20 



KIMBALL : NORMAL ATMOSPHERIC TRANSPARENCY 



Wis.,^ obtained in a similar manner, are plotted in figure 1. The 

 curve that best fits the data shows a rapid fall from above nor- 

 mal between May 1 and June 9, 1912, to 29 per cent below nor- 

 mal in August 1912, followed by a ra,pid rise during the autumn 

 of 1912, and a slower rise in 1913 to normal values at the end of 

 the year. So few radiation measurements have been obtained 

 during winter months that the relative intensities computed for 

 these months are of doubtful value. The intensities measured at 

 Mount Weather on December 13, 1913, areas high as have ever 

 been measured at this observatory with the sun below zenith 

 distance 60°. 



TABLE 2 

 Total Radiation Received on a Horizontal Surface for Hours that were 



1912 



Cloudless. Ratio 



1913 



Mount Weather, Va. 



May 20- June 9 



June 10- July 31 



September 1-October 31 



HOUR ANGLE FROM NOON 



7-5 

 6-7 



0.61 

 1.04 



6-5 

 5-6 



0.87 



1.00 



5-4 

 4-5 



0.96 

 1.05 

 0.96 



4-3 

 3-4 



0.98 

 1.02 

 0.95 



3-2 

 2-3 



1.04 

 0.94 

 0.96 



2-1 

 1-2 



0.94 



1-0 

 0-1 



0.96 



Measurements of the total radiation received on a horizontal 

 surface from the sun and sky have been made at Mount Weather 

 since May, 1912. A comparison of the results for 1912 and 1913 

 for hours when the sky was cloudless is given in Table 2. The 

 results for August are omitted, as a different register was used 

 in August, 1913 from that in operation during the other months. 



There are not mahy hours during the summer, at Mount 

 Weather, when the sky is free from clouds. It can therefore 

 only be claimed that the data in Table 2 indicate that with a 

 cloudless sky the total radiation received on a horizontal surface 

 during September and October, 1912, was about 5 per cent less 

 than that received during the same period in 1913, and that there 

 was much less heat received diffusely from the sky between May 

 20 and June 9, 1912, than during the same period in 1913. This 



^ The observations for Madison, Wis., previous to July, 1912, will be found 

 tabulated in Bull. Mount Weatlier, Obs., 5, 177-181, 1912. 



