Where W^ is the wind speed at the earth's surface, c is the speed of the wind induced drift 

 of the ice. 



On this assumption, I obtained 



_ 0.01 dp 



~ 2 pci) sin 9 dx ' (3) 



and, further, considering the air density up to the height of the geostrophic wind to be constant and 

 equal to 0.0013, Somov and I obtained the following for the Arctic Basin 



c=13.000^, (4) 



Where c is the average drift of ice fields in kilometers per month; dp/dx is the pressure 

 gradient expressed in millibars per kilometer and taken from the monthly pressure map. 



These considerations led me to formulate one more simple rule, namely: over the deep sea, 

 far from the distorting influence of the coast, the speed of pure wind drift of close Ice is directly 

 proportional to the atmospheric pressure gradient and thus it is inversely proportional to the dis- 

 tance between the isobars drawn through the same pressure intervals. 



It seems to me that my laws of the relationship between ice drift, the direction of the isobars 

 and the magnitude of the pressure gradient are a considerable step forward compared with the 

 relationships between ice drift and wind which have been studied heretofore. Of course, in the 

 cycle — pressure gradient creates wind, wind creates drift — it is most difficult and controversial to 

 determine the wind elements (direction and speed), values which change greatly depending on the 

 height at which the observation is made above the ice field. The laws which I have proposed exclude 

 the wind elements which are difficult to determine from the examination and allow one to concentrate 

 on the study of the departures from these rules caused by local conditions. 



UTERATURE: 19, 67, 70, 72, 77, 135. 



Section 137. Comparison of the Computed end Observed Drifts 



As we have seen, the coefficient 13, 000 in formula (4) of section 136 is highly arbitrary and 

 calculations have shown that it is too high. The only way to make it more exact is to compare 

 drifts computed from formula (4) with account taken of the steady current, and the observed drifts. 



Such calculations were made at my request by Somov for the drifting station "North Pole", 

 the icebreaker Sedov and the ship Lenin from the moment their drift began. 



Figure 135 shows the results of the computations. The dashed line indicates the computed 

 drift, the solid line the true drift, and for convenience in comparison the true drift is also shown 

 as total movements for the month. The computed drift was based on mean monthly pressure maps 

 for the northern hemisphere compiled by the Central Weather Institute (1938 and 1939) and by the 

 Interdepartmental Bureau of Ice Forecasting (for 1937). The direction of the isobars was taken 

 from the pressure chart with accuracy up to 10 per cent. 



It should be noted that the isobar plots on the mean monthly pressure charts are very 

 approximate, especially in the central part of the Arctic Basin. Nevertheless, the calculated and 

 observed drifts of the Sedov agree well, despite the unavoidable accumulation of errors inlierent 

 in sequential computation of a theoretical drift. This agreement of the results of the observed two 



373 



