THE DESERT CLIMATE 6 



heat ; and though this conception is not strictly- 

 accurate, I propose to be guided by it. I shall 

 therefore deal first with the supply of water and 

 second with the heat and cold experienced in 

 deserts. The available moisture and the current 

 temperature together determine the amount of 

 water-vapour present in the air, and I shall deal 

 with this next. This will be followed by some facts 

 relative to winds, because air movement is the result, 

 in part at any rate, of the rapid fluctuations in the 

 temperature to which these bare spaces are sub- 

 ject. The discussion of wind leads us naturally 

 to consider evaporation, which results from the 

 presence of moisture and the combined action of 

 heat and of air movements. The meteorological 

 statistics which are quoted do not give very accurate 

 information of the conditions to which the desert 

 plants and animals are exposed. The instruments 

 with which they are taken are generally exposed 

 in a ventilated white-painted screen : the organisms 

 which we are studying live on the surface of rocks, 

 and in holes, and in a variety of other natural 

 habitations. 



ii. Water 



It may be admitted at once that relative lack of 

 moisture is a characteristic feature of all deserts, 

 but it is not possible to state dogmatically that if 

 the rainfall be below a certain figure the land will 

 be desert, because the condition of the land depends 

 not only upon rainfall but on many other climatic 

 factors. It is generally accepted that a rainfall 



