412 



KIMBALL AND MILLER: SOLAR RADIATION 



but little during the warm months of the year, or from May to 

 October, inclusive, the excess of the averages for Madison over 

 those for Washington during the cold months is pronounced. 



This excess cannot be attributed to a difference in the vapor 

 content of the atmosphere at the two stations. The average 

 difference in their respective water vapor pressures in winter does 

 not exceed two millimeters, and as has been shown by one of us, 1 

 this would account for a difference in radiation intensities of only 



TABLE I 



Monthly Means of Solar Radiation Intensity at Madison, Wis., for 

 the Two Years July, 1910, to June, 1912, Inclusive, Expressed in Gram- 

 Calories per Minute per Square Centimeter of Normal Surface. 



MONTH 



January < 



J (p.m. 



February < 



(p.m. 



March ( a - m - 



(p.m. 



April ( a - m - 



(p.m. 



May ( a " m - 



T (a.m. 



June < 



(p.m. 



July ( a - m - 



(p.m. 



August < 



(p.m. 



September. . .. < 



(p.m. 



October < 



(p.m. 



November. . . . < 



(p.m. 



December < ' 



(p.m. 



AIR MASS 



1.0 



1.36 



1.26 

 1.25 

 1.33 



4.5 



1.12 



1.08 



1.06 



5.0 



1.15 



0.74 

 0.73 



1.05 



Note: — The air mass is approximately the secant of the sun's zenith distance. 



1 Jour. Franklin Inst., 171: 339. April, 1911. 



