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it can be supplied from the deeper layers through convection cur- 

 rents, so that ice will form on the surface before the deeper layers 

 have approached the freezing point. Salinity gradients in the sea 

 may also diminish the thermal convection currents. If, because of 

 discharge from rivers or melting of ice, the top layers have a lower 

 salinity, the difference of density may be so great that the surface 

 layer, although cooled to the freezing point, will be too light to sink 

 below the warmer but more saline water underneath. 



A practical outcome of the foregoing is that if a body of water 

 originally of uniform density is losing heat at the surface, ice will 

 be formed most readily in fresh water, less readily in sea water of 

 low salinit}^, and least readily in sea water of high salinity. The 

 greater heat removal required to freeze sea water is due not only to 

 its relatively low freezing point, but also to the increased tendency of 

 the cooled surface water to sink as the temperature of maximum 

 density decreases. 



THE GROWING PROCESS 



On account of its fairly high specific heat and low thermal con- 

 ductivity, water loses heat slowly, so that the surface temperature of 

 a large body of water will lag behind the rise and fall of the mean 

 air temperature. In the Murmansk-White Sea area (lat. 65° to 70° 

 N.), rivers usually freeze about 3 weeks after the mean air tempera- 

 ture falls below 32° F. This phenomenon is probably representative 

 of many similar regions. 



Ice forms first in shallow water, near the coast or over shoals and 

 banks, particularly in bays, inlets, and straits in which there is no 

 current, and in regions with reduced salinity, such as those near the 

 mouths of rivers. It spreads from these areas as centers. Such ice, 

 broken up and carried seaward by winds or currents, starts further 

 ice formation in deeper water, where floating ice that has not melted 

 during the previous season also acts in the same way. Wave action 

 ordinarily hinders the formation of ice to some extent by mixing the 

 waters of the upper layers. Old ice damps sea or swell and, at the 

 same time, by cooling and freshening the water and providing nuclei 

 of ice crystals, assists the beginning of the freezing process. Quickly 

 recurring fresh winds with raised sea will hinder ice formation, 

 breaking it up several times. The greater the depth, with water of 

 salinity greater than 24.7%o, the later is the time of freezing. As a 

 matter of fact, complete freezing may never occur, as in the case of 

 the central part of the White Sea ; hence the necessity for following 



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