THE DESERT CLIMATE 33 



suppose that the evaporation in that particular year 

 was average. Judge, then, how great must be the 

 fluctuations in this important ratio in spots in the 

 Libyan Desert where the rain falls only once in 

 every four or five years, and where the evaporation 

 is believed to be 150 inches or over. 



vii. Light 



The uninterrupted sunHght of Egypt, even in 

 the winter months, is a thing which has arrested 

 the attention of every tourist. Accurate records 

 of the intensity of sunlight in deserts are scarce, 

 but one can at any rate recall a few general truths. 

 The sun's rays consist not only of light rays and 

 heat rays, but also of ultra-violet and other rays 

 which are known to exercise very definite influences 

 on living matter. All these rays, but particularly 

 the ultra-violet rays, pass more readily through 

 dry than through damp air ; their effect on plants 

 and animals is therefore much more potent in summer 

 than in winter, for in summer the days are longest, 

 the sun most nearly vertical at midday, clouds are 

 absent, and the amount of water- vapour in the air 

 at its lowest. 



Viii. SUMMAEY 



We have discussed individual elements of the 

 climate ; let us now try to consider the cHmate 

 as a whole, built up of these and other component 

 elements. If we were to study only the average 

 meteorological statistics for a number of places in 

 the Great Palsearctic Desert we should reahze at 



