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METEOROLOGY FOR MARINERS

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Formation of Ice

Fresh water and salt watcr do not frcczc in the same manner. This is due to the presencc of dissolved salts in the sea water. The salt content, or the salinity as it is known, is usually cxprcsscd in parts per thousand: sea water, typically, has a salinity of 35 parts per thousand though in some areas, cspecially where therc is a considerablc dischargc of rivcr water, the salinity is much less. In the Baltic, for cxamplc, the salinity of the surface water is less than 10 parts per thousand throughout the ycar.

Whcn considering the frcczing process, the importancc of salinity lics not only in its direct effcct in lowcring the freezing temperaturę, but also in its cffcct on the density of the water. The loss of heat from a body of water takes place principally from its surface to the air. As the surface water cools it bccomes morę dense and sinks, being replaccd by warmer, less dense water from below which in tum is coolcd, continuing the proccss of overturning (or convection as it is morę commonly known). The maximum density of fresh water occurs at a temperaturę of 4°c; thus, whcn a body of fresh water is cooled to this temperaturę throughout its depth convcction ccascs, sińce further cooling rcsults in a slight dccrcasc in density. Once this stable condition has bccn reached, cooling of the surface water leads to a rapid drop in temperaturę and ice begins to form when the temperaturę falls to o°C.

With salt water the dclay, due to convcction, in the lowcring of the temperaturę of the water to its freezing point is much morę prolongcd. In some areas where therc is an abundant supply of relativcly waran water at depth, for example off south-west Svalbard, convcction may normally prcvcnt the formation of ice throughout the entire winter despite vcry Iow air temperatures. This delay is, in part, due to the considerablc dcpths of water found in the oceans, but it is mainly due to the fact that the density of salt water continucs to inerease, with cooling, until the surface water frcczcs. In fact the thcorctical maximum density of sea water of average salinity (which can be achieved by supercooling in controlled laboratory conditions) occurs at a temperaturę which is well below its freezing point. Figurę 17.1 shows the relationship bctwccn freezing point, salinity and temperaturę at maximum density.

It can be scen that in water with salinity of less than 24.7 parts per thousand the maximum density is reached bcforc the freezing temperaturę and where the salinity is greater than 24*7 parts per thousand the freezing point is reached before the density attains its theorctical maximum valuc.

The greatest delay in reaching the frcczing temperaturę occurs when the sea water, throughout its depth, is initially at an almost uniform density. In some areas, howevcr, the density profile is not uniform. In thcsc cascs, discontinuities occur where a laycr of lowcr salinity ovcrlies a layer of higher salinity. (At temperatures betwccn 3°c and freezing, variations in density are morę dependent on variations of salinity than on changcs in temperaturę). The incrcascd density at the surface of the upper layer, achievcd by cooling, may still be less than the density of the lowcr layer. The salinity discontinuity bctwccn the two layers then forms a lower limit to convcction and the delay in reaching the freezing temperaturę is then dependent upon the depth of the upper layer. This is particularly so in the Arctic Basin where therc is a salinity discontinuity bctwcen the surface laycr (the Arctic water) and the undcrlying morę salinę Atlantic water. Cooling of the surface water around the periphery of the Basin, and within regions of open water, leads to convcction in a shallow layer which may cxtend to only 50 metres in depth.

FORMA TION AND CHARACTER OF ICE

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The first indication of ice is the appcarance of ice spicules or plates, with maximum dimcnsions up to 2*5 centimetrcs, in the top fcw ccntimetres of the water. These spicules, known as frazil ice, form in large quantities and give the sea an oily appcarance. As cooling continucs the frazil crystals coalcsce to form grease ice which has a matt appcarance. Undcr ncar-frcezing, but as yet ice-free conditions, snów falling on the surface and forming slush may induce the sea surface to form a laycr of ice. These forms may break up under the action of wind and waves to form siiuga. Frazil ice, grease ice, slush and shuga are classificd as new ice. With further cooling, sheets of ice rind or nilas are formed, depending on the ratę of cooling and on the salinity of the water. Ice rind is formed whcn water of Iow salinity freczcs slowly, rcsulting in a thin layer of ice which is almost free of salt, whereas when water of high salinity freczcs, cxpccially if the proccss is rapid, the ice contains pockcts of salt water giving it an elastic property which is characteristic of nilas. This lattcr form of ice is sub-divided, according to thickness, into dark nilas (less than 5 cm thick) and light nilas (5-10 cm thick).

Oncc again the action of wind and wavcs may break up ice rind and nilas into pancake ice which latcr freczcs together and thickens into grey ice and grey-white ice, the lattcr attaining thickncsscs up to 30 centimetrcs. These forms of ice are referred to as young ice. Rough wcathcr may break this ice up into ice cakes or floes.