

UJ 



u 



Figure 79. Accretion of perennial ice. 



the lower surface of the ice , of the heat of crystallization released during the formation of an 

 additional layer of ice of thickness ab. 



The heat absorbed from the atmosphere, however, is used not only to melt a layer of ice ac 

 thick and to raise the temperature within the ice, but also for processes within the ice (fusion) 

 which is reflected by the fact that the upper layers of the ice become completely saturated with 

 water by the end of summer. The importance of this process is clear from the following obser- 

 vations made on the Sed ov. In 1939, the mean daily air temperatures were negative by the 

 beginning of September, but the winter increase of ice thickness did not begin until 20 November 

 after the ice had frozen through. 



It can be seen from table 75 that 1335 freezing degree -days were required before new ice 

 could form in the region of drift of the Sedov , 



If we assume that the amounts of summer ice accretion and melting are constant in the given 

 region, and that the number of free'^ng degree-days is also constant, from formula (6) we get 



, _4(/? — /?') , A/ 



A/ 



25, 



(7) 



where AI = i ' - i " is the total change in the thickness of the ice during the summer and R ' is 

 the number of freezing degree-days necessary for the ice to freeze through. 



From formula (7), it follows that the effect of the summer saturation of ice by water is 

 equivalent to a decrease of the average number of freezing degree-days in a given area, and if 

 this decrease is known, it is not difficult to obtain the maximum ice thickness from both formula 

 (6) and the graph in figure 72 . 



228 



