1916] 



on Illusions of the Upper Air 



611 



is due to the fact that the charted winds show the air at the 

 surface crossing the isobars from high to low ; the observations 

 with pilot-balloons suggest that the effect is peculiar to the surface. 

 If the driving force from high to low were the operative force 

 which produces the wind of a cyclonic depression we should expect 

 to find its operation more strongly marked as we get higher up, 

 because the friction of the surface would not interfere with it, but 

 the fact is quite otherwise. The movement across isobars becomes 

 less and less marked as we ascend : it is relatively much less at 

 Pendennis Castle than it is at Falmouth Observatory a mile away. 

 We cannot be sure that it exists at all at 1500 feet, because we cannot 

 draw the isobars at that level with the necessary accuracy ; the 

 consensus of our observations goes to show that there is no real 

 evidence of convergence at that level. There the centrifugal force 

 of the air travelling over the moving earth, combined with the 



Fig. 3. — Changes in Velocity (V), Pressure (P) and Temperature (T) 

 of the air moving along the path marked E, of Fig. 2. 



centrifugal force due to the curvature of the air's path, are sufficient 

 to balance the force due to pressure, and there is no component of 

 motion towards the centre.* 



What happens nearer the surface is that the friction of the 

 surface converts part of the energy of the motion of the wind 

 into eddy motion, and the air does not move fast enough on the 

 right path to keep up the balance. Consequently it drifts inwards 

 as a pendulum does w^hen its motion is retarded, .but the lower air 

 cannot hold back the air far above it ; the effect of viscosity in that 

 direction was shown by Helmholtz to be negligible. The effect of 

 eddy-motion is very limited in height. 



* See the four reports on wind structure to the Advisory Committee for 

 Aeronautics by W. N. Shaw and J. S. Dines. 



2 s 2 



