MOUNT ST. KATHERINE, 

 AN EXCELLENT SOLAR-RADIATION STATION 



By C. G. abbot 



Secretary, Smithsonian Institution 

 (With Two Plates) 



For many years the Smithsonian Astrophysical Observatory has 

 been engaged in measuring solar radiation on mountain peaks in 

 desert lands, and computing therefrom the solar constant of radia- 

 tion. By that we mean the intensity of the sun's radiation as it would 

 be found by an observer with a perfect instrument, constantly sta- 

 tioned in free space, outside the earth's atmosphere, at the earth's 

 mean distance from the sun. Our object in this work is to determine 

 to what degree the sun's output of radiation is variable, and what 

 effects its variations produce on weather. 



In his " Report of the Mount Whitney Expedition ", Langley 

 speaks strongly of the difficulty of measuring solar radiation any- 

 where as " formidable " , and that of correcting such measurements 

 for atmospheric losses as '' perhaps insuperable " . But over 50 years 

 have passed since Langley made this statement, and new apparatus 

 and new methods have been devised. 



About one million dollars has been spent in making solar measure- 

 ments at the most favorable stations to be found on the earth. The 

 most earnest efforts have been made to conquer the difficulties so 

 forcibly stated by Langley. Many discussions of the sources of error 

 and the degree of their elimination have been published. Tests and 

 tested inferences which indicate very high present accuracy have been 

 disclosed. We have not, indeed, claimed to determine the exact in- 

 tensity of that ultraviolet part of the solar radiation which never 

 reaches the earth because it is cut off completely in the upper atmos- 

 phere by ozone. This is, however, but a very small fraction of the 

 solar constant. This region of the solar spectrum is probably the 

 most variable. Because its rays are lost at high altitudes, its varia- 

 tions do not perceptibly affect the variation of the sun as an agency 

 to be taken account of in weather. 



Meteorologists have, I feel, somewhat neglected our proofs of the 

 accuracy of our work, and have been, I think, somewhat misled by 

 certain criticisms which have appeared in the literature. 



Smithsonian Miscellaneous Collections, Vol. 94, No. 12 



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