i6 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



0.18 cal. for a clear sky. With this type of instrument Lo Surdo * has 

 made measurements at Naples. He observed the radiation during 

 a clear and especially favorable night and found a pronounced maxi- 

 mum about two hours before sunrise. Contrary to Homen he finds 

 a positive access of radiation from the sky even when the sky is 

 clear. The following table gives a brief survey of the results ob- 

 tained by different observers : 



Observer 



Maurer 



Pernter 



Pernter 



Homen 



Exner 



Exjier 



K. Angstrom 

 Lo Surdo. . . 



Date 



June 13-1S 

 Feb. 29, 

 Feb. 29, 

 Aug.. 



July 1, 



May-Nov., 

 Sept. 5-6, 

 A. Angstromj July 10-Sept. 10, 

 1912 



1887 

 1888 

 1888 

 1896 

 1902 

 1902 

 1904 

 1908 



Place 



Zurich 



Sonnblick 



Rauris 



Lojosee 



Sonnblick 



Sonnblick 



Upsala 



Naples 



Algeria 



Mean Value 



0.128 



0.201 



0.I5I 



0.17 



0.19 



0.268 (max.) 



0.155 

 0.182 

 0.174 



If we apply the constant of Kurlbaum a = y.68.io~ 11 , to the law of 

 Stefan-Boltzmann for the radiation of a black surface, we shall find 

 that such a surface at 15 C. temperature ought to radiate 0.526 cal. 

 If the observed effective radiation does not amount to more, for in- 

 stance, than 0.15 cal., this must depend upon the fact that 0.376 cal. 

 is radiated to the surface from some other source of radiation. In 

 the case of the earth this other source of radiation is probably to a 

 large extent its own atmosphere, and in the following pages we shall 

 often for the sake of convenience discuss this incoming radiation 

 as if it were due to the atmosphere, ignoring the fact that a small 

 fraction of it is due to the stars and planetary bodies. 



Then the source of variations in the effective radiation to the sky 

 is a double one. The variations depend upon the state of the radiat- 

 ing surface and also upon the state of the atmosphere. And the state 

 of the atmosphere is dependent upon its temperature, its composition, 

 density, the partial and total pressure of the components, and upon 

 the presence of clouds, smoke, and dust from various sources. 



The present paper is an attempt to show how the effective radia- 

 tion, and consequently also what we have defined as the radiation 

 of the atmosphere, is dependent upon various conditions of the 

 atmosphere. It must be acknowledged that the conditions of the 

 atmosphere are generally known only at the place of observation. 



1 Nuovo Cimento, Ser. 5, Vol. 15, 1908. 



