THE GEOGRAPHY OF DESERTS 



Professor Frank Debenham O.B.E. 

 (Cambridge) 



This paper, being introductory to a series which deals with special aspects of 

 dbserts, can be little more than a review of where deserts occur on the world's surface 

 and why, with some reminders of how many and varied are the factors which may com- 

 bine to produce a desert. 



In broadest outline the origin of an arid zone on land is simple enough, since it 

 is caused by an interruption or suspension of the exchange of water from sea to land 

 via the air in a certain area. They are in fact due to flaws in the fundamental cycle 

 on which all life on land depends, the cycle which raises v/ater by evaporation from 

 the sea, against gravity, moves it over the land, where it is precipitated, and then, in 

 part, brings it back to the sea again by gravity. 



The primary factor in the distribution of deserts is therefore the scheme of world 

 circulation of air, since an on- shore wind can bring water from sea to land whereas an 

 off- shore wind cannot. On the continental scale, indeed, arid zones must occur on the 

 lee side of the land with respect to the prevalent winds. 



That appears at first sight to be a neat explanation of the occurrence of deserts 

 on the western side of continents in the southern hemisphere round about the latitude 

 of the Tropic of Capricorn, but by itself it is by no means an adequate one. It leaves 

 out of account at least two major factors — first, the temperature of the sea which is to 

 yield the water as vapour and, second, the seasonal swing of the zone of Equatorial 

 pressure, which together vv^ith the polar areas appears to govern the circulation of the 

 atmosphere. 



The temperature of the sea, again, is dependent on ocean currents, themselves 

 mainly due to prevalent winds far away from the actual arid zones. These, in the com- 

 paratively simple set of circumstances of the southern hemisphere, tend to bring warm 

 equatorial water to the eastern sides of the continents and cold polar water to the 

 western sides. 



Added to that component we must note too that a continuous off- shore wind tends 

 to bring up cold bottom water to the surface of the sea, while an on-shore wind piles 

 up warmer surface water against the land. The cold water will extract water vapour 

 from the air in the form of fogs and mists, while the warm water on the other side will 

 give itself up to the atmosphere. 



The argument as to the origin of arid regions is already becoming conplicated, 

 yet we have not mentioned several other factors which must bear on the matter, such 

 as the distribution of the land masses, the topography of the land itself, and the in- 

 cidence of those enormous eddies in the atmosphere which we are accustomed to call 

 'depressions' and which appear to upset the neat pattern of world circulation of air in 

 both plan and elevation. It might be wise therefore to leave the argument at that point 

 and view the occurrence of deserts from another and simpler angle. 



