R. Mountford Deeley — Cyclones and Climate. 159 



fell at such a rate as very nearly to establish a condition of con- 

 vective equilibrium. This" condition is one such that air moved 

 from one height to another, owing to the expansion or compression it 

 undergoes, changes its temperature so as to agree with that of the 

 surrounding air. This layer of the atmosphere has been named the 

 troposphere. Above it the temperature conditions are quite different, 

 for the temperature either varies little with increasing height or 

 actually becomes much warmer. This upper layer has been called 

 the stratosphere. 



In the troposphere the conditions of temperature are such that the 

 free flow of the air is possible in re-entrant paths, both horizontally 

 and vertically. But such is not the case in the stratosphere. Here 

 rising air would be chilled and find itself on a level with air warmer 

 than° itself, and air falling would rise in temperature and be on 

 a level with air colder than itself. For this reason vertical air 

 currents of any strength must be almost entirely absent in the strato- 

 sphere, but horizontal movements may take place freely, or the 

 whole may rise or fall bodily. 



Fig. 1. 



Many thousands of self-registering balloons have been sent up and 

 much detailed information obtained concerning the temperature 

 conditions of the atmosphere in different latitudes and at various 

 heights. 



The point of supreme interest is, of course, the temperature 

 difference between cyclonic and anticyclonic areas. Strange to say, 

 there has proved to be little difference between the temperature 

 conditions existing in the great polar cyclones and those of the smaller 

 wandering ones, which affect our climate so much from day to day. 



The most astonishing feature of the cyclone proved to be that, in 

 the troposphere, the centre of the cyclone was colder than its 

 margins, just the opposite to what had been considered likely. Far 

 too much importance was at first attached to this peculiar fact, for it 

 unduly discredited the broad teaching of the "temperature gradient" 

 theory of cyclones. Indeed, Aitkin has shown that although in the 

 troposphere the temperature is lower in the centre of the cyclone 

 than at its margins, owing to its lower pressure, the air is really 

 lighter. 



