THE DESERT CLIMATE 15 



in the production of deserts, but is by no means the 

 only factor, and under certain circumstances places 

 can be utter desert in spite of receiving a liberal and 

 not ill-distributed supply of water. 



iii. Heat 



The temperature is of fundamental importance to 

 the flora and fauna, not only for its own sake, but 

 because it influences the humidity of the air and the 

 evaporation of moisture. It is generally realized 

 that the climate of a desert may be intensely hot ; 

 it is equally true, though not so well known, that it 

 may be extremely cold. Considerable daily and 

 yearly range of temperature is a characteristic of 

 the chmate of all great land areas, and we are 

 justified in presuming that the extremes of tempera- 

 tures of the desert are partly due to the fact that 

 most desert areas lie in the centre of large land 

 masses. This presumption is supported by the fact, 

 to which we shall return later, that desert areas 

 which lie close to the sea are not subjected to 

 extremes of heat and cold. But we must not 

 attribute the range of the temperature exclusively 

 to the continental position of deserts, because the 

 extremes encountered in deserts are much greater 

 than they are in other continental areas ; and we 

 beheve that the very great fluctuations of tempera- 

 ture are due very largely to the bareness of the soil 

 and the clearness of the sky. The radiant heat of 

 the sun is not impeded either by cloud or by vegeta- 

 tion from striking directly on the soil, which is 



