HUMPHREYS: METEOROLOGICAL PARADOXES 1 63 



THE COLDEST AIR COVERS THE WARMEST EARTH 



This paradoxical statement refers to the air of the stratosphere, 

 with respect to which it is a well-known truth whatever the ex- 

 planation may be. 



It has doubtless been known since the dawn of intelligence 

 that the top of a mountain is colder than the adjacent valleys, 

 and that the highest among neighboring mountains has the coldest 

 top. And for much more than a century, actually since No- 

 vember 30, 1784, it has been known from observations by bal- 

 loonists, that the temperature of the free air also decreases with 

 elevation, at least up to such altitudes as were attained by manned 

 balloons. About the close of the last century, however, it be- 

 came evident, through records obtained with sounding balloons, 

 that in middle latitudes the temperature of the atmosphere con- 

 tinuously decreases, on the average, with increase of altitude up 

 to only 10 or 12 kilometers above sea level, and then becomes 

 substantially constant. Numerous subsequent records obtained 

 at many places have shown the additional surprising fact that 

 this isothermal region, or stratosphere as it is generally called, 

 begins at a higher level, and is colder, over equatorial regions 

 than over any other part of the world. Indeed, it seems to be 10° 

 to 15° C. colder over the equator, where its average temperature 

 is roughly —70° C, than, for instance, over the polar circles. 



The temperature of tlie stratosphere appears to be determined 

 chiefly by the intensity of the outgoing radiation from the earth 

 and the intervening water vapor and hence it seems to follow 

 that this radiation must be less intense over regions near the 

 equator than over those of the middle and higher latitudes; 

 a conclusion that merely shifts the burden of explanation from 

 one paradox to another. 



Obviously, the earth as a whole must emit, on the average, 

 the same amount of radiant energy that it absorbs, but the dis- 

 tribution of the two certainly is different. In equatorial re- 

 gions the upward movement of the atmosphere is so general 

 and so strong that high haze, cirrus, and other types of clouds 

 are exceedingly common, and the atmosphere necessarily humid 



