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 CHAPTER I 



FORMATION AND GROWTH OF SEA ICE 



An understanding of the formation, growth, and decay of sea ice 

 is desirable for comprehension of many of the problems in ice seaman- 

 ship. The climatic factors bearing on the formation of ice naturally 

 vary from place to place and from season to season. However, a 

 knowledge of the basic physics involved will be of great assistance to 

 the mariner, enabling him to recognize certain salient features of ice 

 and take advantage of its properties. 



CAUSE OF FREEZING 



In temperate and tropical latitudes, the ocean acts as a storehouse 

 of radiant heat from the sun. The visible and infra-red wave lengths 

 are largely absorbed in the surface layers, and the heat so stored is 

 given off to the air at night and at other periods w^hen the air is 

 colder than the sea surface. In higher latitudes, however, as the 

 nights begin to grow longer in the autumn, insufficient heat is stored 

 in the short daylight period to compensate for the losses at night, and 

 the temperature of the surface waters is therefore lowered. As the 

 season progresses, the altitude of the sun becomes lower day by day ; 

 less radiation is received, and more is reflected from the sea surface 

 owing to the low^ angle of incidence of the rays. Finally, the water 

 reaches the freezing point and further loss of heat results in the 

 formation of ice. 



Conditions then become even less favorable for the retention of 

 radiant heat from the sun since, as will be discussed more fully in 

 a later chapter, ice reflects much more of the visible radiation than 

 does water. Cooling of the air in contact with the ice is accelerated, 

 and as this cold air spreads, more ice is formed. 



INFLUENCE OF SALINITY 



Fresh water freezes at 32.0° F,, but the salt present in sea water 

 causes it to remain liquid until a lower temperature is reached. The 

 greater the salinity, the lower the freezing point. Ordinary sea 

 water, with a salinity of 35%o (35 parts per 1,000), does not begin 

 to freeze until it has been cooled to 28.6° F. 



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