101 



HADIATION HIOLOGY 



tive to the inaximuin intensity at KiOO A the intensities at 5, 8, and 14 /x 

 are l.i) X 10 •', 3 X 10-', and 3.7 X l()-'\ respectively, hut the idea that 

 io is approximate to or is less than tiic intensity of a hlack body at ()()00°K 

 must not !)(' pushed too far because, as brought out in the preceding sec- 

 tion, both in the region of X rays and of radio waves the solar energy is 

 such as to indicate the existence of emissive regions of the sun, probably 

 in the corona, which are at temperatures much greater than ()000°K. 



In Table 3-1 is given the energy in several portions of the solar spec- 

 trum on top of the atmosphere calculated from the areas under the aver- 

 age of the curves of Figs. 3-3 and 4. In obtaining the values in column 



Table 3-1. Spectral Distribution of Soi.ar Radiation 



3 of the table the solar constant, which is the flux density of total solar 

 radiation on top of the atmosphere at the earth's mean distance from the 

 sun, was taken to be 1.94 cal/cm- sec. It is seen from Table 3-1 that 

 about half the total solar radiation lies in the visible and ultraviolet 

 regions below 7000 A and about half in the infrared. 



SOLAR ULTRAVIOLET RADIATION AT THE EARTH'S SURFACE 



The intensity of the solar ultraviolet radiation that reaches a particu- 

 lar point on the surface of the earth depends on the amount of ozone, air 

 haze, and clouds between that point and the sun. Of these materials, 

 ozone is important because of the absorption of its great ultraviolet band 

 which begins at about 3400 A and increases rapidly for shorter wave 

 lengths. Air, haze, and clouds atteiuiate the rays of the sun mainly by 

 scattering with little true absorption; the scattering, of course, causes the 

 sky. For ultraviolet radiation, true absorption of the oxygen of the air 

 does not set in appreciably until the wave length becomes less than about 

 2700 A. 



The absorption coefficient a of ozone, observed by Tsi-Ze and Shin- 

 Piaw (1932, 1933) is plotted in curve I, Fig. 3-5, against the wave length; 

 about one-fifth the observations are shown by the dots. The absorption 

 coefficient a is defined by 



i = z>-«, (3-5) 



