-1111111115,0006,0007,0008,0009.0001,0002.0003.004,00010,000ADCUMURCECREEDAYS(O'CBAEETEDFROSTEFigure 3404b. Relationship between accumlated frost degree days (°C) and ice thickness (cm).Various empirical formulae have been developed based onwhen temperatures remain below freezing. The relationshipthis premise.All appear to perform better under thin ice con-between frost degree day accumulations and theoretical iceditions when the temperature gradient through the ice isgrowth curvesatPointBarrow,Alaska isshown inFigurelinear, generally true for ice less than 100 centimeters thick3404a.Similar curves for otherArctic stations are containedDifferencesinpredictedthicknessesbetweenmodelsgener-inpublications availablefromtheU.S.NavalOceanographicallyreflect differencesin environmental1parametersOfficeandtheNationalIceCenter.Figure3404bgraphically(snowfal, heat content of the underlying water column, etc.)depicts the relationship between accumulated frost degreeat the measurement site.As a result, suchequations must beconsidered partially site specific and their general use ap-days (°C)and ice thickness in centimetersproached with caution.For example,applying an equationDuring winter, the ice usually becomes covered withderived from central Arctic data to coastal conditionsortosnow,which insulates the ice beneath and tends to slowAntarcticconditionscouldlead tosubstantial errors.Forthisdown its rateofgrowth.This thickness of snowcover variesreasonZubov's formula is widely cited as it represents an av-considerably from region to region as a result of differingerageofmanyyearsofobservationsfromtheRussianArctic:climatic conditions.Its depth may also vary widely withinh+50h=80very short distances inresponse to variable winds and ice to-pography. While this snow cover persists, about 80 to 85where h is the ice thickness in centimeters for agiven dayandpercent of the incoming radiation is reflected back to space. is the cumulative number of frost degreedays indegreesEventuallyhowever,the snowbeginstomelt,as theairtem-Celsius since the beginning of thefreezing seasonperaturerises above o°C inearly summer and the resultingAfrost degree day is defined as a day with a mean tem-freshwater forms puddles on the surface. These puddles ab-perature of obelow anarbitrarybase.The base mostsorbabout90percentof the incomingradiation and rapidlycommonly used is the freezing point of freshwater (O°C).If,for example, the mean temperature on a given day is 5°be-enlarge as they meltthe surrounding snow or ice.Eventuallylowfreezing,then fivefrostdegree days arenoted for thatthe puddles penetratetothebottom surfaceof the floes andday.Thesefrost degree days are then added to those noted theas thawholes. This slow process is characteristic of ice innextdaytoobtainan accumulatedvalue,whichisthenaddedthe Arctic Ocean and seas wheremovement is restricted byto those noted the following day.This process is repeatedthe coastline or islands.Where ice is free to drift into warmerdaily throughout the ice growing season.Temperatures usu-waters (e.g.,the Antarctic, EastGreenland,and theLabradorallyfluctuateaboveandbelowfreezingfor several daysSea),decay isaccelerated in responseto wave erosionasbefore remainingbelowfreezing.Therefore,frostdegreedayaccumulations areinitiated on the firstday of the periodwell as warmerairand seatemperatures

ICE IN THE SEA 461 Various empirical formulae have been developed based on this premise. All appear to perform better under thin ice con￾ditions when the temperature gradient through the ice is linear, generally true for ice less than 100 centimeters thick. Differences in predicted thicknesses between models gener￾ally 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 ap￾proached 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 av￾erage of many years of observations from the Russian Arctic: where h is the ice thickness in centimeters for a given day and φ 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 tem￾perature 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° be￾low 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 usu￾ally 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 to￾pography. 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 tem￾perature rises above 0°C in early summer and the resulting freshwater forms puddles on the surface. These puddles ab￾sorb 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. Figure 3404b. Relationship between accumlated frost degree days (°C) and ice thickness (cm). h 2 + 8 50h = ∅

462ICEINTHESEA3405.SalinityOf Sea Icesity of the water in which it floats. Thus, if an iceberg ofdensity0.920floats inwaterofdensity1.028(correspondingto a salinityof35parts per thousand and a temperatureofSea ice forms first as salt-free crystals near the surface-1°C),89.5 percent of its mass will be below the surface.ofthesea.Astheprocesscontinues,thesecrvstalsarejoinedtogether and, as they do so, small quantities of brine areThe height to draft ratio for a blocky or tabular icebergtrapped withintheice.Onthe average,new ice15 centime-probablyvariesfairlycloselyabout 1:5.This averageratio wasters thick contains 5to10 parts of saltper thousand.Withcomputed for icebergs south of Newfoundland by consideringlowertemperatures,freezingtakesplacefaster.Withfasterdensityvalues andafewactual measurements,andbyseismicfreezing,agreateramountof salt is trapped inthe ice.meansatanumberoflocationsalongtheedgeoftheRossIceShelf near LittleAmerica Station.It was also substantiated byDepending upon the temperature, the trapped brine may eidensity measurements taken in a nearbyholedrilled throughtherfreeze or remain liquid,but because its densityis greaterthe 256-meter thick ice shelf. The height to draft ratios of ice-than that of the pure ice, it tends to settle down through the purebergs become significant when determining their drift.ice.As it does so, the icegradually freshens, becoming clearer,stronger, and more brittle. At an age of1 year, sea ice is suff-cientlyfreshthat itsmeltwater, iffound inpuddlesofsufficient3407.DriftOfSeaIcesize.andnotcontaminatedbysprayfromthesea.canbeusedtoreplenish the freshwater supply of a ship. However, ponds ofAlthough surface currents have some affect upon thesufficientsizetowatershipsareseldomfoundexceptiniceofdriftof packice,theprincipalfactoriswind.Dueto Corio-great age, and then much of the meltwater is from snow whichlis force,ice doesnotdrift in the direction of the wind, buthas accumulated onthesurfaceofthe ice.When seaicereachesvariesfromapproximately18toasmuchas 90ofromthisanageofabout2vearsvirtuallyallofthesalthasbeeneliminat-direction,depending upontheforceofthe surfacewindanded.Icebergs,havingformed from precipitation,containno salt,theicethickness.IntheNorthernHemisphere,thisdriftisanduncontaminatedmeltwaterobtainedfromthemisfreshto the right of the direction toward which thewind blows.The settling out of the brinegives sea ice a honeycomband in the Southern Hemisphere it is toward the left.Al-structure which greatly hastens its disintegration when thethough earlyinvestigators computed average angles oftemperaturerises abovefreezing.In this state,when itisapproximately 280or290for the drift of close multiyearcalledrottenice,muchmoresurface isexposedtowarmairpack ice,large drift angles were usually observed withlowandwater,andtherateofmeltingisincreased.Inaday'srather than high, wind speeds.The relationship betweentime,afloeofapparentlysolid ice several inchesthick maysurface wind speed, ice thickness, and drift angle was de-disappearcompletely.rived theoreticallyforthe drift of consolidated pack underequilibrium(abalanceofforcesactingontheice)condi-3406.DensityOf Icetions, and shows that the drift angle increases withincreasing icethickness and decreasing surface wind speedA slight increase also occurs with higher latitude.The density of freshwater ice at its freezing point is0.917gm/cm3.Newlyformed sea ice,due to its saltcontent,Sincethecross-isobar deflection of the surfacewindismoredense,0.925gm/cm3beinga representativevalueovertheoceans is approximately20°,thedeflection of theThe densitydecreases as the icefreshens.Bythe time it hasicevaries,fromapproximatelyalong the isobarstoasmuchshedmostofitssalt.seaiceislessdensethanfreshwateras7oototherightoftheisobars,withlowpressureontheice, because ice formed in the sea contains more air bub-left and high pressure onthe right in the Northern Hemi-bles.Icehavingnosaltbutcontainingairtotheextentof8sphere.The positions of the low and high pressure areas are,percentbyvolume(an approximatelymaximumvalueforof course,reversed in the Southern Hemisphere.seaice)hasadensityof0.845gm/cm3Therateof drift depends upon the roughness of theThe density of land icevaries over even wider limitssurface and the concentration of the ice. PercentagesThat formed by freezing of freshwaterhas a densityofvaryfromapproximately0.25percenttoalmost8per-0.917gm/cm3,asstatedabove.Muchofthelandice,howev-cent of thesurfacewindspeedasmeasureder, is formed by compacting of snow. This results in theapproximately 6 meters above the ice surface.Low con-entrappingofrelativelylarge quantities ofair.Neve,a snowcentrations of heavily ridged or hummocked floes driftwhichhasbecomecoarsegrainedandcompactthroughtem-fasterthanhighconcentrationsoflightlyridgedorhum-peraturechange,forming the transition stage to glacier icemockedfloeswith the samewind speed.Sea ice of 8tomayhaveanaircontentofasmuchas50percentbyvolume9 tenths concentrations and sixtenths hummockingorBy the time the ice of a glacier reaches the sea, its densityclose multiyear ice will drift at approximately2 percentapproachesthatoffreshwaterice.Asampletakenfromanof the surfacewind speed.Additionally,the responseicebergontheGrandBankshadadensityof0.899gm/cm3.factorsof1and5tenths ice concentrations,respectively,Whenicefloats,partofitisabovewaterandpartisbelowareapproximatelythreetimes and twicethemagnitudethesurface.Thepercentageofthemass belowthesurfacecanoftheresponsefactorfor9tenthsiceconcentrationswithbefound bydividingthe averagedensity oftheicebythe den-the same extent of surface roughness.Isolated icefloes

462 ICE IN THE SEA 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 centime￾ters 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 ei￾ther 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 suffi￾ciently 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 eliminat￾ed. 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 bub￾bles. 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, howev￾er, 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 tem￾perature 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 den￾sity 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 ice￾bergs 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 Corio￾lis 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. Al￾though 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 de￾rived theoretically for the drift of consolidated pack under equilibrium (a balance of forces acting on the ice) condi￾tions, 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 Hemi￾sphere. 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 per￾cent of the surface wind speed as measured approximately 6 meters above the ice surface. Low con￾centrations of heavily ridged or hummocked floes drift faster than high concentrations of lightly ridged or hum￾mocked 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

463ICEINTHESEA903085650080ASSUNPTION-EGUILIBRUNCONDITIONST5ANDS/BICECONCENTRATION000COMPUTED FOR 66*SO'LATITUDE70AFTERSHULEIKIN,I9SS(MOOIFIED)6scoLie'0ssICETHICKNESS CURVES505o(FEET)*34340403s3330305252020.1501On-40505220222426283032343638O1610041SURFACE WIND SPCED [KNOTS)Figure3407.Ice drift directionforvarying wind speed and ice thicknesshavebeen observed todrift as fast as 10percent to12per-riety of factors such as horizontal pressure gradients owingcentof strong surface winds.to density variations in the water, rotation ofthe earth,grav-itationalattractionofthemoon,and slopeofthe sea surface.The rates at which sea ice drifts have been quantifiedWind not only acts directly on an iceberg,but also indirect-through empirical observation, The drift angle, however,ly by generating waves and a surface current in about thehasbeendeterminedtheoreticallyfor10 tenths ice concen-same direction as the wind.Because of inertia, an icebergtration.This relationshippresentlyisextendedtothedriftofmay continuetomovefrom the influence ofwind for someall iceconcentrations,duetothelackofbasicknowledgeoftimeafterthe wind stops orchangesdirection.thedynamicforces thatactupon,andresult inredistributionof seaice, inthepolar regions.The relative influence of currents and winds on thedriftof an iceberg varies according to the direction and magnitude3408. Iceberg Driftof theforces acting on its sail areaand subsurface cross-sec-tional areaThe resultant force therefore involves theproportions ofthe iceberg above and below the sea surface inIcebergs extend a considerable distance below the sur-relation to the velocity and depth of the current, and the ve-face andhave relatively small sail areas"compared to theirlocityanddurationofthewind.Studiestendtoshowthat.subsurface mass.Therefore, the near-surface current isgenerally,wherestrong currents prevail,thecurrent is domi-thoughttobeprimarily responsiblefordrift; however, ob-nant. In regions of weak currents,however,winds that blowservations have shown that wind canbe thedominant forcefora number of hours ina steady direction materially affectthat governs iceberg drift at a particular location or time.the drift of icebergs.Generally, it can be stated that currentsAlso, the current and wind may contribute nearly equally totend to have a greater effect on deep-draft icebergs, whiletheresultantdriftwinds tend to have a greater effect on shallow-draft icebergs.Two other major forces which act on a drifting icebergAs icebergs waste through melting, erosion, and calv-are the Coriolis force and, to a lesser extent, the pressureing, observations indicate the height to draff ratio maygradient force which is caused by gravity owing to a tilt ofthe sea surface, and is important only for iceberg drift in aapproachl:1 during theirlast stageof decay,when they aremajor current. Near-surface currents are generated by a va-referred to as valley, winged, horned, or spired icebergs

ICE IN THE SEA 463 have been observed to drift as fast as 10 percent to 12 per￾cent 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 concen￾tration. 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 sur￾face 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, ob￾servations 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 va￾riety of factors such as horizontal pressure gradients owing to density variations in the water, rotation of the earth, grav￾itational attraction of the moon, and slope of the sea surface. Wind not only acts directly on an iceberg, but also indirect￾ly 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-sec￾tional 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 ve￾locity and duration of the wind. Studies tend to show that, generally, where strong currents prevail, the current is domi￾nant. 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 calv￾ing, 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. Figure 3407. Ice drift direction for varying wind speed and ice thickness

464ICE INTHE SEAThe heightto draft ratiosfound for icebergs in their variouscurrent,withthe angle increasingfrom approximately3°at10knots,to20°at30knots,and to73°at 60knots.As a lim-stages are presented in Table 3408a.Since wind tends tohaveagreater effect on shallowthanondeep-draft ice-iting case for increasing wind speeds, drift may beapproximatelynormal (to theright)to the winddirection.bergs, the wind can be expected to exert increasinginfluence on iceberg drift as wastage increases.This indicates that the wind generated current is clearlySimpleequations whichpreciselydefine iceberg driftdominating the drift. In general, the various models usedcannot be formulated at present because of the uncertaintydemonstrated that a combination of the wind and currentin the water and air drag coefficients associated with ice-was responsible for thedrift of icebergs.bergmotion.Valuesfor theseparameters not onlyvary3409.ExtentOf IceInTheSeafrom icebergto iceberg,butthey probably changefor thesame iceberg over its period of wastage.When an area of sea ice, no matter what form it takesPresent investigations utilize an analytical approachor how it is disposed, is described, it is referred to as packfacilitated by computer calculations, in which the air andice.Inbothpolarregions thepack iceis a verydynamicfea-water drag coefficients arevaried within reasonable limits.ture, with wide deviations in its extent dependent uponCombinations of these dragvalues arethen used in severalchanging oceanographic and meteorologicalphenomenaincreasingly complex watermodels that try toduplicate ob-In wintertheArctic pack extends over the entireArcticserved iceberg trajectories.The results indicate that with aOcean,and for a varyingdistance outward from it,thelim-wind-generatedcurrent, Coriolisforce,anda uniform wind,its recede considerably duringthe warmer summer months.but without a gradient current, small and medium icebergsThe average positions of the seasonal absolute and meanwill drift with thepercentages of the wind asgiven in Table3408b.The drift will be to the right in the Northern Hemi-maximumand minimumextents of sea icein theArctic re-gionareplotted inFigure3409a.Each year a largeportionsphere and to the left in the Southern Hemisphere.ofthe icefrom the Arctic Ocean movesoutward betweenWhen gradient currents are introduced, trajectoriesGreenland and Spitsbergen (Fram Strait) into the North At-vary considerably depending on themagnitudeof the windlanticOcean and isreplaced bynew ice.Becauseof thisand current, and whetherthey are in the same or oppositedirection. When a 1-knot current and wind are in the sameconstant annual removal and replacement of sea ice,rela-tivelylittle of theArcticpack ice is more than10yearsolddirection,drift is to the right ofboth wind and current withdriftangles increasing linearlyfromapproximately5°at10Icecoversa largeportion of theAntarctic waters and isprobably thegreatest singlefactorcontributingto the isola-knots to22°at 60knots.When the wind and a1-knot cur-rent are inopposite directions, drift isto the leftofthetion ofthe Antarctic Continent.Duringthe austral winterIcebergtypeHeight to draft ratio1:5Blocky or tabular1:4Rounded or domed1Picturesque or Greenland (sloping)Pinnacled or ridged1:1Horned, winged, valley, or spired (weathered)Table3408a.Heighttodraft ratios for various types of icebergsWind Speed (knots)IceSpeed/WindSpeed (percent)Drifi Angle (degrees)Small BergMed. BergSmall BergMed. Berg1023333.612853333203.814798.344.14.450344.53.6604.93.7Table3408b.Drift of icebergaspercentageofwindspeed

464 ICE IN THE SEA 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 ice￾bergs, 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 ice￾berg 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 ob￾served 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 Hemi￾sphere 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 cur￾rent 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 lim￾iting 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 fea￾ture, 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 lim￾its 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 re￾gion 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 At￾lantic Ocean and is replaced by new ice. Because of this constant annual removal and replacement of sea ice, rela￾tively 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 isola￾tion of the Antarctic Continent. During the austral winter Iceberg type Height to draft ratio Blocky or tabular 1:5 Rounded or domed 1:4 Picturesque or Greenland (sloping) 1:3 Pinnacled or ridged 1:2 Horned, winged, valley, or spired (weathered) 1:1 Table 3408a. Height to draft ratios for various types of icebergs. Wind Speed (knots) Ice Speed/Wind Speed (percent) Drift Angle (degrees) Small Berg Med. Berg Small Berg Med. Berg 10 3.6 2.2 12 69 20 3.8 3.1 14 55 30 4.1 3.4 17 36 40 4.4 3.5 19 33 50 4.5 3.6 23 32 60 4.9 3.7 24 31 Table 3408b. Drift of iceberg as percentage of wind speed