SUNLIGHT AND ITS MEASUREMENT 169 



characters because of the laws under which the phenomena 

 are produced, so that once we are enabled to recognize these 

 characters we may be able to draw sufficiently accurate con- 

 clusions regarding the effects of the entire complexes. 



For example, Abbot 33 found that the high volcanic dust en- 

 countered in the late summer of 1912 reduced the intensity of 

 the direct beam of the sun by as much as 20% and yet the effect 

 on the various portions of the spectrum was very nearly the 

 same for all wave lengths in the visible spectrum. It does not 

 follow that this will be true for all dust clouds. In the article 

 referred to just above, Abbot quotes Arago to the effect that 

 in the year 1831 the volcanic eruptions reduced the intensity 

 of the light very greatly and the sun was exhibited to the un- 

 aided eye as azure, greenish or emerald. The same diversity 

 in the character of the radiation in a cloudy sky may be ob- 

 served frequently, some clouds appearing reddish while neigh- 

 boring ones are perhaps deep blue. 



Finally it may be noted that the upper surfaces of fog clouds, 

 as distinguished from the high cirrus clouds, apparently re- 

 flect about 65 % of the radiation incident upon them so that 

 by reflection alone such clouds reduce the intensity of the 

 radiation reaching the earth by about 65%. 34 



The discussion may be concluded with a statement of the 

 average (so far as experience goes) effects of the conditions 

 that have been mentioned. 



Losses from incoming solar energy occur in seven ways: 



(1) General scattering by the permanent gases of the 

 atmosphere. 



(2) General scattering by water vapor. 



(3) Selective (banded) absorption by the permanent gases. 



(4) Selective (banded) absorption by water vapor. 



(5) Absorption and reflection by clouds. 



(6) Absorption and reflection by dust. 



(7) Absorption in chemical reactions. 



It is at once evident that the effect of each of these conditions 



33 Ann. Astrophys. Obs. Smithsonian Inst. 3: 216. 1913. 



34 Ann. Astrophys. Obs. Smithsonian Inst. 2: 136-145. 1908. 



