THE DESERT CLIMATE 11 



years, and it is probable that throughout a great 

 part of the Libyan Desert and of the Western Sahara 

 the rainfall is so slight and so irregular in its occur- 

 rence that it makes little difference to the plant 

 Hfe. Such plants as exist at all, and they are very- 

 few, are dependent on subsoil water. 



Prolonged droughts have been recorded from 

 many other deserts. As we have seen, the rainfall 

 in the American deserts normally falls twice yearly, 

 and is distributed through nearly every month : in 

 spite of this it is on record that no measureable 

 rain fell at Bagdad, California, from October 3, 1912, 

 to November 8, 1914 ; a drought of over a year 

 occurred at Indio, California, from November, 1893, 

 to January, 1895. 



It may be stated as a general rule that the rainfall 

 in desert places, even in places in which it is relatively 

 regular, is liable to very great variation from the 

 monthly and annual averages. This variation in- 

 tensifies the struggle between the plants and animals 

 and their environment, and is an important factor 

 in the production of a desert. Angot suggested that 

 the ratio between the maximum and the minimum 

 annual rainfall of a spot could be used as an index 

 of its agricultural possibilities. He believed that 

 conditions were very unfavourable to agriculture in 

 places in which the annual minimum rainfall was 

 less than one-third or one-half of the annual maxi- 

 mum. It is known, however, that an even greater 

 disproportion than this can occur in places which 

 are by no means desert, and we cannot say that a 

 great irregularity in the amount of annual rainfall 



