METEOROLOGICAL PARADOXES HUMPHREYS. 189 



experience — than that vigorous stirring establishes a uniform tem- 

 perature throughout the agitated medium. And indeed this conclu- 

 sion is quite correct in respect to the particular things we are likely 

 to have in mind, but it does not apply to the open atmosphere. In fact, 

 if the temperature of the atmosphere were uniform through any con- 

 siderable altitude, a complete stirring of it would immediately destroy 

 this uniformity. 



Let, then, the atmosphere, whatever its initial temperature distribu- 

 tion, be thoroughly mixed without the addition or subtraction of heat. 

 This will bring it into such state (that of neutral equilibrium) that 

 any portion of it on being adiabatically moved to a different place 

 will, on arriving at that place, have the same temperature as the then 

 adjacent air at the same level. That is, it will have the same potential 

 temperature throughout, or same actual temperature when subjected 

 to the same pressure. The truth of the above statement is obvious 

 from the fact that an}^ temperature difference that might be developed 

 by a transfer of the kind mentioned clearly could be reduced by fur- 

 ther mixing. 



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

 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 1° C. per 100 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 temperature 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 10 kilometers, is very puz- 

 zling 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. Essentially, however, this tempera- 

 ture distribution depends on the following facts : 



1. The atmosphere, as we know from observation, is so diather- 

 manous that half, roughly, of the effective radiation received 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 absorption is concentrated, and not the 

 atmosphere, through which absorption is diffused, that is most 



