THE DESERT CLIMATE 



31 



wind. Figures are available from a limited number 

 of desert stations giving the number of inches of 

 water which would be evaporated from an open 

 surface of water by the month or by the year. These 

 figures do not represent what actually happens in 

 the desert, because very soon after a fall of rain the 

 surface of the soil is dried and evaporation practically 

 ceases. But if they are taken in conjunction with 

 the figures for annual mean rainfall they represent 

 very vividly the discrepancy between the water 

 upon which the plants can draw and the desiccating 

 effect of the desert climate and sun. This ratio 

 between possible evaporation (e) and actual rainfall 

 (r) is of very great interest to biologists, and it is 

 probably one of the most important factors in 

 determining whether a place will or will not be 

 desert. So far as we know it is high in all desert 

 regions, and it is not high in regions which are not 

 desert. 



The following examples show the very high pos- 

 sible evaporation in deserts, and the ratio between 

 this and the actual rainfall : 



Place. 



Yuma, Arizona . 

 Piute Dam, Utah 

 Tucson, Arizona . 

 Mohave . 

 Cahuilla, California 



Rainfall 



(Mean), 



Inches (r). 



3-26 

 8-61 



11-58 

 4-97 



17-35 



Evapora- 

 tion, 

 Inches (e) 



100 



61-2 



90 



95 

 116 



e/r. 



30-7 

 7-1 

 7-8 



19-1 

 6-7 



Cannon has given the following figures, which 

 show that at Laghwat and Ghardaia the discrepancy 



