An arid region is one where the precipitation is much less than the world average 

 and therefore where the amount of water as vapour in the air is much less than the 

 average also. This state of affairs can come about in two ways — either the air has 

 lost the water it originally had or else it never had a very good supply. To put this in 

 a rryare scientific way, the air over an arid region is far below saturation point, and it 

 may reach that state because its temperature has been raised after losing its original 

 supply or because its original temperature was low and therefore it never had a good 

 supply. 



We may illustrate this by referring to the conditions on that narrow but very ex- 

 treme desert of the Namib which stretches along the coast of South-west Africa for 

 hundreds of miles. In the rainy season for Southern Africa, roughly from November to 

 Kiarch, the easterly winds from the Indian Ocean bring warm and nearly saturated air 

 inland. Forced up over the Drakensberg range and the plateau behind it is cooled and 

 parts with much of its vapour as rain. When it reaches the so-called Kalahari Desert 

 it still has enough vapour to produce some thunderstorm rain, and if it does not fall 

 there it will fall further west where the higher land of South - west Africa cools it. The 

 air then descends to the coast and warms up as it goes so that at sea- level it is so 

 far below saturation point that even the cold Benguela current can only produce an oc- 

 casional fog. 



At other times of the year there are occasional drifts of air from the Atlantic in- 

 land over the Namib, but it has come from the cold surface current and again is so far 

 below saturation point that it cannot do better than an occasional mist. We should 

 note that this fogginess may be, and in the Namib often is, a very important factor for 

 the biology of that area; but nevertheless it remains a very severe desert. 



We may therefore think of most deserts as largely due to their occupying vast 

 areas of rain- shadow, that is to say, areas which are on the lee side of land which 

 has robbed the air of most of its precipitable moisture. Narrow coastal deserts such 

 as those of South- west Africa and Chile add the effect of a cold current off-shore and 

 are more arid still. This simple explanation of the distribution of deserts will, no 

 doubt, be amplified by papers later in this symposium. 



Yet no desert is completely and permanently without water, just as the air above 

 it is n^ver completely dry, and it is as well that biologists should realise just how 

 moisture does reach the ground to sustain such desert life as exists. 



Dry air means clear skies and clear skies mean excessive insolation by day and 

 radiation by night — the two .processes which are mainly responsible for some degree 

 of precipitation. The rapid heating of the land by the sun by day induces rising cur- 

 rents of air, which are usually local in extent. On the smaller scale these produce the 

 familiar dust- whirlwinds which have so many curious names in different parts of the 

 world. On the larger scale these upward currents will take the air high enough to cool 

 down by adiabatic expansion and even reach dewpoint, so that clouds are formed — 

 usually of the cumulus type, since the release of heat within them still further accel- 

 erates the rate of ascent of air. These often produce rain, as their fuzzy under- sur- 

 faces show, but it is rain which rarely reaches the ground, and indeed one can see it 



