516 F. W. Very — Transmission of Terrestrial Radiation. 



I endorse, at least as a good average summer value ; but 

 the " conclusion that about - 07 gr. cal. or only about 14 

 per cent of the radiation from the earth's surface will escape 

 the absorbing atmosphere and go out to space "* does not fol- 

 low. For, let it be noted, the observed radiation of 0*15 to the 

 sky is the radiation immediately to space from the surface of 

 the earth. The restriction that " a part of the observed noc- 

 turnal radiation is not transmitted entirely through the atmos- 

 phere, but merely to its colder layers,"! can not be admitted. 

 The " nocturnal radiation " which is " observed " gets through 

 to space. The statement made that, because certain observers 



had given the value of 0*12 — ^ — ^— for ' : atmospheric radia- 



cm min. l 



tion " at a height of 3000 m , we must therefore " consider 

 about 0*06 gr. cal. [assumed to be the mean " atmospheric 

 radiation " of an air layer 3000 m deep] of the nocturnal radi- 

 ation as intercepted by colder layers of the lower atmosphere":}: 

 is certainly incorrect. Actually, *49 — ■ 15 = '34 gr. cal., or 

 between five and six times as much, has been intercepted ! 

 The " colder layers " of the atmosphere could be obliterated 

 completely without altering the transmission of terrestrial 

 radiation appreciably, for it is the lower warm and moist layers 

 in contact with the surface which do the work. The amount 

 specified by Angstrom could be easily intercepted in the first 

 100 meters of air (this is known from the experiments with the 

 distant radiator), and almost all of the radiation which the 

 atmosphere is capable of absorbing will be taken out long 

 before the level of 3000 m is reached, because, after the readily 

 absorbable rays have been struck out, the passage through sub- 

 sequent layers adds little to the absorption. 



A layer of air extending from the surface up to 3000 m is 

 assumed by Mr. Angstrom to be " the effective radiating layer 

 of the atmosphere." But the lower layers of the atmosphere 

 are prevented from radiating directly to space on account of 

 the absorption of their peculiar radiations by higher layers. 

 Still less can air raised some hundreds or thousands of meters 

 above the surface radiate downward to the ground to any 

 appreciable extent, because the lower layers have greater 

 absorbent power for the air's own peculiar radiation, and 

 besides, the lower layers are apt to be hotter than those 

 above. Moreover, all of the lower layers of air lose their heat 

 to some extent in other ways than by immediate radiation. 

 For example, a good deal of the hotter air is cooled by simply 

 being mixed with colder air, being transported as wind to 

 distant localities where the temperature is lower. UsTot that 



* Astrophysical Journal, vol. xxxviii. p. 200, September, 1913. 

 fOp. cit., p. 199. % Op. cit., p. 199. 



