90 



ANALYSIS OF THE ENVIRONMENT 



radiation in the remote infra-red supplies 

 an insignificant amount of energy. On some 

 days the infra-red energy between about 

 20,000 to 30,000 A and 7700 A, the begin- 

 ning of visibiUty, may be greater than that 

 carried by visible Hght. In general, 50 to 58 

 per cent of radiant energy lies in the visible 

 range, and 1 to 5 per cent Ues in the ultra- 

 violet region, with less than 0.1 per cent 



tive solar energy is such that the entire field 

 of the ultraviolet gives only a small fraction 

 of the caloric energy to reach the earth, 

 while the nonvisible, infra-red rays carry 

 about one-half of the heat received. Data 

 from the latitude of Cleveland, Ohio, are 

 summarized in Figure 8. The maximum in- 

 tensity of the sun's energy as it reaches the 

 earth Hes at 4700 to 5000 A. with the sun 



0.000 



1 IIIILIZ3Z: 



I niiiEziz: 



I ninrzs: 



Fig. 8. Spectral distribution in percentage of total solar radiation at the latitude of Cleve- 

 land, Ohio. I, Below 3100 A; II, below 3250 A; III, below 3500 A; IV, below 4000 A; V, 

 4000 to 7000 A. ( Drawn from data reported by Forsythe and Chiistison. ) 



in wave lengths shorter than 3130 A 

 (Bracket, 1936; Kimball, 1924; Ellis and 

 Wells, 1941). 



Figure 7 gives the annual variations and 

 total radiation received on a horizontal sur- 

 face with a clear sky or with average cloud 

 conditions for three widely separated 

 stations in the United States. The record 

 for Washington, D. C, which is also given, 

 shows weekly variations as great as 50 per 

 cent of the normal values and from 30 to 

 40 per cent of the mean solar constant. 



Spectral distribution of ecologically eflFec- 



near the zenith. The region of the greatest 

 intensity is displaced toward the longer 

 wavelengths with decreasing altitude of the 

 sun and is located at about 7000 A at 80° 

 incidence when the rays are passing through 

 nearly six air masses; the shortwave Umit 

 is similarly shifted toward the red under 

 these conditions (Forsythe and Christison, 

 1930; Bundesen, Lemon, et al, 1927). 



Approximations are sometimes more re- 

 vealing than exact statements. Roughly one- 

 third of the radiation reaching the earth's 

 atmosphere is thrown oflF into outer space 



