536 F. IF. Venj — Note on Atmospheric Radiation. 



where we must presume cither that atmospheric radiation is 

 itself impeded by vaporous absorption, or that convection 

 plays a larger part in the denser air. The actual mean direct 

 radiation between neighboring air masses, which lies at the 

 foundation of the successive absorption and radiation of the 

 indirect process, must he smaller than that assumed. The exten- 

 sion of an observed air radiation for a layer 1 meter thick, at an 

 excess of 10°, to a layer 1000'" thick with the same excess, or a 

 proportional one, is only justifiable on the supposition that in 

 the passage of heat from molecule to molecule by radiation, 

 the small distances traversed in successive steps compensate for 

 the small differences of temperature and feeble radiations of 

 the separate interchanges whose integral finally reaches a sum 

 total of energy equivalent to that of a direct transmission. 

 Whatever doubt one might have as to the legitimacy of the 

 assumption that the two processes are even moderately equiva- 

 lent,' is now set at rest by the near agreement of the exponent 

 of T for the indirect process with the exponent (4) required by 

 Stefan's law. This seems to prove that, after all, the process 

 is a radiative one, rather than convective. 



The potential radiation function which Bigelow derives from 

 the internal energy of the air by the formula 



", - «0 



where U, and U are the internal energies at the top and bot- 

 tom of a layer, and v l — v„ is the change of volume of unit mass 

 of air in the same interval, at an altitude of 18,000 meters, is 

 about 0*16 of that at the earth's surface, which is - tf of that 

 for a black body.* This ratio is not improbable considering the 

 variation of the composition of the air in a vertical column ; 

 but a distinction must be preserved between the passage of. 



* With Kurlbaum's constant of radiation, R=5 - 32 x (10) -4 x T 1 joules /sq . 

 m. sec, a black body radiates at 289° and 225° abs. C. (which, are tem- 

 peratures observed in Europe at and 18,000 meters) 3,711,000 and 1,363,000 

 mechanical K. M.S. units, or 0'532 and 0'196 small calories per sq. cm. per 

 min. (Eatio = 2'Tl : 1). Bigelow obtains a ratio of potential air radiation = 

 6'37 : 1. The sum of the values of the radiant potential (K) between the 

 surface and 18,000 meters altitude is about one-third of the difference for 

 black radiators at the gnen temperatures, which may mean that the emission 

 bands of the air spectrum cover about one-third of the entire range. On 

 rising in the air some of the vapor bands become narrower and drop out, 

 and thus the potential radiant function diminishes more rapidly than that 

 of the black body for the same fall of temperature. The air radiation from 

 laboratory measurements agrees with the average AQ, which therefore repre- 

 sents the actual air radiation to space and increases upwards with the 

 progressive removal of the outer obstructing layers. The surface layers 

 of air have large K and small AQ of necessity, for it is only because these 

 layers are not free to radiate to space in accordance with their high 

 potential that the lower air can maintain its relatively high temperature. 



