224 



BIOLOGICAL EFFECTS OF RADIATION 



from measurements made at Washington, D. C, Madison, Wis., and 

 Lincoln, Neb., have led us to adopt a = 0.22, giving for the equation 



Q, = Qo[0.22 + 0.78,S] 



The value of a must vary with the character of the cloud layer and 

 perhaps also the surface conditions, as for instance, with a snow cover, a 

 surface covered with vegetation, or a sandy surface. 



Join. Feb. Mar. Apr. 

 II ? 31 10 ?0 2 m? 1 11 ?! 



May June July Aug. Sept. Oct Nov. Dec. 

 11 21 31 10 20 30 10 20 30 9 19 29 8 18 28 8 18 28 7 11 27 7 17 27 



Fig. 2. — Curves for Twin Falls, Washington, and Chicago, showing the annual variation 

 in the total radiation received on a horizontal surface with a clear sky and with average 

 cloud conditions. 



Figure 2 gives curves for Twin Falls, Washington, and Chicago, show- 

 ing the annual variations in the total radiation received on a horizontal 

 surface with a clear sky and with average cloud conditions. Superposed 

 upon the latter for Washington is an irregular trace containing weekly 

 averages for the year 1925. These weekly values show variations as 

 great as 50 per cent of the normal values and from 30 to 40 per cent of the 

 value at the outer limit of the atmosphere. The importance of cloudi- 

 ness in its effect upon the radiation received at the surface of the earth is 

 therefore apparent. 



SUMMARY 



The following may be mentioned as of special interest : At the three 

 most northerly stations, Sveanor, Treurenberg and Green Harbor, the 



