F. W. Very — Sky Radiation and Isothermal Layer. 369 



Art. XXXIII. — Sky Radiation and the Isothermal Layer ; 

 by Frank "W. Very. 



Introductory. 



During the hours of sunshine, light from the sky, and to a 

 smaller extent invisible infra-red radiation from the same 

 source, fall upon the earth's surface in addition to the direct 

 rays of the sun. This secondary source of modified sunlight, 

 diffused by the air and its floating dust, constitutes during 

 most of the day only a fraction of the total radiation received. 

 The heat communicated to the earth's surface by this incoming 

 radiation makes the earth a source of infra-red radiation of 

 extreme wave-length ; but since of the two sources of supply 

 that from the sky is much the smaller, the incoming radiation 

 from the sky is less than the outgoing radiation to the sky 

 which has been derived from the total heat due to the com- 

 bined primary and secondary sources. Consequently, the sky 

 behaves like a cold body by day as well as by night, its effective 

 temperature may be measured, and something may be learned 

 from the phenomena of sky radiation in regard to the consti- 

 tution of the earth's atmosphere and the processes by which 

 the earth parts with the radiation which it receives from the 

 sun. 



General Phenomena of Atmospheric Radiation. 



In the experiments described in my treatise on " Atmos- 

 pheric Radiation,"* I found that some of the more absorbent 

 vapors are incapable of direct radiation through a distance of 

 more than a few centimeters. It follows that the aqueous 

 vapor and carbon dioxide of the lower atmospheric layers can 

 not radiate directly to space notwithstanding their great radiant 

 power. 



But in addition to this, the more transparent gases, the 

 oxygen and nitrogen of the air, are opaque to their own radia- 

 tions in layers of some meters thickness and cannot radiate to 

 outer space by the direct way until the limit of altitude for 

 these gases is approached, where the remaining material is 

 insufficient to produce complete absorption. The depths of the 

 layer beyond which direct radiation ceases will vary, not only 

 for the different constituents of the atmosphere, but also for 

 different radiant lines in the spectrum of the same gas or vapor. 

 Each emission line in a spectrum has a different coefficient of 

 absorption by subsequent layers of the emitting gas. Some 

 lines are of such feeble intensity that a depth of many miles of 



* Bulletin G, U. S. Weather Bureau, 1900. 



Am. Jour. Sci. -Fourth Series, Vol. XXXV, No. 208.— April, 1913. 

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