AIR CURRENTS IN RELATION TO AVIATION 357 



thick (fig. 6). The temperatures on the absolute scale may be 

 represented by the numbers 280 , 275 , 270 written against the 

 different levels. The clouds may seem to float very peacefully 

 in the empyrean like so many balloons. But suppose that, either 

 by some mechanical process or the thickening of the surface 

 layer, the whole layer is slowly lifted through a hundred metres 

 and in that way or in some other the pressure above the cloud 

 layer is reduced by the corresponding amount : consider what 

 happens. By the diminution of pressure over the whole surface 

 by about a third of an inch of mercury, the temperature of the 

 cloudless air falls i°, that of the cloud globule by only |° be- 

 cause of the condensation. The cloud becomes warmer than its 

 surroundings by |°; it will therefore begin to rise; but its 

 place cannot be taken by the air just above and around it, 

 because the descending air would be warmed i° and be too hot 



270° A 



_0l-O. . o_.r\_o -^O^JTA — Z75°A 



280°A 



Fig. 6. — Layer of detached clouds between two layers of clear air. 



for its environment, so the descending air will have to be dis- 

 tributed over a large area in order that no portion may be 

 warmed more than |°. The ascent is localised, the descent 

 must be distributed. Hence the cloud patches persist and in- 

 crease. Reverse the process and compress the layers : on 

 account of the evaporation the cloud is warmed less than its 

 environment, the cloud mass will sink locally, the displacement 

 must be distributed. 



So clouds must be regarded as evidence of regions of 

 ascending or descending air, according as they are forming or 

 evaporating : thus the complexity of vertical structure may 

 be considerable. 



Let us take another case of local significance, the cliff eddy, 

 which can easily be demonstrated in the laboratory by a wooden 

 cliff-shaped structure and an air-blast. Suppose we have a 



