F. W. Very — Sky Radiation and Isothermal Layer. 371 



perature-in version, a layer many kilometers deep, which is truly 

 the radiant layer of the atmosphere. 



The radiation from the first sources forms a continuous 

 spectrum, but that from the third, or atmospheric source, is 

 discontinuous. The absorption of solar rays by the upper 

 layers of the atmosphere is not only the cause of the extra heat 

 in the layer of permanent temperature-inversion, but variations 

 in the distribution of absorbent ingredients within the layer 

 also occur and are accompanied by simultaneous fluctuations 

 of temperature. Thus this region is only approximately "iso- 

 thermal," and is so more especially on account of the very slow 

 change in its vertical temperature-gradient as compared with 

 the steep gradients of the lower air, and from its seasonal 

 constancy. 



" The greatest changes of temperature noted from one obser- 

 vation to the next," says Mr. W. R. Blair,* " all levels consid- 

 ered, take place at the surface of minimum temperature " at an 

 altitude of about 17 km. This observation by the staff of the 

 Mount Weather Observatory confirms the supposition on p. 123 

 of my work on "Atmospheric Radiation" that the elevated 

 regions of the atmosphere up to at least 20 km. undergo much 

 larger temperature variations than the more deeply situated 

 layers of the free air owing to the larger incipient absorption 

 of the incoming solar rays by the moisture in the upper air. 

 The altitude of 20 km. assumed in my former publication as an 

 approximate limit of the aqueous atmosphere within which 

 thermal conservation is produced through absorption of solar 

 radiation by aqueous vapor, was derived from a preliminary 

 charting which 1 made of some of Glaisher's observations, and 

 must be extended perhaps to about the same height as the 

 atmosphere of oxygen or nitrogen. A measurable amount of 

 aqueous vapor is still found at 30 km. 



The lowering of the level of the more potent aqueous absorp- 

 tion in descending air movements gives temperature-maxima 

 at the level of greatest temperature variation (about 17 km.) ; 

 and conversely, ascending movements bring minimum temper- 

 atures at this point, because the level of maximum thermal incre- 

 ment through incipient absorption has been elevated, while at 

 the same time locally transported air has been cooled by expan- 

 sion. Temperature-inversions associated with bodies of especially 

 moist or especially absorbent (that is, nearly moisture-saturated) 

 masses of air occur at all levels, but are sometimes most marked 

 at an elevation of 2000 or 3000 meters during strong descend- 

 ing movements of the atmosphere, the two causes (namely, 

 lowering of the level of incipient absorption, and presence of 



* Bulletin Mount Weather Observatory, vol. iv, p. 203, 1912. 



