30 ANIMAL LIFE IN DESERTS 



more devastating ; in 1918 a very large camp at 

 Balad Ruz, north-east of Baghdad, was struck by a 

 dust-devil of unusual size, which carved its way 

 through the camp, leaving a lane in which not a 

 single tent was standing. Heavy articles of kit were 

 blown as much as 200 yards through the air, and 

 an officer of my acquaintance was scooped out of 

 his tent with all his camp furniture, and dropped 

 twice, the second time with several broken ribs. 



The effect of wind on other climatic conditions 

 is various. A hot dry wind causes a rapid and 

 considerable drop in the relative humidity, and 

 generally a rise in temperature. Often, however, 

 the wind causes a drop in temperature, because it 

 mixes the lower layers of the atmosphere, which 

 have been heated by contact with the hot earth, 

 with the higher layers which have not been so heated. 

 A wind heavily laden with sand or dust, and blowing 

 for many hours on end, may cause a most marked 

 drop in the day's temperature, by shielding the land 

 from direct sunlight. 



vi. Evaporation 



Though on one hand the rainfall is meagre, on 

 the other the evaporation is excessive in all deserts 

 and at nearly all seasons. The rapidity with which 

 water evaporates, whether from the superficial layers 

 of the soil or from hollows in which it has accumu- 

 lated, is caused by the high diurnal temperatures, 

 by the direct rays of the sun beating down from a 

 cloudless sky on bare earth unprotected by a covering 

 of vegetation, and by the unimpeded action of the 



