SUNLIGHT AS A SOURCE OF RADIATION 



113 



clearness of atmosphere and sky as those that occurred in the United 

 States. 



A few measurements have been made of the ratio of the ultraviolet 

 radiation on a horizontal plane from the total hemisphere of the sky 



200 



E 

 u 



o 

 o 



CM 

 rO 



V 



u 

 o 

 <t- 



r< 



100 



200 



E 

 u 



o 



o 



V 



^ 



100 



8 9 10 II 12 I 2 3 4 5 6 

 AM NOON PM 



Fig. 3-12. Calculated values of Eh for wave lengths less than 3200 A through the day 

 for various seasons and latitudes. 



to that from the sun, i.e., Eh (sky)/A\ (sun). For clear days in Cleve- 

 land in 1936 and 1937 for wave lengths less than 3022 A the ratio was 

 1.0, 1.1, 1.5, 2.2, 4, and 10 for zenith angles 30°, 40°, 50°, 60°, and 

 70°, respectively. During the mid-day hours of June 13, 1928, a very 

 clear day, for zenith angles about 20°, Pettit (1932) found that, for wave 

 lengths less than 3200 A, the ratio was 1, 0, 0.55, and 0.43 at altitudes 

 above sea level in the vicinity of Pasadena, Calif., of 845, 3400, and 5700 

 ft, respectively. In general, these ratios are greater than the correspond- 

 ing values for visible light; the ratio for visible light is about 0.2 for a 



