DESERT WIND STORMS. 361 



course subsequently forced to continue its march, as 

 their water supply had run short. * 



The occurrence of these furious squalls and tempests 

 is, however, only a natural consequence of the sudden 

 and extreme alternations of heat and cold, to which 

 all very dry regions are subject. When we remember 

 that the winds themselves are caused by changes of 

 temperature, it must be evident that these phenomena 

 of sudden storms are only what might be expected to 

 occur everywhere throughout the desert zone. These 

 questions have, however, been fully commented upon in 

 the Section on " Climates and Temperatures, " to which 

 we beg to refer the reader. 



The period of the day when the solar heat is at its 

 maximum, it may be desirable to repeat, is generally 

 in the afternoon, say about three p.m., and conse- 

 quently these storms are much more common in the 

 afternoons than in the mornings, though of course they 

 may occasionally occur at any hour, especially during 

 the prevalence of the simoom or hot wind. 



Such storms usually, however, come up shortly before 

 sunset,f because it is at that hour that the changes 

 of temperature begin to be very marked, but the wind 

 does not in general attain its greatest intensity until 

 after the night has fallen, because it is not till then 

 that the fall in the temperature has become consider- 

 able the reduction in the temperature after nightfall 

 being generally very rapid. Count D'Escayrac de 



* See Le Grand Desert, Itineraire (Tune Caravane du Sahara au 

 Pays des Negres, par Eugene Daumas (French General), Paris 1848, 

 pp. 299301. 



y Le Desert et Le Soudan, par M. Le Comte D'Escayrac de Lauture, 



1853, P- 38. 

 Ibid., p. 36. 



