4. Ice, even in very thin discs, is completely nontransparent to long-wave rays. In this 

 respect, ice is similar to glass, and a thin ice cover develops a "green house" effect, analagous 

 to the same effect of glass. 



This fact was noted by Melloni who as far back as 1832 showed that a plate of ice 2,6 mm 

 thick passes 6 per cent of the energy incident on it from a source whose temperature was 1200°, 

 and passes no radiant energy from a source fhaving a temperature of 100° . 



The "green house" effect of ice is a very important factor. Due to it, ice in thin discs not 

 only preserves the layers of water lying beneath it from cooling but with a sufficient intensifica- 

 tion of illumination, it even assists in gradually warming them. 



As for the penetrability of sea ice to the total flow of radiation, Chernigovskii presents the 

 average results of corresponding measurements inTiksi Bay (table 54). 



TABLE 54. THE AVERAGE PENETRABILITY OF SEA ICE BY TOTAL FLOW 

 OF RADIATION IN PER CENT FROM 19 APRIL THROUGH 4 MAY 

 1937 (TIKSI BAY) 



Chernigovskii also presents data on the penetrability of the snow cover for radiation from 

 which it can be seen that not more than 10 per cent of the radiation passes through a layer of snow 5 

 cm thick, not more than 5 per cent through a layer of snow 10 cm thick, and not more than 1 per 

 cent thi'ough a layer of snow 15 cm thick. 



During intensive melting, however, when icing of the snow mass occurs simultaneously, the 

 penetrability of the snow cover is considerably increased. Thus, according to the observations 

 made at the end of June, 1935 on Uedineniie Island 23 to 30 per cent of the radiation penetrated 

 through snow 5 cm thick, and 6 to 13 per cent was retained in the snow, 26 per cent of the radia- 

 tion penetrated through a layer 10 cm thick, and 11 per cent was absorbed; through a 16 cm layer, 

 7 per cent of the radiation penetrated. Other polar stations also obtained amounts on the same 

 order . 



LITERATURE: 62, 73, 80, 137. 



Section 70. Illumination of the Sea Under the Ice 



The question of under-ice illumination has a dual significance. First, this illumination 

 determines the biological productivity of the sea which depends on the photo synthetic activity of 

 plants. Actually, if the sea is covered by a solid ice cover, and, therefore, is isolated from a 

 direct exchange with the atmosphere, the only source for covering the expenditure of oxygen is 



177 



