254 



Ice in the Sea 



Deeper in the ice the thermal conductivity approaches a limiting value of 5-0 x 10~^ 

 which corresponds to the value obtained for clear freshwater ice without air bubbles. 

 Malmgren's determination of the physical constants of sea ice are of considerable 

 importance in questions of the heat balance in polar regions, since they allow the de- 

 termination of the amount of heat gained by the surface of the ice in polar regions and 



% 

 o 



I 2 



Fig. 113. Changes in thermal conductivity in sea ice with depth. 



thus also by the atmosphere immediately above it from the water below. For the greater 

 part of the year the water underneath is warmer than the ice cover and the air above it, 

 and therefore there is a continuous flux of heat upwards. Such a calculation can be 

 made with the temperature observations of the "Maud" over a period of a year. 

 The total amount of heat passing through the different depth-levels in a year amounts 

 on the average to 6800 g cal/cm^ and should be the same for all levels. This amount 

 of heat is released to the atmosphere above the ice year after year. In the cold season 

 of the year when the temperature gradient is several times larger this flux of heat is 

 greater; in summer it may even be reversed but is then never very large. Taking a 

 depth of 0-75 m as representative for the entire layer of ice, the amount of heat, 

 W^o-75 passing through this level per cm^ and month can be calculated, knowing the 

 temperature gradient for each month during the colder season of the year. Part of this 

 heat serves to raise the temperature of the 0-75 m thick surface layer. If the tempera- 

 ture difference between the beginning and the end of the month is Jr then the heat 

 gained by the atmosphere during that month is 



W, 



= f^o-75 - IScaAr = fFo.75 - 34-4 J T. 



The values calculated by Malmgren using this equation for the months from Septem- 

 ber 1923 to April 1924 show that during the cold season of the year the atmosphere 

 receives the very large amount of 76,700 kg cal/cm^, which is sufiicient to melt 96 cm 

 of ice. However, large as this may appear, it is only a ninth part of the heat that the 

 European Mediterranean, for example, provides to the atmosphere (676,000 kg 

 cal/m^). However, in the polar regions its eff'ect is none the less still important. During 

 the cold part of the year there is a thin layer of cold air over the Polar Sea, extending 

 to a height of about 150 m (Sverdrup, 1926). This layer of air has such a stable 

 stratification that it mixes only to a very small extent with the air above. The heat from 

 below is thus imparted almost entirely to this layer and prevents a decrease of the 



