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



the absorbent may be needed to give them sensible strength, 

 and such lines may be radiated from depths which are approxi- 

 mately equal to the entire atmosphere, but other lines are so 

 intense that they are absorbed by the substance in exceedingly 

 attenuated form and by very thin layers. Many of the lines 

 and bands of aqueous vapor are of this character. 



Now the distribution of vapor in the atmosphere is condi- 

 tioned by temperature, and as long as there remains a quantity 

 of even the less absorbent gases sufficient to preserve a tem- 

 perature above the point at which vaporization can take place, 

 vapors of the more condensible substances will continue to be 

 found. It is not necessary that the temperature shall be above 

 the melting point of the substance. Ice vaporizes at tempera- 

 tures far below the freezing point. Carbon dioxide is an 

 example of a substance which vaporizes directly from the solid 

 state without previous melting, even when heat is applied. 



Some vapors, and notably that of water, have extraordinary 

 powers of radiant absorption. At a temperature of —30° C, 

 when the surface air contains less than 0*3 gram per cubic 

 meter of aqueous vapor, I found that the great H band in the 

 solar spectrum still produced an almost complete extinction of 

 the spectrum between 5 ^ and 8 /jl. In the upper isothermal 

 layer where the temperature falls to —60° Centigrade, the 

 aqueous vapor is reduced to about one-tenth of the above 

 quantity, but there are many kilometers of air at this nearly 

 constant temperature, and sometimes with a relative humidity 

 as high as 50 per cent up to 25 kilometers.* Thus the total 

 depth of absorbent vapor is by no means inconsiderable in this 

 extensive upper layer, and the intensity of the great 3 band is 

 so extraordinary that it will continue to be a feature of the 

 infra-red spectrum at these extreme elevations. Here also the 

 atmosphere first begins to part with a portion of its heat by 

 radiation to outer space. 



The Loci of Terrestrial Radiation. 



There are three principal loci of terrestrial radiation, namely: 

 (1) A thin heated superficial layer of the terraqueous globe ; (2) 

 the solid or liquid particles suspended in the air, constituting 

 haze, smoke, or cloud ; and (3) the upper static layer of tem- 



* See W. R. Blair, " Free Air Data," Bulletin of the Mount Weather 

 Observatory, vol. 4, part 4, 1912. 



For example : Sept. 11, 1910, Rel. humidity = 56$ at 24899 m (p. 245). 

 Sept. 13, 1910, " " 69 " 17258 (p. 247). 



Feb. 16, 1911, " " 60 " 14248 (p. 258). 



March 3, 1911, " " 50 " 19050 (p. 270). 



March 4, 1911, " " 55 "17575 (p. 271). 



