51S F. IF". Very — Transmission of Terrestrial Radiation. 



faces ; but the isothermal layer in its more attenuated parts, 

 where at last the air molecules are free to radiate directly to 

 space, is the final dispenser of this accumulated energy. 



We reserve the term " transmission of terrestrial radiation " 

 for that portion of the thermal energy at the earth's solid or 

 liquid surface which, being transformed into radiation of such 

 wave-lengths that the air cannot absorb it, escapes directly to 

 space. That this is what happens to the radiation emitted 

 from the earth to the sky is made certain by the following 

 facts : In my paper* on " The Probable Range of Temperature 

 on the Moon " I gave 48 per cent as the transmission, or 52 

 per cent as the atmospheric absorption of the extreme infra- 

 red radiation emitted by the moon (from winter measures). 

 The earth radiates at a lower temperature than the moon, 

 because the earth reflects the larger part of the sun's rays, fall- 

 ing on it,f and therefore somewhat longer wave-lengths pre- 

 ponderate in its spectrum. These fall outside the region of 

 greatest aqueous absorption, and are therefore more freely 

 transmitted than lunar radiation whose maximum falls within 

 the great H baud of aqueous vapor. Hence, on the nights in 

 question, a larger percentage of terrestrial radiation must have 

 been escaping than the percentage of lunar radiation then 

 undoubtedly entering through the same absorbent atmosphere; 

 and it is not at all exceptional that I obtain from winter meas- 

 ures of terrestrial radiation the value given in my paper in 

 Science on " The Transmission of Terrestrial Radiation by 

 the Earth's Atmosphere in Summer and in Winter," namely, 

 an absorption of 42 per cent, which is comparable with the 

 value given by Dr. Nils Ekholm for absorption of terrestrial 

 radiation by atmospheric aqueous vapor in more northern 

 lands, and is fully confirmed by the same, notwithstanding Mr. 

 Angstrom's opinion. " In Sweden the absorption amounts to 

 about 45 per cent and only in the polar regions does it fall 

 below 30 per cent," says Dr. Ekholm \\ that is, transmissions of 

 55 to TO per cent are the rule in these northern lands. 



In describing his method which presupposes radiation to an 

 entire hemisphere, Mr. Angstrom says :§ " It must always be 

 regarded as a dangerous approximation to calculate the radia- 

 tion to the whole sky from the radiation to a limited part, 

 assuming a certain standard distribution of radiation." This 

 may also be true as regards radiation to the sky in other direc- 



* Astrophysical Journal for November and December, 1898; cf. p. 275. 



t See my determination of " The Earth's Albedo," Astronomische Nach- 

 richten, Nr. 4696, November, 1913 ; and a more elaborate, or definitive 

 determination in A. N., Nr. 4819-20. 



% Om Naturens Varinehushallning, K. Yetenskapsakademiens Arsbok 1914, 

 p. 314. 



§ Astrophysical Journal, vol. xxxvii, p. 315, June, 1913. 



