378 KIMBALL AND MILLER! CLOUDS AND RADIATION 



sun, and then fell rapidly to a value somewhat lower than was 

 recorded before the cloud came up. The record of February 3, 

 1912, a day without clouds, affords a good comparison curve, 

 as apparently the atmospheric transmission was about the same 

 on these two days, disregarding the effects due to clouds. The 

 value of the sun and sky radiation on the 3d, at the time the peak 

 was recorded on the 5th, was 0.79 calories, so that the radiation 

 of the 5th, augmented by the cloud effect, was 41 per cent in excess 

 of the radiation with the clear sky of the 3d. The cloud was a 

 relatively thin sheet, and the temperature prevailing at the time, 

 12°F. at the surface, was low enough to make certain that the 

 cloud was made up of ice particles. Its brightness was therefore 

 doubtless due in part to light received upon the upper surface, 

 and transmitted thru the cloud by refraction and reflection. 



On July 28, 1912, shortly after 8 a.m., a similar sheet of alto- 

 cumulus clouds formed rapidly over the Blue Ridge at Mount 

 Weather. An imperfect solar corona was visible soon after the 

 clouds covered the sun. The record made by a horizontally 

 exposed Callendar pyrheliometer shows that as the edge of the 

 cloud sheet approached the sun the concentration of the solar 

 rays caused an increase in the recorded radiation of 13 per cent 

 over what would have been recorded had the clear sky radiation 

 curve been continued. After a short interval of clear sky, clouds 

 again obscured the sun. At this time they were on all sides of 

 it, and just before the sun was obscured the record shows an 

 increase in radiation intensity of 20 per cent above what would 

 have been received had the sky been clear. 



At the same time a record was obtained from a Callendar pyr- 

 heliometer mounted equatorially in a diaframed tube, and kept 

 pointed towards the sun by clockwork. The angular opening 

 from the center of the pyrheliometric receiving surface to either 

 side of the outer square diafram was about 4°. But little sky 

 radiation could therefore be admitted to the pyrheliometer, and 

 the record shows scarely a trace of increased radiation intensity 

 as the clouds approached the sun. This seems to disprove the 

 assumption made by some writers that the sky is exceptionally 

 clear between clouds. 



