22 ANIMAL LIFE IN DESERTS 



At Tucson, Arizona, in June, the temperature 

 ranges daily through about 17° F. (10° C.) at a 

 depth of 6 inches (15 cm.) in the soil, and over about 

 2° F. (1° C.) at a depth of 10 inches (25 cm.) ; at 

 the same season the shade temperature ranges over 

 about 57° F. (37° C). Therefore at 6 inches (15 cm.) 

 the daily fluctuation is about one-quarter of that 

 of the air, and at 10 inches (25 cm.) it is only about 

 one-thirtieth. Results which are essentially similar 

 have recently been obtained by C. B. Williams in 

 the desert near Cairo, Egypt, in July. The same 

 investigator sends me the following data taken in 

 April, 1922, at Luxor, Egypt, on cotton land which 

 had been irrigated three weeks previously : air 

 temperature 86° F. (30° C), soil surface 131° F. 

 (55° C), soil at 3 inches (7-5 cm.), 81° F. (27° C.) ; 

 at the same time the relative humidity of the air 

 was about 15 per cent., and the relative humidity 

 3 inches deep in the soil 98 per cent. These observa- 

 tions may be taken as typical of others which have 

 been made in South Algeria and other deserts. It 

 will be seen that an animal would very easily reach 

 equable conditions by burrowing, and that it would 

 pass through a very great range of temperature and 

 humidity as it entered or left its burrow during the 

 middle of the day. 



iv. Relative Humidity 



The rainfall and the temperature of the desert 

 have been discussed : upon their interaction depends 

 the relative humidity of the air. It is defined as the 

 amount of water- vapour which the air contains com- 



