Chapter VIII 



Ice in the Sea 



Extensive icefields cover the polar seas. The outer boundaries where the ice borders 

 upon the warmer surrounding waters of lower latitudes are subjected to a constant 

 change due to the freezing and melting process. They may take a wide variety of forms 

 depending on the given external conditions. A plastic and lively description of the 

 magic of the polar ice world has been given by Weyprecht (1879). Besides the so- 

 called sea ice, formed by the freezing of sea-water, other floating ice is introduced to 

 the sea from the neighbouring land by the great rivers (river ice), and in addition 

 icebergs from the glaciers reach the sea. Floating river ice is comparatively unim- 

 portant, except in coastal Siberian and North American waters, therefore; sea ice 

 and icebergs dominate ice conditions in the Arctic and the Antarctic, and may be 

 carried by ocean currents to warmer oceanic regions. This ice drift prolongs the 

 existence of the winter ice barrier in the polar regions into spring and summer, and is 

 thus of considerable importance for navigation. 



1. Formation and Terminology' of Sea Ice 



Ice crystals are formed in the water either on crystallization nuclei, which are the 

 smallest possible particles of organic or inorganic origin that are always present, or at 

 an aggregation of several molecules which meet each other grouped more or less by 

 chance giving a configuration favourable for crystal formation (Nernst, 1909). It 

 appears that the triplex molecules are decisively engaged in the first phase of ice 

 formation. Besides the crystallization nuclei, supercooling of the water is also necessary. 

 The greater the purity of the water and the less disturbed it is, the more supercooling 

 is needed. In natural waters there are always sufiicient crystalhzation nuclei present, 

 and the water is usually in movement so that a very small degree of supercooling of 

 only some hundredths of a degree Celsius is required to initiate ice formation. How- 

 ever, supercooling has to be continuous for the formation of ice crystals. 



Since the formation of ice releases a latent heat of 80 g cal/g, for a change of water 

 into ice heat must be continually removed by an amount greater than the latent heat. 

 The more intensive the cooling and the less disturbed the water, the smaller are the 

 ice crystals so formed, which show a needle-like structure. If the water is in movement 

 then the forming ice particles lose this needle-like character, looking then like 

 flat plates with irregular rounded edges, about 2-A cm long, 0-5-1 cm wide and 

 0-1-1 mm thick. They usually accumulate and form muddy clumps. 



The dependence of the freezing point on the salinity is discussed on p. 45. Only 

 pure water is involved in the actual freezing process. Part of the salt content of the water 

 is separated during the formation of the ice and, as a more or less concentrated salt 

 solution, fills the small separating layers between the ice crystals which themselves 



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