LIGHT AND VEGETATION 41 



In Paris, about 20 per cent of the solar energy is absorbed 

 by water vapour, i.e., nearly as much as is affected by atmo- 

 spheric diffusion. These figures relate to an air mass of 3-5, 

 when the sun's rays arrive obUquely at an angle of 73 J° from 

 the vertical and the thickness of precipitable water is 1-5 cm., 

 which are average conditions. 



But whereas diffusion particularly affects the radiations 

 of short wave-length — blue, violet and ultra-violet — absorp- 

 tion by water vapour suppresses more especially the infra-red 

 radiations. 



Absorption by Particles in Suspension in the Air 



Particles of all kinds, soUd and Hquid, are suspended in 

 the air; without them, a cloudless sky would always be very 

 blue. They act by diffusion, more than by true absorption, 

 and as they diffuse ahnost equally the radiations of different 

 colours they give the sky a whitish or greyish tint which is 

 superposed on the blue caused by molecular diffusion. 



The particles vary greatly in kind: there are microscopic 

 droplets of water which seem to remain Uquid even when the 

 atmosphere is not saturated, dust of all sorts, smoke from 

 towns, spores, pollen, volcanic ash, dust raised by the wind and 

 carried sometimes over long distances, and small grains of salt 

 from the evaporation of droplets of sea- water in coastal districts. 



Their linear dimensions seem to be of the order of a few 

 tenths of a micron and the viscosity of the air is sufficient to 

 prevent them from falUng more than the small particles in 

 tobacco smoke. 



We do not know how to calculate directly the energy lost 

 by direct radiation through the diffusion caused by particles. 

 But as we know how to calculate the loss due to the two other 

 factors, atmospheric diffusion and absorption by gases, we 

 can deduce this unknown quantity by the difference between 

 the measured total absorption and that calculated. In winter 

 at Saint-Maur, near Paris, this loss is nearly equivalent to 

 that due to absorption by water vapour; in summer it is a 

 Uttle less. 



