405] F. M. HiUebrandt 207 



of using the graph and a weather observer's report for estimat- 

 ing sunshine will be best shown by an example. 



Suppose it is desired to estimate the average daily sunshine 

 intensity for some station in the general region of Mount 

 Weather, for the first week of August. The average ordinate 

 value for this week is first obtained from KimbalPs graph. 

 For periods as short as a week or two this may be done by 

 averaging the values for the first and last days of the period, 

 since the curve may be taken as a straight line for such short 

 intervals. From the report of the weather observer at the 

 place in question, the number of clear, partly cloudy, and 

 cloudy days is next determined for the days August 1 to 

 August 7, inclusive, and some arbitrary weighting is given 

 to each kind of day. We may, for instance, call clear days 

 whole days of sunshine, partly cloudy days half days of sun- 

 shine, and assume that cloudy days are days without any sun- 

 shine. The scheme of weighting adopted must, of course, be 

 adhered to in all the estimates made for different periods and 

 stations. The system of weighting given above was used in the 

 studies for which this method of approximating sunshine was 

 developed. By summing these weighted daily values a num- 

 ber is obtained which represents the equivalent number of 

 clear days for the period considered. Suppose, in the example 

 selected, that this equivalent number of clear days is 3.5, 

 which is 0.5 of the total number of days in the period. The 

 latter value may be termed " the coefficient of clear weather." 

 By multiplying the average daily intensity value for clear 

 days, as obtained from the curve, by this coefficient of clear 

 weather a number is secured that is a rough approximation 

 of the average daily sunshine intensity for the week. 



While it is certain that solar radiation affects plants in 

 other ways than through its heating effect, it is no less cer- 

 tain that by far the greater part of the energy of sunshine 

 absorbed by plants is converted into heat (largely as latent 

 heat of the vaporization of water), and it seems probable that 

 the other effects produced upon the plant may be more or less 

 proportional to the total energy equivalent of sunshine. The 



