34 PHYSICAL PROPERTIES OF SEA WATER 



the temperature rises, no matter how low it may be. However, the 

 amount of heat needed for melting sea ice of a given temperature and 

 salinity can be stated. Thus, 68 g cal are needed for melting sea ice of a 

 temperature of —2° and a salinity of 6°/oo, and 55 g cal are needed for 

 melting sea ice of a temperature of — 1° and the same salinity. 



The latent heat of evaporation of sea ice probably equals that of pure 

 ice and depends upon whether the ice volatilizes directly to vapor or 

 melts first. In the former case the latent heat of evaporation is about 

 600 g cal/g, and in the latter case it is about 700 g cal/g. The vapor 

 pressure over sea ice cannot depart much from that over pure ice, which 

 is given in meteorological tables. 



Pure ice contracts when cooled, but sea ice may expand, depending 

 upon its temperature and salinity, because, when the temperature is 

 lowered, more ice is formed, and because the transformation of brine into 

 ice is accompanied by expansion. Thus, for sea ice of salinity 6°/oo the 

 coefficient of thermal expansion at —2° is equal to —69.67 X 10~^, at 

 -12° it equals 0.00, and at -22° it equals 0.93 X 10-1 The negative 

 sign indicates that the ice expands when the temperature is lowered. 



The coefficient of thermal conductivity of pure ice is about 5 X 10~^. 

 For sea ice the thermal conductivity is in general decreased, owing to the 

 presence of air bubbles in the ice. Values ranging between 1.5 X 10~^ 

 and 5 X 10~^ have been recorded. 



The absorption of radiation in sea ice has not been examined, but from 

 general experience it can be stated that sea ice has a high transparency for 

 visible radiation. The temperature radiation of sea ice nearly equals that 

 of a black body. 



The albedo of sea ice — that is, the fraction of incoming short-wave 

 radiation which is reflected from the surface of the ice — varies with the 

 character of the surface. For ordinary grayish sea ice the albedo is 

 about 0.50, but for sea ice covered by rime it may be as high as 0.80. 



