1916] 



on Illusions of the Upper Air 



613 



show that a pressure-difiFerence of 26mb. exists at the level of 

 10 kilometres where convection has ceased to exist. The difference 

 is accentuated to the extent of 21mb. as the surface is reached, by 



the existence of the high pressure transmitted from above in spite of 

 the relative coldness of the air at the lower pressure. The diagram 

 included in Mr. Dines's paper, which is reproduced with some 

 modifications in Fig. 4, showed that there is a remarkable change 

 at the top of the troposphere. Above the level for which values are 

 given in the table the high is colder than the low, reversing the state 

 of things in the troposphere. 



We cannot resist the conclusion that the pressure-differences of 

 cyclone and anticyclone are not local surface effects at all ; we must 

 seek their origin in the upper air, where there is no convection. 

 They are little affected by the lower stratum of 9 kilometres, which 

 roughly marks the range of the effect of heating at the surface. 



The idea of warm air in the lower layer causing the low pressures 

 which are recorded on our barometers is therefore an illusion. 



Thus it will be seen that the observations of the upper air have 

 proved that all the vital parts of the facile description which was the 

 accepted theory of cyclones and anticyclones are quite illusory. 

 What it took for guidance in forming a picture of the structure was 

 the accidental character of motion near the ground. We now feel 

 that the motion of air in the lowest kilometre had better be dis- 

 regai*ded, or, better still, be handed over to students of turbulent 

 motion, while we, as meteorologists, consider the normal state of the 

 atmosphere as motion under balanced forces. Instead of a natural 

 flow from high pressure to low pressure we have a natural flow 

 without any change of pressure ; the motion of a heavenly body 



