1916] on Illusions of the Upper Air 609 



driving* force, whatever be the area covered by the circulation. The 

 observations of the upper air have made us famiUar with certain facts 

 about the height of the atmosphere that make such an idea too 

 improbable. The convective atmosphere is only about 10 kilometres 

 thick. The region in which convection can operate is therefore a 

 thin skin represented by a centimetre in the case of a map on the 

 millionth scale, on which 1000 miles is about ~ 6 feet in length. A 

 cyclone is often regarded as a towering structure which may produce 

 curious effects by tilting its axis, but that is clearly illusory ; the idea 

 that descending air over Xorthern France is operating in conjunction 

 with rising air over Iceland to produce a flow of air along the line 

 joining them is an unproductive way of representing the facts. 



The idea of the ordinary cyclones and anticyclones in our 

 latitudes as foci of centripetal and centrifugal motion is an illusion. 

 In all ordinary cases of cyclone the convergence of the paths of air 

 towards the centre is itself an illusion, because the motion of the 

 cyclone makes them miss their apparent aim, and we get in actual 

 fact paradoxical cases of air which, always seeking a place of 

 lower pressure, yet makes its way to a place of higher pressure, 

 because the pressure has been raised over its path ; and though it 

 always seeks the centre, in reality it goes further away from it. If 

 it wanted to reach it, it was a mistake to aim at it ; if it wanted to 

 get near, it should have aimed to get away. There certainly is 

 convergence and convection, but it is local and not general over the 

 cyclone. The idea which is conveyed by convergence in spiral paths 

 to the centre of a moving cyclone is an illusion. It did not even 

 require observations of the upper air to tell us that.* 



Take the time required for the operating forces to produce any 

 such wind velocities as we find in actual experience. In one hour 

 an ordinary pressure-difference would produce a velocity of 1000 

 metres per second, if it were free to act. The time required to 

 generate a velocity of, say, 10 metres per second, is infinitesimal 

 compared with the time during which we see the forces in opera- 

 tion : these last for hours, or even days, while a minute would 

 suffice for the production of all the velocities exhibited ; the motion 

 of the air which we register on anemometers is not accelerating 

 motion but uniform motion, except for the effect of turbulence 

 and local convection ; so we must picture to ourselves the air of 

 cyclones as being under the operation of balanced forces, not un- 

 balanced forces. I wish to suggest that the idea of air being 

 accelerated by the forces we see on the map is another illusion, 

 so far as the upper air is concerned. 



The ostensible reason for supposing that the distribution of 

 pressure created by convection is pushing air from high to low 



* See "Life-History of Surface Air-currents," by W. N. Shaw and R. G, K. 

 Lempfert, M.O. publication, No. 174. 



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