NO. 4 THE BRIGHTNESS OF THE SKY MOORE AND ABBOT 7 



Observations on clear skies. — The major portion of the observa- 

 tions on the entire hemisphere of clear skies was made in connection 

 with auxihary curves to investigate the sky radiation during holo- 

 graphs. These were run quite regularly with holographs from 

 August 18, 1917, to November 26, 1917. They were originally 

 intended to offer a means of obtaining a very close approximation 

 to the solar constant by constructing an empirical curve showing the 

 relation between intensity of sky radiation, and the ratio of the true 

 solar constant to the apparent value obtained by exterpolating the 

 pyrheliometer values to zero air mass. Such a curve was found 

 to be quite erratic and of no value for solar constant work, but the 

 results obtained are quite interesting in themselves and a resume 

 will be given here. 



Twenty-eight sets of observations were taken in all. In each of 

 these the total sky calories (exclusive of the sun) were obtained at 

 known air masses. Plots were made betwen sky calories and air 

 masses, and similar plots between solar calories and air masses as 

 obtained by the pyrheliometer. The areas underneath these respec- 

 tive curves were read between the limits of the air masses obtain- 

 ing during the course of the holograph. The ratio of these areas 

 was plotted against the ratio of true to apparent solar constant. 

 As stated above this investigation did not prove useful for solar 

 constant determination. 



The main interest attaching to these observations in the present 

 discussion has to do with the general intensity of sky radiation and 

 the effect of the altitude or air mass of the sun on the same. To 

 best show the results obtained by these observations a table has been 

 prepared giving the air mass (M) and zenith distance (Z) of the 

 sun at a certain chosen point in each holograph, the sky calories (H) 

 at this air mass, the solar calories (S), the sum of solar and sky 

 radiation on a horizontal surface, the ratio of solar radiation to sky 

 radiation, and a general description of the character of weather pre- 

 vaiHng during the holographs. The column headed " Pyrh." gives 

 the solar calories measured on a surface at right angles to the path 

 of the beam, while 5* is the same reduced to a horizontal surface, or 



5"=—^^' The units in H, P}^^!., and 5" are calories per sq. cm. 



per min. 



