40 LIGHT, VEGETATION AND CHLOROPHYLL 



the short wave-lengths are markedly predominant and produce 

 its blue colour. 



By contrast, we shall now discuss real absorption. Ozone 

 and water vapour (by water vapour, we mean an invisible 

 gas, and not droplets which forms clouds) are the two 

 principal gases of the atmosphere capable of absorbing a 

 large part of the solar radiation. 



Ozone exists in a proportion which is nearly constant; 

 the quantity of atmospheric ozone is evaluated by the thick- 

 ness of the gas layer, this gas being assumed to be collected 

 in a uniform pure layer at atmospheric pressure and at a 

 temperature of 15° C. The thickness of ozone is of the order 

 of 3 mm. (the thickness of the atmosphere expressed in the 

 same way would be 10 km., i.e., 3 milUon times more). 



Water vapour exists in a proportion which is very variable 

 with place and time; its quantity is measured by the thickness 

 of the layer of liquid water that would be produced by con- 

 densing all the vapour at the surface of the ground. In Paris, 

 the average is 1-84 cm. 



We have seen already that ozone stops completely all the 

 solar ultra-violet radiation of a wave-length less than 2,900 A. 

 This is by far the most important characteristic of atmospheric 

 absorption and will be discussed in the chapter concerned with 

 the properties of the ultra-violet. All vegetation, if it were still 

 able to survive, would be profoundly different from the vegeta- 

 tion that we know if this thin screen of ozone did not protect it 

 from the fatal action of ultra-violet rays of short wave-length; 

 our skin and eyes would also suffer grievously from it. 



It is estimated that about 5 per cent of the solar energy 

 is thus absorbed by ozone in the ultra-violet. This proportion 

 is rather small because the sun radiates relatively little in that 

 spectral region. 



Ozone also absorbs 0-5 per cent of the visible radiation 

 and 0-1 per cent of the infra-red. 



The atmosphere contains a very variable proportion of 

 water vapour which absorbs certain radiations situated in 

 narrow bands of wave-length, especially in the infra-red. 



