the isobar, calculated as the arlthematic mean of 317 observations conducted on board ship, was 

 28° to the left of the isobar. 



Let us recall that if the steady current is excluded, the mean drift angle of the ice, according 

 to Nansen is 28° to the right, while I found an average drift angle (also excluding the steady cur- 

 rent) of 29° in my analysis of the drift of the Sedou. Thus the Arctic Institute confirmed my 

 assertions. * 



It should be emphasized that the movement of ice along the isobars (in the general case, 

 moving in space) means that at each given moment the actual drift of ice at each point in the sea is 

 tangential to the isobar passing through the given point. Consequently, at each given moment the 

 isobars are the flow lines of the ice drift. 



LITERATURE: 64, 70, 72, 77, 84, 156. 



Section 136. The Rate of Drift Along tfie Isobars 



In 1938, after establishing the law of ice movement along the isobars, I raised the question 

 of the relationship between the drift of ice and the pressure gradient. 



We know that in tlie case of a geostrophic wind the pressure gradient is balanced by the 

 Coriolis force: 



Where w is the angular velocity of the earth's rotation, W is the geostrophic wind velocity, (p the 

 geographic latitude, p^ the air density and dp/dx the horizontal gradient of atmospheric pressure. 



From formula (1) we get 



1 dp 



W=-- : /• (2) 



2 cop sin 9 dx 



If we know the conversion factor for calculating the geostrophic wind in terms of wind at the 

 earth's surface, formula (2) would allow us to calculate the wind drift speed from ordinary pressure 

 maps. 



However, the question of the wind speed at the earth's surface is a very complex problem. 



According to Brent, the wind speed at the earth's surface is about 0. 7 in the case of weak 

 winds and 0. 6 of the geostrophic value in the case of strong winds. As Khromov has observed, 

 the aerological observations in northern Europe have shown that the wind speed at the earth's sur- 

 face is 0. 46-0. 48 of its speed at 1, 000 m. Finally, as we have seen from Efremov's observations, 

 in the layer closest to the surface of the ice, the wind speed changes so much that in the final 

 analysis doubts arise as to just which wind speed should be considered. 



As a first approximation I calculated that 



Wo = 0.5 W, 

 c = 0.02 Wo, 



*As Gordienko has reported to me, the drift of ice along the isobars has been confirmed by 

 numerous instrument observations in the Chuckchee Sea. 



372 



