156 



RADIATION BIOLOGY 



varies between 400 and 600 cal cm-^ depending upon climatic conditions. 

 For the entire year in the United States the average varies with locality 

 from 250 to 450 cal cm-^. Of this energy, only about 40 per cent is 

 within the range 400-700 m/x (Table 3-8). For an average cloudy sky. 

 Kimball and Hand (1936) have shown that the visible irradiance varies 

 from a maximum of about 1500 ft-c to a minimum of about 500 ft-c 

 during the middle of the day. 



One striking feature of solar-irradiance data is the slight difference 

 shown in maximum intensity and spectral composition between clear 



Table 3-9. Time from Sunrise to Sunset on the Twenty-first Day of 



Each Month for Each 10° of Latitude 



(Eckert and Clemonce, 1946.) 



" L, continuous daylight; D, continuous darkness. 



summer days at any latitude within the tropical and temperate zones. 

 The important differences are chiefly due to the greater proportion of 

 clear days in one region as contrasted with another. Ultraviolet studies 

 (Coblentz and Stair, 1944) have resulted in similar conclusions. The 

 middle-ultraviolet irradiance, about 320 m^, may be nearly as great on 

 a clear day at latitude 62° N (Alaska) as at 39° N (Washington). In 

 the tropics the ultraviolet intensity may be 20 per cent higher than in the 

 temperate zone for clear days and the same solar angle, owing to lower 

 ozone concentration. Weather data on the proportion of overcast skies 

 and information on atmospheric pollution in the vicinity of large cities 

 can be used as an approximate means of comparing the relative amounts 

 of solar energy to be expected at any two locations. 



Day Length. The flowering of many plants and the sexual behavior of 

 certain animals are influenced by the length of the daylight period. In 

 the northern hemisphere the longest days occur on June 21 and the short- 



