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 
U. 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’ 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 
+ 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). 
