46 LIGHT, VEGETATION AND CHLOROPHYLL 



Diffused Solar Radiation 



So far in this chapter we have been considering the direct 

 radiation of the sun. But even a pure atmosphere diffuses 

 this radiation and clouds diffuse it still more. 



This diffused hght sometimes constitutes almost the whole 

 of the Ught available; thus Stockholm, in December, receives, 

 on an average, 93 per cent of diffused Ught, against 7 per cent 

 of direct sunlight. 



In Paris, the fraction of the sky covered by clouds in the 

 course of the year is greater than the fraction of blue sky, 

 58 per cent against 42 per cent. Thus, first, the duration of 

 illumination by diffused Ught, and, second, the quantity of 

 energy received from it, have an importance that should not 

 be forgotten. 



The general average, calculated over a year, of the energy 

 received on the ground and coming from a blue sky, or from 

 a cloudy sky in the form of diffused radiation, represents in 

 Paris about 40 per cent and in Helsinki 44 per cent of the 

 total radiation; the rest comes directly from the sun without 

 diffusion. 



