HUMPHREYS: MBTEOROLOGICAI^ PARADOXES l6l 



That is, it will have the same potential temperature throughout, 

 or same actual temperature when subjected to the same pres- 

 sure. The truth of the above statement is obvious from the 

 fact that any temperature difference that might be developed by 

 a transfer of the kind mentioned clearly could be reduced by 

 further mixing. 



But as a mass of this air is carried to higher levels it continu- 

 ously expands against the diminishing pressure — diminished 

 by the weight of the air passed through — thereby does work at 

 the expense of its own heat energy and correspondingly cools 

 to lower temperatures. The ratio of this cooling to increase of 

 altitude evidently depends upon the nature of the gas and the 

 change of pressure. In the case of our own atmosphere it is 

 approximately i ° C. per loo meters. 



Although, therefore, stirring does bring an incompressible 

 liquid to a uniform actual temperature, it brings the atmosphere 

 only to a uniform potential temperature, or an actual tempera- 

 ture that is very non-uniform. 



THE NEARER THE SUN THE COLDER THE AIR 



The familiar fact that with increase of elevation and consequent 

 approach (during the daytime) to the sun, the air nevertheless 

 gets rapidly colder, at least through the first lo kilometers, is 

 very puzzling to the average person if he tries to explain it. 

 Nor, indeed, is the explanation of this phenomenon quite so 

 simple and obvious as we sometimes are asked to believe. Es- 

 sentially, however, this temperature distribution depends on 

 the following facts: 



(i) The atmosphere, as we know from observation, is so dia- 

 thermanous that half, roughly, of the effective radiation re- 

 ceived from the sun, that is, half of the portion absorbed and 

 not lost by reflection, goes directly to heating the surface of the 

 earth. Consequently, it is this surface, where the energy ab- 

 sorption is concentrated, and not the atmosphere, through which 

 absorption is diffused, that is most strongly heated by insolation. 

 The heated surface in turn warms the air above it, partly by con- 

 tact, and partly by the long wave-length radiation it emits, and 



