368 ENVIRONMENTAL CONDITIONS. 



From the records of the Marvin sunshine recorder, the United States 

 Weather Bureau has derived data of the normal number of hours of 

 sunshine, for each day of the year, for each of the stations included in 

 the sunshine study. These numbers are expressed as percentages of 

 the possible daily hours of sunshine in each case, the possible number 

 of hours being, for each day and station, the number of hours between 

 sunrise and sunset. Through the kindness of Professor P. C. Day, 

 of the United States Weather Bureau, we have been able to obtain 

 these data of the percentage of possible hours of sunshine for each 

 month and for 57 stations in the United States, and these have been 

 employed as basis for our sunshine computations. 



From the present point of view the percentage of possible hours of 

 sunshine is quite without interest; what affects plant activities is, of 

 course, simply the amount of sunshine, and, if we consider this in terms 

 of hours of light intensity above the threshold of the sunshine recorder, 

 it is the actual number of hours of sunshine which should attract our 

 attention. Our first step was, then, to calculate the normal number 

 of hours of sunshine for each month, in each case. This was done by 

 finding the number of possible hours of sunshine for the latitude of 

 each station and for each month included in the period of the average 

 frostless season, from Marvin's Sunshine Tables,^ and then multiplying 

 this number by the corresponding percentage of the possible, con- 

 sidered as hundredths. The next step was to sum the numbers thus 

 obtained for all whole months occurring in the average frostless season 

 for the station in question, and to add to this sum quantities calculated 

 to represent the fractions of a month with which the average frostless 

 season generally begins and ends. The final sum represents the normal 

 number of hours of sunshine occurring in the period of the average 

 frostless season, for the particular station in question. The sums thus 

 obtained are given in table 21, the summations being plotted on a 

 chart, with isoclimatic lines drawn in the usual way. The chart 

 is given as plate 69. It is obvious, from the small number of stations 

 for which data are available, that this chart is very crude and super- 

 ficial. Nevertheless, a rational and self-consistent arrangement of cli- 

 matic zones is here brought out, and this zonation is very similar to that 

 based on temperature conditions. The stations receiving the most sun- 

 shine (measured in terms of hours by the Marvin recorder) are in the 

 extreme Southwest, while those receiving the least lie near the northern 

 boundary of the country or in the mountain regions. The lines of the 

 western portion of this chart are shown as distinct from the rest, to 

 suggest the greater uncertainty with which they have been placed. 



1 Marvin, C. F., Sunshine tables, Edition of 1905, giving the times of sunrise and sunset in 

 mean solar time and the total duration of sunshine for every day in the year, latitudes 20° to 50° 

 North, U. S. Dept. Agric, Weather Bur., 1905 (numbered "W. B. No. 320"). 



