154 



RADIATION BIOLOGY 



as the sun approaches the horizon until at 5° it is 10.4 and at 1° it is 27. 

 The air mass at small solar angles is larger than would be derived from 

 purely geometrical considerations, because the variation in density of the 

 atmosphere causes the rays to be refracted toward the horizon. This 

 accounts for the rapid attenuation at sundown and the orange-to-red color 

 when the sun is close to the horizon. In passing close to the earth's 



300 



500 



1100 



700 900 



WAVE LENGTH, mjl 



Fig. 3-6. Spectral energy distribution of solar energy for air masses from to 5. 

 (Fro77i Moon, 1940.) 



surface, the rays traverse dust-laden air, which, together with the other 

 constituents, severely scatters the shorter wave lengths. 



Spectrum. Figure 3-6 gives the spectral energy distribution of solar 

 radiant energy at air masses from to 5, as summarized by Moon (1940). 

 The curve for zero air mass corresponds to the spectral energy distribu- 

 tion outside the atmosphere. It will be noted that, as the air mass 

 increases, the wave length of maximum energy X,„ shifts from about 

 470 mn, at an air mass of 1, to 650 m/x, at an air mass of 5. The sky- 

 radiation curve has a maximum in the blue and falls rapidly with increas- 

 ing wave length (Abbott et al., 1923; Taylor and Kerr, 1941). These 

 data are summarized in Table 3-8. Most of the energy is within the 

 range 700 1100 m/x; relatively little energy, less than 10 per cent, is in 

 the infrared beyond 1500 m^u. The color temperature of the solar disk 

 varies between 6000° and 6800°K for a clear sky. The average for the 



