THE DESERT CLIMATE 27 



The ordinary statistics of relative humidity pub- 

 lished by official meteorologists are very unsatis- 

 factory for the biologist. In many places readings 

 are only taken in the morning and evening, and the 

 full stress of extremely low humidity which oppresses 

 the flora and fauna at midday passes unrecorded. 

 Furthermore, readings are often taken at a place 

 convenient to the observer, seldom out in the desert, 

 and the readings of humidity suffer perhaps more 

 than any others because of the variations which 

 occur between places a few yards inside or outside 

 an oasis. It is also to be remarked that readings 

 are sometimes taken at 8 a.m. and 8 p.m., the two 

 times at which the humidity is changing most rapidly. 

 If, therefore, the reading is taken a few minutes 

 too soon or too late a considerable error may be 

 produced. 



V. Wind 



Rapid fluctuation in the temperature of the surface 

 of the desert, and of the air above it, results in air- 

 currents, which are generally greater in summer than 

 in winter. At any rate, in the Great Palaearctic 

 Desert a calm during the middle of the day is rare, 

 and in many places winds of constant direction are 

 experienced at this season. The sirocco is a hot 

 easterly or southerly wind which blows in Palestine, 

 Algeria, and other parts of the Mediterranean littoral ; 

 the shamal, in Mesopotamia, blows from the north- 

 west for many days at intervals all through the 

 summer. During June, July, and August, which 



