538 F. II'. Very — Note on Atmospheric Radiation. 



Although the radiation function in the lower air, where 

 only indirect radiation exists, can he fairly well represented by 

 a radiation formula with the exponent of T (the absolute 

 temperature) equal to 1, so that indirect radiation is still 

 satisfied by Stefan's law with small changes in the constant,* 

 vet in the upper isothermal layer where free radiation ought, 

 if anywhere, to be in control, the exponent of the radiation 

 formula suddenly becomes negative. This discrepancy, how- 

 ever, is more apparent than real, and appears to be due to the 

 superposition of several effects attributable to the progressive 

 removal of absorbent vapor in the upward direction, combined 

 with a special absorption of solar rays and the limitation of 

 long-waved radiation. 



The observed value of air radiation has an immediate 

 application to the phenomena of the nocturnal deposition of 

 dew where the atmospheric cooling concerns an air layer only 

 a few meters thick; but this value may require modification 

 when applied to the conditions of the elevated isothermal 

 region. Here the diminished radiant mass in unit volume has 

 the outflow of its radiation of smaller wave-length than 15/a 

 proportionally less obstructed through absorption by its own 

 substance, which tends to give constant radiation in spite of 

 varying density, and thus there is a new freedom of radiation 

 from the upper surface which was not possessed by the air at 

 lower levels, while at the same time there is a strong accession 

 of thermal energy from absorption of the sun's rays by the first 

 portions of aqueous vapor encountered. This absorbent layer 

 immediately becomes a radiant one and its extra heat is dissi- 

 pated by radiation to outer space. (The region of the accession 

 of freer atmospheric radiation to outer space is about 3000 

 meters higher in the torrid zone than in temperate latitudes 

 and all of the data are correspondingly modified.) This seems 

 to be the meaning of the sudden increase in the loss of heat on 

 entering the isothermal layer, which passes, according to 

 Bigelow's computation, from —140 in mechanical Iv.M.S. 

 units per 1000 meters for the lower layers, to —362 per 1000 

 meters for the isothermal layer. 



Westwood Astvophysical Observatory, 

 Westwood, Massachusetts, ' 

 May, 1912. 



* We knew already that, within the very limited range of its selective 

 radiation, the maximum radiant layer of a gas behaves like a black body. 



