190 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1020. 



strongly heated by insolation. The heated surface in turn warms 

 the air above it, partly by contact, and partly by the long wave-length 

 radiation it emits, and of which the atmosphere is far more absorp- 

 tive than it is of the comparatively short wave-length solar radiation. 



2. Furthermore, and this is an equally vital part of the explana- 

 tion, 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 main- 

 tain 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 outermost 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. Beyond 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 

 radiation became equal. 



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

 the sun the colder the air — because (1) 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 radiation than 

 it absorbs. These together so affect the density of the atmosphere 

 as to induce vertical convections, and thereby to establish and main- 

 tain, throughout the region in which they are active, a rapid decrease 

 of temperature with increase of elevation. 



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 explana- 

 tion 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 



