206 Sunshine Intensity [404 



but the means thus far available for measuring sunshine in- 

 tensity are difficult to apply in field studies. A method is 

 here presented by which a roughly approximate index of sun- 

 shine intensity during any period for any station may be 

 made from records such as are kept by the observers of the 

 II. S. Weather Bureau. 



The total heat equivalent of the actual sunshine for any 

 given period at a station is primarily a function of three 

 terms: (1) the maximum possible number of hours of sun- 

 shine (determined by latitude and season) ; (2) the mean 

 intensity of full sunshine for the period and station, ex- 

 pressed in terms of heat; (3) the condition of the sky> 

 whether overcast, partly overcast or clear. The daily values 

 for the first two of these terms vary in a regular manner 

 throughout the year at any given place, and the ones for the 

 third term are roughly stated in the observer's records, as just 

 mentioned. It was desired to combine these three terms so 

 as to get approximations of sunshine intensity for a number 

 of different stations in Maryland for the summer of 1914, 

 in order to make comparisons of the summer march of sun- 

 shine intensity with that of corresponding measurements of 

 plant growth. This has been accomplished in the manner 

 described below. 



The first two terms are combined in the ordinates of the 

 graph given by Kimball 1 for the maximum possible total 

 radiation received per day at Mount Weather, Virginia. 

 Since this station is at about the same latitude as the stations 

 in Maryland, the ordinate values may be taken as approximate 

 measures of the total radiation intensity for the corresponding 

 dates at any place in the state. These values represent the 

 total amount of heat, expressed in gram-calories per square 

 centimeter of horizontal surface exposed, received from the 

 sun and sky on clear days at Mount Weather. The method 



1 Kimball, Herbert H., " The total radiation received on a hori- 

 zontal surface from the sun and sky at Mount Weather. Monthly 

 Weather Rev. 42: 474-487. 1914. (See especially fig. 8, p. 484). 



