414 Tables 130 and 131 



SOLAR RADIATION OUTSIDE THE ATMOSPHERE 



From a long series of measurements by the Smithsonian Institution the value of the 

 solar constant, the total solar radiation at normal incidence outside the atmosphere at 

 the mean solar distance, was calculated in 1913 x to be 1.9408 ly. min." 1 (1 langley (ly.) 

 =. 1 gram-calorie per square centimeter (cal. cm." 2 )). Later investigations 2 showed that 

 the standard used in these measurements gave results about 2.4 percent too high and that 

 the adjusted value is about 1.89 ly. min." 1 In determining the solar constant, corrections 

 must be made for ultraviolet and infrared radiations which cannot be measured at the 

 base of the atmosphere. Recent observations (see below) indicate that the correction 

 employed for the unmeasured ultraviolet radiation may have been too low; also solar 

 radiation in the infrared region beyond about 2.5/* is still imperfectly known. In view 

 of this uncertainty and since the value 1.94 ly. min." 1 has long been used in meteorological 

 literature as the best value of the solar constant, it has been retained in this volume for 

 all calculations involving the solar constant. It should be clearly understood that this 

 value is subject to revision as more data concerning the ultraviolet and infrared portions 

 of the solar spectrum are obtained. It should also be pointed out that there is evidence 3 

 that the solar constant fluctuates as much as ±1.5 percent. In addition, the varying 

 distance between the sun and earth (see Table 169) produces a change in the actual solar 

 radiation at the top of the atmosphere of about ± 3.5 percent from the mean value. 



On the basis of the Smithsonian and other observations, Moon * in 1940 proposed a 

 spectral solar radiation curve at normal incidence outside the atmosphere at the mean 

 solar distance. More recently a rocket observation 6 has given a direct measurement 

 (at 55 km.) of the ultraviolet spectrum of the sun at wave lengths below 0.34/i. Since 

 less than 1 percent of atmospheric ozone is above this level, this observation should be 

 closely representative of ultraviolet solar radiation at wave lengths above 0.22/t at the 

 top of the atmosphere. Tables 130 and 131 are therefore constructed using Moon's values 

 for wave lengths above 0.33/j. and data from the rocket observation for wave lengths 

 below 0.33/*. 



Table 130 gives for various wave lengths the intensity of solar radiation outside the 

 atmosphere in relative units. Table 131 gives the energy contained in various narrow 

 spectral bands at normal incidence, in cal. cm. -2 min." 1 , assuming a solar constant of 1.94 

 cal. cm." 2 min." 1 



1 Smithsonian Inst., Ann. Astrophys. Obs., vol. 3, p. 134, 1913. 



1 Aldrich, L. B., and Abbot, C. G., Smithsonian pyrheliometry and the standard scale of solar 

 radiation, Smithsonian Misc. Coll., vol. 110, No. 5, 1948. 



8 Abbot, C. G., Solar radiation and weather studies, Smithsonian Misc. Coll., vol. 94, No. 10, 1935. 



* Moon, P., Journ. Franklin Inst., vol. 230, p. 583, 1940. 



8 Hulbert, E. O., Journ. Opt. Soc. Amer., vol. 37, p. 405, 1947. 



(continued) 



SMITHSONIAN METEOROLOGICAL TABLES 



