THE DESERT CLIMATE 23 



pared with the amount which it would contain if it were 

 saturated with moisture from a flat surface of water 

 at the same temperature. Relative humidity might, 

 of course, be expressed as a vulgar fraction ; we could 

 say that it was " one-third," meaning that the air 

 contained a third of the amount of water-vapour 

 which it could hold if saturated at the particular 

 temperature under discussion. But it is generally 

 more convenient to express the humidity in per- 

 centages, the figure 100 representing saturation. 

 The amount of water- vapour which the air can hold 

 varies very greatly with the temperature, and it 

 rises constantly as the temperature rises, so that a 

 much greater amount of water-vapour is required 

 to saturate a certain volume of air when it is warm 

 than when it is cold. Suppose, for instance, that 

 the air is saturated with water-vapour in the cold 

 early morning, and that the actual amount of 

 moisture in the air remains constant throughout the 

 day : then the relative humidity will fall steadily as 

 the temperature rises, because the air when warm 

 could contain much more moisture than it actually 

 does contain. 



This is what happens in most parts of the world 

 on most days in the year. Among masses of dense 

 vegetation and in damp places it is hardly noticeable 

 because, however warm the air becomes at midday, 

 it can still saturate itself from the wet soil and the 

 vegetation. But in deserts the difference between 

 midday with its low relative humidity, and the night 

 with its high relative humidity, is extremely pro- 

 nounced, because, as we shall see later, fl.uctuations 



