WATER-VAPOR TRANSPARENCY TO LOW-TEMPERA- 

 TURE RADIATION 



By F. E. FOWLE 



The main purpose of this research was to determine the trans- 

 parency of water vapor, mider atmospheric conditions, to radiation 

 such as the warm earth sends toward space. Upon the absorptive 

 property of water vapor rests in part the virtue of the atmosphere 

 as a conservator of the heat which the earth receives from the sun. 

 Radiation from the sun reaches the earth's surface diminished by 

 a certain portion scattered toward space and certain other portions 

 absorbed in the gases and vapors of the atmosphere. The return of 

 the energy of this radiation back to space is an indirect process. The 

 warmed earth is cooled partly by convection currents playing over 

 its surface and partly by direct and indirect radiation through the 

 constituents of its atmosphere. Of these the principal hindrances 

 to free radiation are aqueous vapor and carbonic acid gas. 



The radiation from the sun, at an apparent temperature of 6,000 

 to 7,000° K.,^ passes through the atmosphere wnth comparatively 

 little true absorption. Nearly all of the radiation of a body of this 

 temperature lies at wave-lengths shorter than 2 fi. At sea-level on a 

 clear day when the sun is in the zenith only about 6 to 8 per cent is 

 absorbed from the direct solar beam within the great infra-red bands 

 par, ij/, (f> and O in its passage to the surface of the earth.' The amount 

 scattered from the direct solar beam by the dust and molecules of the 

 air amounts to considerably more but after subsequent reflections in 

 considerable part reaches the earth." 



The radiation from a body of the temperature of the earth, which 

 may be taken as about 287° K., is of wave-lengths nearly all greater 

 than 2 /i, and is hindered by quite a different series of absorption 

 bands in its passage outward through the air. These absorption 

 losses are caused principally by the water vapor and carbonic acid 

 gas present in the atmosphere. This present research will treat 



^ In what follows the symbol K. denotes absolute temperature in centigrade 

 degrees. 



^ Astrophysical Journal, 42, p. 406, 1915. 



" See Annals of the Astrophysical Observatory of the Smithsonian Insti- 

 tution, Vol. 3. 



Smithsonian Miscellaneous Collections, Vol. 68 No. 8 



