NO. 4 THE BRIGHTNESS OF THE SKY MOORE AND ABBOT 33 



February 22. 



Ratios : 



b/a = .7h b/c = .so, d/c = . 54, e/f = .38. 



The readings disagreed considerably among themselves, the sets 

 on clouds being much less satisfactory in this respect than those on 

 the snow, as will be seen by an inspection of sets a, b, and g, which 

 were typical. 



Sets 



The greater uniformity of the snow observations is probably due 

 to the fact that the snow is comparatively a rough surface and a 

 considerable proportion of the radiation came originally from por- 

 tions of the sky very distant from the clouds observed directly ; like- 

 wise, a large part of the radiation coming from the clouds in ques- 

 tion, failed to reach the pyranometer, not being absorbed but reflected 

 in other directions. Imagine, now, that the radiation from the clouds 

 observed was reduced by one-half, while the rest of the sky remained 

 the same. The effect observed from such a spot on the snow that a 

 directly reflected ray from our clouds would strike the pyranometer. 

 would, at the same time, be reduced not one-half but by one-half 

 of that proportion which our clouds contributed to the whole radia- 

 tion coming to the instrument from that spot on the snow. And 

 though the radiation from the whole sky was far from uniform, the 



