21 



Light. 

 The only records of light conditions tha^ were availahle 



for all of the stations of this study 'jrere the daily ocular 



estimates of cloudiness furnished by the cocnera*:ive weather 



observers. To rakke use of these estimates it was, of course, 



first necessary to bring these daily percentages of clear sky 



together for each culture period, so as to derive for each 



period a single value that might be taken to represent the 



Intensity of the light condition. The method employed to ac- 



complish this is p esented in the next following paragraphs. 



^ The presentation of this method is here ^practically 

 the same as that previously published. See; Hildebrandt, ?.F. , 

 A methcd for approximating sunshine intensity from ocular ob- 

 servations of cloudiness. Johns Hopkins Un'v. Circ. 

 March, 1917. 



The total heat equivalent cf the actual sunshine for 

 any given period at a given st ."^ion is primarily a function 

 of three terras: (1) the maximum possible number of hours of 

 sunshine (determined by latitude and season); (?.) the me-m 

 intensity of full sunshine for the period and station, which 

 may be expressed In terms of heat received per unit of a 

 horizontal surface; (3) the condition of the sky, whether 

 overcast, partly overcast or clear, '"he daily values for the 

 first two of these temms vary In a regular manner throughout 

 the year at an:/ given place, and the ones for the thJi rd term 

 are roughly stated in the observer's records, as Just mention- 

 ed. 



■^he first two terms are combine.i ' -•^. ^he or-linates of •'■he 



