KIMBALL AND MILLER: CLOUDS AND RADIATION 379 



On May 17, 1912, a strato-cumulus cloud sheet formed over 

 the Shenandoah valley to the west of Mount Weather, and at 

 9.15 a.m. advanced rapidly over the mountain. In the five min- 

 utes preceding the time at which the sun was obscured the re- 

 corded radiation intensity on a horizontal surface increased 12 

 per cent above the very regular curve that had been made with 

 the clear sky previously prevailing. 



At Mount Weather on June 9, 1912, cumulus clouds formed 

 rapidly after 11 a.m., and between 11.03 a.m. and 11.19 a.m., 

 the recorded radiation intensity increased 11 per cent. On July 

 8, at 10.00 a.m., a thin fracto-cumulus cloud that formed between 

 the zenith and the sun increased the radiation intensity by 8 per 

 cent. 



In all these cases the zenith distance of the clouds was less than 

 the zenith distance of the sun, and the clear sky that had preceded 

 the clouds made possible rather definte measurements of the 

 increase in radiation intensity due to the clouds. When these are 

 in rather dense masses, such as cumulus or strato-cumulus form- 

 ations, the condensation of the solar rays must be attributed to 

 reflection from the cloud surfaces. 



There are also many cases in which the cloud effects, while 

 marked, cannot be accurately measured. A good example is 

 the record obtained at Madison on June 3, 1912. The sky was 

 clear until 9.30 a.m., when small cumulus clouds advanced from 

 the northwest and covered the sky by 10.20 a.m. The alternate 

 sunshine and shadow caused the recording pen to traverse the 

 sheet rapidly, for the most part below the normal for that time 

 of the day with a clear sky; but at 11.30 a.m. the relative distri- 

 bution of clear sky and cloud was such that the unusual concen- 

 tration of the solar rays caused a radiation intensity of 1.84 calor- 

 ies- to be recorded. 



Apparently the intensity at this time l^ad the sky been clear 

 would have been 1.12 calories, which gives an excess of radiation 

 of 64 per cent due to reflection from cloud surfaces. 



Another notable example of the effect of cloud reflection is the 

 record for June 17, 1912, at Madison, when thin alto-cumulus 

 or alto-stratus clouds prevailed thruout the day. The radiation 



