METEOROLOGICAL PARADOXES HUMPHREYS. 191 



the highest among neighboring mountains has the coldest top. And 

 for much more than a century, actually since November 30, 1781, 

 it has been known, from observations by balloonists, that the tem- 

 perature 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 became evident, through records 

 obtained with sounding balloons, that in middle latitudes the tem- 

 perature of the atmosphere continuously 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 surpris- 

 ing 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 the 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 spectral distri- 

 butions of the two certainly differ. In equatorial regions 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, and, therefore, highly absorptive 

 of earth radiation, to great altitudes, especially as anticyclones with 

 their extensive regions of descending air are there unknown. Clearly, 

 then, a large part of the radiation through the stratosphere of this 

 region must come from the clouds and from water vapor that are 

 very high and correspondingly cold, and therefore its intensity, it 

 would seem, must be correspondingly feeble. The pent up heat 

 below can find an outlet through horizontal circulation and radiation 

 from lower and warmer levels in higher latitudes. 



This, perhaps, is at least the partial explanation of why the mini- 

 mum temperature of the stratosphere occurs over the tropical 

 regions — why the coldest air covers the warmest earth. 



AS THE DAYS GROW LONGER THE COLD GROWS STRONGER. 



This old proverb paradox expresses the well-known fact that our 

 lowest temperatures do not occur at the time of the shortest days, or 

 when the heat supply from the sun is least, but some time afterwards, 



