THE DESERT CLIMATE 29 



sand upon rock is illustrated in Fig. 11 (facing 

 p. 42). 



The hot air which rises from contact with the 

 heated desert sometimes makes its way upwards in 

 vortices, or dust-devils. The dust-devil is a rotating 

 column of air, and is generally seen on very hot days 

 on which a moderate breeze is blowing. It is very 

 sharply delimited, and owing to its load of debris, 

 dust, and bushes, it is easily visible. It moves 

 slowly about in the desert, following the direction 

 of the wind rather irregularly. The height of the 

 vortex is variable ; a century ago Lane measured 

 one of 750 feet in Egypt and saw others taller, but 

 the majority were shorter ; they were continuous 

 columns reaching up from the ground for several 

 hundreds of feet. Normand observed dust-devils 

 at Samarrah in Northern Mesopotamia, some of 

 which were at least 300 metres high, though only 

 about 5 metres wide at the base. Sometimes these 

 vortices lose contact with the ground, but maintain 

 their existence at very great heights. Flying officers 

 in Mesopotamia have encountered vortices of this 

 sort whirling upwards at a height of 5,000 feet above 

 the ground and still carrying dust and debris. Even 

 a very small dust-devil is violent ; I have had my 

 shirt stripped from my back by a vortex which 

 seemed to be only a dozen feet in diameter, and 

 during the operations in Mesopotamia and North- 

 West Persia it was no uncommon sight to see a 

 particular tent struck and emptied of its contents 

 by a small vortex, which left untouched other tents 

 only a few yards away. Larger vortices are much 



