These examples show that formulas (6) and (10) could be extended to the entire Arctic Basin 

 with a certain approximation. This is very important since it is a very laborious task to deter- 

 mine ice thickness under polar conditions and it is very easy to compute the number of freezing 

 degree-days. * 



It should merely be emphasized that formula (6) is applicable only under more or less calm 

 hydrological conditions. For example, the ice in river rapids is always thinner than in calm 

 stretches, and in certain rivers the rapids sometimes do not freeze at all during the entire winter. 

 Such phenomena can also be observed in transverse ice survey profiles. The ice is always thicker 

 at some points than at others. This is explained by the alluvial and erosional action of the current, 

 which causes accretion and erosion of the ice along the lower surface of the ice cover in a manner 

 quite similar to and according to the laws by which these same streams distribute suspended soil 

 particles on the bottom of the river. 



Figure 73 shows the ice survey profile through the Severnaya Dvina near Solombala, made 

 on 9 December 1941. In the figure, attention is focused on the rafted floes near the shores and 

 near the path made by the icebreaker and an accumulation of sludge ice 400 m from the right bank 

 which had no apparent cause. Undoubtedly, such an accumulation was caused by a convergence of 

 current streams at this point, and, as a result, by a certain deceleration. 



.AXIS OF CHANNEL MADE BY 

 /^THE ICEBREAKER 



SLUDGE 



Figure 73 . Ice survey profile through the Severnaya Dvina . 



These same phenomena are observed at sea in areas of strong tidal currents. For example, 

 in the narrowest part of Matochkin-Shar, near Cape Uzkii (Mys Uzkii), where the tidal currents 

 reach a speed of five knots, the middle part of the strait freezes only during the most severe 

 winters. It is clear that in such cases, special "local" formulas should be worked out for the 

 relationship between ice thickness and the number of freezing degree-days. Furthermore, it is 

 clear that, in general, in order to apply formula (6), freezing degree-days should be counted from 

 the moment ice appears in a given region and never from the initial moment of negative air 

 temperatures. 



The number of freezing degree-days varies within wide limits from year to year at each 

 separate point of the arctic, and of course this affects the thickness of the ice formed during a 

 given winter in a given region. For example, during the period 1921 to 1936, the maximum num- 

 ber of freezing degree-days on Dickson Island (Ostrov Dikson) was 4, 780 (in 1927-1928) and the 

 minimum was 3, 595 (in 1931-1932); the difference was about 25 per cent of the maximum value. 



*These formulas were checked by the Hydrometeorological Institute for the White and 

 Caspian Seas and were found to be completely satisfactory. 



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