WIND VELOCITY AND SURFACE PRESSURE GOLD 121 



ient of pressure would be as above, except that the effective gradient 

 would be the maximum gradient multiplied by the cosine of a, the 

 angle between the path and the isobars. The corresponding velocity- 

 would be approximately v cos a, except in cases of considerable 

 curvature. In^the majority of the observations the curvature was 

 small, and we should therefore expect the surface wind to be nearly 

 v cos a, so that the numbers in Column 8 would be nearly unity. 

 This is far from being the case; but the change of the station of 

 observation from Berlin to Lindenberg is accompanied by a cor 

 responding change in the ratio of the surface wind velocity to v cos a. 



This suggests that the effect of the surface, apart from the purely 

 frictional effect, is to reduce the velocity in a given direction in a 

 constant ratio depending on the locality, and that departures in the 

 observed velocities from those corresponding to this ratio are to 

 be associated with unsteady meteorological conditions. 



The last column 1 gives approximately the ratio of the volume of 

 air crossing the isobars at the surface to the volume crossing at 

 iooo meters. 



The ratio appears to be nearly constant ; the change in December 

 is probably due to the exceptional conditions which prevailed during 

 part of the month, when the air was considerably warmer at iooo 

 meters altitude than at the surface. 



'Or, the wind component, perpendicular to the isobars at sea level divided 

 by the analogous component at iooo meters. 



