CHAPTER II 



SOLAR RADIATION 



The solar radiation reaching the earth's surface is variable. 

 Its composition, and particularly its intensity, are affected 

 by its passage through the atmosphere. Even when the air is 

 pure and the sky clear, certain parts of the radiation are 

 absorbed; ozone suppresses the ultra-violet rays of short 

 wave-lengths and other parts, situated particularly in the 

 infra-red, are suppressed by water vapour. 



The atmospheric absorption also depends on the height 

 of the sun above the horizon, the luminous rays being obliged 

 to pass through a greater and greater air mass as the sun 

 sinks; in addition, the diminishing inclination of the rays 

 reduces the illumination on the ground in proportion to the 

 cosine of the angle that they make with the vertical. 



Such are the principal factors which, with cloudiness, 

 determine the nature and the intensity of the radiation on 

 the ground. We shall study them separately and we shall also 

 see how this radiation is distributed in time, according to the 

 length of the day and night, for it is very important to plants 

 that they should receive hght in periods of a suitable length, 

 even if the total quantity available remains constant. Finally, 

 we shall study the diffused radiation which comes to us when 

 the sun is hidden behind clouds and which also exists when 

 the sky is clear. 



Solar Constant 



Although the atmosphere is in a continuously variable 

 state, the solar radiation which reaches it appears to be very 

 constant. The sun is an incandescent gaseous mass whose 

 temperature at the surface is of the order of 6,000° C. 



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