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 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. 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 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 (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 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
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-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. 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 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 anothFigure 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 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 thick-