2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 69 



feet) thick at the start and 180 meters (600 feet) thick at the close 

 of the work. 1 



The sky above was cloudless and very clear. Under these condi- 

 tions the following" experiments were made. 



OBJECT AND METHOD OF THE EXPERIMENTS 



It was desired to determine what proportion of the rays of the sun, 

 including sun rays scattered by the sky, is reflected upward from a 

 level layer of cloud of indefinite extent. For this purpose a pyran- 

 ometer 2 having a glass hemispherical cover was to be exposed in one 

 series of experiments in its inverted position to measure the rays 

 coming up from fog in the hemisphere below, and on a similar day 

 in the usual position to measure the rays from the sun and sky 

 in the hemisphere above. The glass cover served as a screen to sift 

 out for observation rays lying between 0.3 microns and 3.0 microns 

 in wave length. These rays comprise practically all rays of rela- 

 tively appreciable intensity in the solar spectrum. The glass excludes 

 rays of more than 3.0 microns in wave length such as the earth, 

 the clouds, and the atmosphere emit by virtue of their proper tem- 

 peratures. In order to determine whether the reflecting power of a 

 wide sheet of cloud differs much with the angle of incidence of the 

 rays, it was desirable to begin the experiments at low sun and con- 

 tinue them till the sun reached high altitude above the horizon. Ex- 

 periments reported in Volume II of the Annals of the Smithsonian 

 Astrophysical Observatory s of course show that the reflection varies 

 in azimuth and nadir distance greatly with the angle of incidence. 

 But it was not shown certainly whether the total intensity of the 

 reflected rays summed up over all azimuths and nadir distances 

 within a hemisphere would change much with the angle of incidence 

 of the rays upon the cloud layer. 



1 In passing up and down through the layer of fog the observer reported 

 as follows : 



Pacific standard time 

 Level of bottom (feet) 

 Level of top (feet) 



6 hr. 55 min. 

 1,000 

 2,600 



9 hr. 00 min. 

 1,800 

 2,600 



10 hr. 55 min. 

 2,000 

 2,600 



Such a thinning of the fog from the bottom without much change in its 

 upper level seems curious and is probably unusual. 



2 See Smith. Misc. Coll., Vol. 66, Nos. 7 and II, 1916. 



3 For further discussion of the theory of the method of observing see the 

 figure and explanation given in Addenda to Annals Vol II, entitled, " Note 

 on Reflecting Power of Clouds." 



