II I 



RADIATION HIOLOGY 



dear sky at sea level and Z = 30°. The ratio, either for ultraviolet 

 radiation or for \isil)lc light, increases with increasing haze, and in thick 

 haze or cloud oxcrcast, when the direct rays of the sun are reduced to 

 zero, the ratio becomes infinite because there is still light from the sky. 

 The distribution of ultrax'iolet radiation for \va\-c lengths less than 

 3200 .\. o\'er the sky was measured by Pet tit (1932) duiing the spring of 

 1928. His average values of the ultraxiolet sky brightness at Pasadena 



I 



o 

 m 



10 



90° 



60° 



30° 



30° 



60° 



90° 



HORIZON ZENITH HORIZON 



Fig. 3-13. Sky brightness for ultraviolet radiation and visible light. Curve I. for 

 ultraviolet wave lengths less than .■?200 A in California. Curves II and III. for 

 visible light in Brazil and Switzerland. 



on a meridian through the sun with the sun at an altitude of 50° are 

 shown by the solid-line curves of Fig. 3-13. The values varied con- 

 siderably from day to day because of changes in haze. For comparison 

 two curves of sky brightness for visible light are plotted in Fig. 3-13, one 

 observed in Brazil (Richardson and Hulburt, 1949) and one in Switzer- 

 land (Dorno, 1919, Table Illd). The curves refer to a clear sky, to a 

 meridian through the sun, and to the sun at an altitude of 50° ; the three 

 curves are adjusted to pass through the same point at the zenith. The 

 difference in the two visible-light curves was probably due to the haze 

 conditions in the respective atmospheres. The curves show the well- 

 known lirightness of the sky near the sun and indicate that the sky near 

 the horizon was relatively darker for ultra\iolet radiation than for \'isiblc 

 light. 



SOLAR INFRARED SPECTRUM AT THE EARTH'S SURFACE 



In Fig. 3-14 is gix-en the solar spectral-intensity curve /H on top of the 

 atmosphere and the tiansmission cur\'es of th<' important terrestrial 

 al)sorber, water, in liciuid and \apor form as ol)ser\ ed with spectrometers 

 of low resolving power. The transmission curves for litiuid water refer 



