26 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 7I 



From the above it is seen that with a very hazy sky, with the sua 

 ranging in altitude from 9° to 53° (the haze remaining practically 

 constant), the radiation from the region near the sun's limb de- 

 creases 45 per cent, while the whole sky radiation increases 118 per 

 cent. 



OBSERVATIONS ON CLOUDED SKIES 



As might well be expected, the observations made with the pyrano- 

 meter on the radiation from cloudy skies, furnish a large range of 

 values dependent upon the character of the clouds present in the 

 sky, their position, and also the position of the sun with reference 

 to the clouds. Quite a series of observations upon cloudy skies was 

 taken at Hump Mountain, and under quite varying conditions. A 

 resume of these observations will be given and in most cases a brief 

 statement as to the conditions obtaining at the time. Usually a mean 

 of the values obtained will be included rather than the many pages 

 of values as recorded. That is to say, the observations of each day 

 will be divided into groups, of similar values in each group, and the 

 mean given with the number of values entering into the mean. 



Taking .0700 calories as a fair average of the intensity of the 

 radiation on a horizontal surface from a cloudless sky at hour angles 

 of the sun of 3 to 4, it will be seen that for cloudy skies the values 

 are from four to nine fold for average clouds, and from one to four 

 fold for very heavy clouds. Very often, just preceding the precipita- 

 tion of rain, the radiation drops very considerably and very rapidly. 



With low fog the observations unfortunately are few, but the 

 indications are that the radiation is ten fold or more that of clear 

 skies. 



An average cloudy sky, if the clouds are not too thick, lets through 

 about as much radiation (measured on a horizontal surface) as do 

 the sun and a clear sky combined with the sun at an altitude of 

 about 15°. The radiation from a low fog is about the same as from 

 the sun and a clear sky at 30° sun. 



