METEOROLOGICAL PARADOXES HUMPHREYS. 185 



No matter, therefore, how paradoxical it may be, air pushed north 

 does blow east (in the Northern Hemisphere), pushed east blows 

 south, pushed south blows west, pushed west blows north; while 

 in the Southern Hemisphere it blows exactly contrariwise. 



RAIN OKIES THE AIR. 



As everyone knows there is continuous and often rapid evapo- 

 ration from practically all parts of the earth's surface. Neverthe- 

 less the atmosphere as a whole never becomes even approximately 

 saturated. Water, as just stated, is always evaporating into the 

 air and thus constantly tending to saturate it ; but, on the other hand, 

 the air is forever being dried by the precipitation out of it of rain, 

 snow, and other forms of condensation. Whatever the temperature 

 and relative humidity of a given mass of air at any place along its 

 convectional route, the total of water vapor it then contains obviously 

 is less, in general, than when 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 precipitation that fell out of it during its convectional 

 journey. In short, as the paradox puts it, rain does dry the air — 

 does prevent it from becoming and remaining everywhere reekingly 

 and intolerably humid, as it otherwise would be. 



JtfQRE AIR GOES VP 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 what- 

 ever goes up must come down. And, indeed, to make the explana- 

 tion of it entirely clear and definite, it will be necessary to consider it 

 independently under two heads : a, when the air is measured in terms 

 of volume, and, &, when it is measured in terms of mass. 



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

 culation of the atmosphere is only a gravitational phenomenon con- 

 sisting of the sinking of relatively cold, and, therefore, also rela- 

 tively dense air, and its consequent lifting or forcing up of adjacent 

 air that happens to be comparatively warm and light. In short, con- 

 tracted 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 restricted to a comparatively small volume and 

 mass moving rapidly, while the descent extends to a relatively large 

 volume and mass settling slowly. On the average, though, con- 



