100 110 m m M m m w 



700 100 1400 2100 fl 

 11 (1 



Figure 132. Distribution of atmospheric pressure over the Arctic Basin 

 for the first ten days of December 1938. 



The rule about the direction of the ice drift along the isobars, which I developed empirically, 

 also has a theoretical basis. 



We know that at heights of 500 to 1, 000 m above the earth's surface, as indicated by direct 

 observations, the wind direction coincides with the isobars (geostrophic wind). On approaching the 

 earth's surface, the wind approximates more and more the pressure gradient because of friction 

 with the earth's surface. 



Theoretically, at the earth's surface the angle between the wind and the gradient is 45°. 

 Consequently, if the theoretical calculations are valid, clearly the theoretically pure wind surface 

 current and the pure wind drift of the ice should pass along the isobars, because the pure wind 

 surface current, according to Ekman, does not depend on latitude and deviates 45° to the right of 

 the wind direction at the earth's surface in the northern hemisphere, while the wind direction at 

 the earth's surface, according to Taylor, deviates 45° to the left of the isobars. 



Hesselberg conducted painstaking theoretical and empirical investigations of the deviation of 

 the wind from the isobars. He pointed out that movement of the pressure systems will cause 

 variations of the drift angle, depending on the position of the observation point witli respect to the 

 center of the given pressure system. Table 104 gives some results of Hesselberg's investigations. 



369 



