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the deep-water route in order to reach high latitudes during the season 

 of ice formation. 



The first sign of freezing is an oily or opaque appearance of the 

 water, due to the formation of needle-like spicules and thin plates of 

 ice about one-third of an inch across, known as frazil crystals. These 

 consist of fresh ice, free of salt, and increase in number until the 

 sea is covered by slush of a thick, soupy consistency. 



Snow, falling into water, aids freezing by cooling and by providing 

 nuclei for ice crystals. Except in sheltered waters, an even sheet of 

 ice seldom forms immediately ; the slush, as it thickens, breaks up into 

 separate masses and frequently into the characteristic pancake form, 

 the rounded shape and raised rim of which is due to the fragments 

 colliding with each other. The formation of slush damps down sea 

 or swell, and if the low temperature continues, the pancakes adhere 

 to each other, forming a continuous sheet. 



RATE OF GROWTH 



Sea ice may grow to a thickness of 3 to -1 inches in the first 24 

 hours, and from 2 to 3 more in the second 24 hours. Ice is a poor 

 conductor of heat and the rate of its formation drops appreciably 

 after the first 4 to 6 inches have formed; a snow cover, if present, 

 still further reduces the conductivity, as shown in figure 2, Once 

 a layer of ice is formed, snow falling on the surface retards growth 



TEMPERATURE - DEGREES FAHRENHEIT 



Am 



SNOW 



ICE 



WATER 



THIN ICE 



THICK ICE 



SNOW-COVERED ICE 



Figure 2. — Idealized diagrams illustrating the distribution of temperature with air at 0° F. 

 and sea water at its freezing point of 30° F. The rate at which heat is conducted through 

 the ice from the water to the air (which, neglecting radiation, is proportional to the rate of 

 freezing of the water or the rate at which the ice increases in thickness) is proportional to 

 the slope of the thermal gradient in the ice away from the vertical. 



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