1 62 HUMPHREYS: METEOROLOGICAL PARADOXES 



of which the atmosphere is far more absorptive than it is of the 

 comparatively short wave-length solar radiation. 



(2) Furthermore, and this is an equally vital part of the ex- 

 planation, the lower atmosphere (below about 10 kilometers), 

 under all ordinary conditions emits more radiant energy than it 

 absorbs — the difference being supplied by conduction. It is 

 these two phenomena, (a) the surface heating (warming below), 

 and (b) the net loss of heat by radiation (cooling above), that 

 together establish and maintain the vertical convections of the 

 atmosphere under which, since the descending portions grow 

 warmer through compression, and the ascending colder through 

 expansion, the whole of the convective region is made to decrease 

 in temperature with increase of elevation. 



But since the coefficient of absorption of the air, as of other 

 objects, changes but little if at all with the temperature, while 

 its emissive power decreases rapidly as it grows colder, and since 

 the intensity of the incident terrestrial (including atmospheric) 

 radiation remains roughly constant up to an altitude of many 

 kilometers, beyond the first 4 or 5, it follows that the upper limit 

 of the convective region is not, as formerly supposed, the outer- 

 most extent of the atmosphere, but at that elevation (10 to 12 

 kilometers above sea-level) at which the temperature is so low 

 ( — 55° C. roughly) that the loss of heat by radiation is no longer 

 in excess of, but now equal to, its gain by absorption. Be- 

 yond this level temperature does not decrease, or does so but 

 slightly, with increase of elevation; nor would it so decrease 

 (at least at anything like the present rate) beyond any level above 

 the thin conducting surface layer, at which absorption and radia- 

 tion became equal. 



In short then, the air grows colder with elevation — the nearer 

 the sun the colder the air — because (i) owing to its transparency 

 to solar radiation it is heated mainly at the surface of the earth, 

 and (2) because, at ordinary temperatures, it emits more radia- 

 tion than it absorbs. These together so affect the density of the 

 atmosphere as to induce vertical convections, and thereby to 

 establish and maintain, throughout the region in which they are 

 active, a rapid decrease of temperature with increase of elevation. 



