32 LIGHT, VEGETATION AND CHLOROPHYLL 



In reality, a hazy, cloudy, dust-laden or very humid, 

 atmosphere absorbs 90 per cent and therefore radiates so 

 much more into space and towards the ground. As a result, 

 the temperature of the ground may be higher, since it will 

 receive an increased atmospheric radiation. 



Nocturnal Cooling by Radiation 



The radiation from the ground is specially interesting, 

 since dew, white frost, etc., are phenomena connected with 

 it; it occurs particularly at night. 



The coohng by radiation which is not compensated by 

 the low radiation of a clear and dry atmosphere causes the 

 condensation of water vapour into dew or its crystallization 

 into white frost. 



It is of great advantage if we can control this radiation ; 

 two methods are possible, either to reflect its own radiation 

 on to the ground, or to increase the atmospheric radiation. 



In the first case, the radiated heat is directed downwards, 

 either by using taut screens (linen cloth, straw mats, glass) 

 or by producing a smoke screen. These screens act by their 

 reflection factor for the infra-red, and also by substituting 

 their own radiation for that of the atmosphere, which they 

 mask. This is all the more advantageous as their temperature 

 remains higher. 



Walls, heated up by the sun during the day and cooling 

 slowly, form a partial screen for the area near to them; they 

 mask a part of the sky and radiate abundantly during the 

 night. In this way they are accumulators of energy. 



In the second case — the increase of the atmospheric 

 radiation — a very effective local action is obtained by hghting 

 fires; the hot gases and particles proceeding from the com- 

 bustion emit a radiation which is added to that of the 

 atmosphere, even if the smoke produced is not abundant. 

 Some fuels are shown to be more eff*ective than others. 



Glasshouses and glass-covered frames are a very interesting 

 appHcation of the properties of glass in the presence of 

 radiations. It is very transparent to the visible and the near 



