2IO 



NATURE 



[May 4, 191 6 



ILLUSIONS OF THE UPPER AIR.^ 



A Review of Progress in Meteorological Theory 

 IN England since 1866. • 

 Structure of the Atmosphere according to the 

 Observations of the Upper Air. 



BUT if the ideas which were common in meteoro- 

 logical practice fifty years ago are now to be re- 

 garded as illusory, let us consider what we have in 

 their place. We go back to the three elements : the cir- 

 culation, the convergence, and 1;he convection. As to 

 the circulation, we now think of it as it is exhibited 

 in the upper air, and instead of regarding it as an 

 incidental disturbance of the motion from high to 

 low, we regard it as the foundation of atmospheric 

 structure ; as the motion of air which is persistent 

 because the pressure-gradient is balanced by the centri- 

 fugal action of the earth's rotation, which we may 

 call the geostrophic compyonent, and of the curvature 

 of ffie path over the earth's surface, which we call 

 the cyclostrophic component. If the balance between 

 velocity and pressure is not perfect, the difference from 

 perfection can oe only infinitesimal, because in the 

 free atmosphere the air must always begin to adjust 

 itself to the strophic balance from the moment that 

 any infinitesimal change becomes operative, and the 

 power of adjustment arising from the extreme mobility 

 of the air prevents any finite perturbation being set 

 up, except temporarily in those regions where violent 

 convection is operative. It is only through the mobile 

 air that perturbation can be transmitted. We no 

 longer picture to ourselves the air as being somehow 

 held firm without moving until a pressure distribution 

 is set up and then let go; the first symptom of 

 pressure-difference will be the occasion of motion, 

 the distribution and velocity grow together; they 

 adjust themselves automatically. The whole history 

 of the general motion of the atmosphere is the story 

 of the constant pursuit of the strophic balance, the 

 adjustment of velocity to pressure, constantly disturbed 

 by infinitesimal changes. 



Near the surface things are much more complicated, 

 because there is turbulence due to the interference of 

 the surface and the obstacle which it offers to the 

 steady progress of air. The air loses some of its 

 motion, and is exposed to the pressure without the 

 velocity that is required to balance it. It must, there- 

 fere, fall away towards the low pressure, taking out 

 of the pressure the energy necessary to provide for 

 the loss by friction. Thus the convergence which we 

 have to account for is only that shown near the surface 

 within half a kilometre. We need not trouble our- 

 selves about a supposed convergence and convection 

 over the whole area in the upper air. The second 

 element of our specification disappears. After years 

 of contemplation of the motion of the air from high 

 to low as produced in a quiescent atmosphere by the 

 operation of pressure-difference and kept within 

 bounds by friction, we now regard the motion from 

 high to low as actually caused by the friction which 

 retards the velocity required to maintain the strophic 

 balance. To base the theory of motion of the upper 

 air upon the idea of a given distribution of pressure 

 setting a quiescent atmosphere in motion is as great 

 an error as to begin the lunar theory by supposing the 

 moon to start from rest under the force of the earth's 

 attraction, and only to find out after it had started 

 that the earth was moving. 



As to convection, there is certainly convection 

 wherever there is instability or the juxtaposition of 

 air of different densities. It takes a great variety of 

 forms ; it is very common in cyclones, but it is not a 

 necessary attribute of them. Possibly it is set up there 

 more easily because the air travels so much faster in 



1 From a discourse delivered at the Royal Institution on Friday, March lO, 

 by Sir Napier Shaw, F.R.S. Continued from p. 194. 



NO. 2427, VOL. 97] 



cyclonic areas than it does in anticyclones, and adjoin- 

 ing localities are fed from different sources of supply. 

 Apart from a certain interference due to change of 

 latitude, the convection is probably the one disturbing 

 cause of the strophic balance of velocity and pressure. 

 So we regard the troposphere as a layer of about 9 

 kilometres thick, always striving to arrange its motion 

 according to the pressure, and perpetually baffled in its 

 endeavours by the ubiquity of convection. But since 

 all the changes proceed by infinitesimal steps, there 

 is never a time when we can identify a state of finite 

 divergence from the balance between velocity and 

 pressure. From this point of view the centre of a 

 cyclonic or anticyclonic system has no special 

 dynamical importance. It becomes a notable feature 

 on the map when for any reason the cyclostrophic 

 component is the chief element in balancing the pres- 

 sure. That is seldom the case in our maps, which 

 more often consist of isobars of complicated shapes. 



The Dominance of the Stratosphere. 



Further than this, Mr. Dines has thrown a new 

 light upon the origin of differences of pressure at 

 the surface by obtaining the correlatiort coefficient 

 between corresponding deviaticns of pressure from 

 the normal at the level of 9 kilometres and at the 

 ground, and has obtained results "ranging from o'67 

 for the last available set of a hundred soundings on 

 the Continent to 088 for soundings in England 

 grouped for the winter season." Moreover, the standard 

 deviations are of the same order of magnitude at 

 both levels — that is to say, both levels are subject to 

 similar changes. At the same time, the correlation 

 coefficient between the pressure at the surface and 

 the mean temperature of the 9-kilometre column is 

 small ; in other words, the temperature of the lower 

 strata of the atmosphere has, on the whole, little 

 to do with the general distribution of surface-pressure 

 in this country. Its effects are local. 



We must therefore regard the general flow of air, 

 except in so far as it is disturbed by convection, as 

 governed not by what happens at the surface, but by 

 what is imposed upon it from the stratosphere above. 

 It is from there that the general control of the dis- 

 tribution of our pressure comes. It is only modified 

 by what happens below. The upper air, the strato- 

 sphere, is the operator, and the lower air the subject 

 operated on. After fifty years of strenuous endeavour 

 to regard the surface as the operator and the upper 

 air as the subject, the exchange of rdle is very dis- 

 turbing, but it has its compensations. There are 

 many things which can easily be explained by opera- 

 tion from above, but only with the greatest difficulty 

 by operation from below. Let us indulge in some 

 speculations which follow from supposing that the 

 stratosphere operates upon the troposphere. It makes 

 the troposphere as tuneful as an organ under the 

 alternating rarefaction and compression caused by the 

 changes in the stratosphere. Every cloud is the subject 

 of its action. One can imagine them being developed, 

 showing first the region of greatest humidity, like 

 the development of a photographic plate, which further 

 develops into loss of stability, and so into cumulus- 

 cloud and a shower. And let us not forget that 

 each several cloud means the disturbance of the 

 normal circulation ; the condensation will alter locally 

 the horizontal distribution of temperature, and there- 

 fore that of pressure and wind. On the table are two 

 autochrome photographs of the western sky at 

 Ditcham Park, with a quarter of an hour's interval, 

 on a September evening in 191 1, with gradually red- 

 dening clouds that gradually vanished as they ap- 

 proached from the west. Nothing could be more 

 attractive than to speculate upon such changes in 

 relation to the changes of pressure in the strato- 

 sphere 



