38 LIGHT, VEGETATION AND CHLOROPHYLL 



when the sun is at 30° above the horizon. The following 

 table gives the relation between m and the zenith distance. 



There are three reasons why solar radiation is weakened 

 by its passage through the atmosphere. 



1. Molecular diffusion, which removes from the direct 

 radiation a part varying with the wave-length and which 

 disperses it in all directions. As this effect varies inversely as the 

 4th power of the wave-length, it is particularly perceptible, as far 

 as the rays of the visible spectrum are concerned, in the blue; 

 it is this diffused blue radiation which gives its colour to the 

 sky. All the gases in the atmosphere contribute to the diffusion. 



2. Absorption, properly speaking, by gases, principally 

 by ozone and water vapour; the energy thus absorbed is 

 transformed into heat. Each gas selectively absorbs well- 

 defined spectral bands which characterize it. As water vapour 

 exists in very variable proportions in the atmosphere, its 

 absorption is also variable. 



3. Weakening, particularly by diffusion, due to particles 

 of all sorts — dust, smoke and droplets — in suspension in the 

 air; this is also very variable and is often very important in 

 certain regions. 



Molecular Diffusion 



Molecular diffusion disperses the direct radiation of the 

 sun just as a turbulent medium would do. We shall consider 



