156 HUMPHREYS: METEOROLOGICAL PARADOXES 



it left the surface of the earth by the amount of precipitation in 

 the meantime abandoned by it. That is, on the average, air 

 descends to the earth drier than it was when it ascended, and 

 drier solely because of, and in proportion to, the amount of pre- 

 cipitation that fell out of it during its convectional journey. 

 In short, as the paradox puts it, rain does dry the air — does pre- 

 vent it from becoming and remaining everywhere reekingly 

 and intolerably humid, as it otherwise would be. 



MORE AIR GOES UP THAN EVER COMES DOWN 



This is, perhaps, about as incredible a paradox as can be 

 found, for it seems flatly to contravene the well known dictum 

 that whatever goes up must come down. And indeed to make 

 the explanation of it entirely clear and definite, it will be neces- 

 sary to consider it independently under two heads : a, when the 

 air is measured in terms of volume, and, b, when it is measured 

 in terms of mass. 



Measured in terms of volume. — -As everyone knows, the vertical 

 circulation of the atmosphere is only a gravitational phenomenon 

 consisting of the sinking of relatively cold, and, therefore, also 

 relatively dense air, and its consequent lifting or forcing up of 

 adjacent air that happens to be comparatively warm and light. 

 In short, contracted air descends and expanded air ascends (is 

 buoyed up by the descending denser air) . Hence, mass for mass, 

 the volume of the ascending air is always larger than that of the 

 descending air. The ratio between the actual ascending and 

 descending volumes, however, or masses, may be anything, as 

 illustrated by chimney circulation, in which the ascent is re- 

 stricted to a comparatively small volume and mass moving rapid- 

 ly, while the descent extends to a relatively large volume and 

 mass settling slowly. On the average, though, considering 

 both velocity of vertical movement and volume occupied, or 

 velocity times volume, the atmosphere as a whole is always 

 ascending, a fact not only interesting itself, but also of some 

 importance to both the aeronaut and the aviator. 



Measured in terms of mass. — Whatever the volume relations 

 between ascending and descending air may be, it would seem 



