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



the spectrum, but it does not seem probable that an atmospheric 

 feature so seasonally constant as the isothermal region should 

 be dependent on sucli a seasonally variable thing as atmospheric 

 aqueous vapor, and we may rather anticipate that the conser- 

 vation will be found to lie in some of the permanent gases, 

 which here exchange roles with the aqueous vapor of the lower 

 air. In fact if we try to apportion the three factors of atmos- 

 pheric temperature — absorbent, conservator, radiant — we shall 

 find that the solid earth is the absorbent from which the lower 

 air derives its heat by transference, the aqueous vapor is the 

 conservator, being highly opaque to its own radiation and there- 

 fore compelling the lower air to radiate slowly by its poorest 

 radiators, the air molecules. The latter constitute the radiant 

 for the lower moist air. 



In the isothermal region, aqueous vapor is the chief absorb- 

 ent by reason of its numerous bands in the visible infra-red 

 spectrum. Oxygen and carbon dioxide help a little, and ozone 

 may contribute something towards the retention of the short- 

 est solar ultra-violet waves. If the suggestions in the preced- 

 ing paragraph are correct, carbon dioxide and some of the 

 permanent gases are the chief conservators here, and the radiant 

 is aqueous vapor through its emission bands between B and A, 

 and to some extent through 3 (although this emission is rather 

 feeble at this lower temperature) together with ozone in the 

 region from 9'2/u to lOOyu. Beyond great A, emission ceases, 

 except that near the extreme outer limits of the layer the 

 opaque gases become sufficiently attenuated to again act as 

 radiators, when the temperature rapidly falls to absolute zero. 



The most important fact derived from this research is that 

 apparently the effective temperature of the sky never descends 

 below the temperature of the isothermal layer even in the cold- 

 est and driest weather and with the purest skies. This fact 

 favors the supposition that this layer is almost completely 

 absorbent for radiation beyond about 14/*, that the effect is one 

 of emission rather than of almost total reflection, and that the 

 apparent effective temperature of the sky is the actual tempera- 

 ture of the upper air and its clouds, reaching, when the sky is 

 clear and the lower air sufficiently transmissive, the tempera- 

 ture of the isothermal layer itself. 



Westwood Astrophysical Observatory, 



Westwood, Massachusetts. 



August 22, 1912. 



