would need many centuries of records. Indeed on this time scale the mean rain regime 

 may never be constant. And a small climatic change would have a very marked biolo- 

 gical effect. Using the rough rainfall scale I have mentioned earlier, an increase of 

 8m/m only in the mean effective annual rainfall (say 24m/m by gauge) would make 

 nomad life possible over most of the now lifeless core of the Libyan Desert. Thirty 

 to fifty year country would get an effective storm every other year. This must roughly 

 have been the condition in Neolithic times, may -be until as recently as 2000 BC, over 

 the southern half or more of what is now dead land. Significantly one finds their camp 

 sites concentrated towards the sands. 



Effect of Cloud and the Season of Rain. Effect of Vi/ind Direction 



Excluding the monsoon deserts of Asia it is a general rule that the tropical fringe 

 of a dry desert gets summer rain whereas the temperate fringe gets its rain in winter. 

 But in spite of the higher temperature it seems easier for a general herb cover to re- 

 vive under conditions of infrequent summer than of winter rains. The likely explana- 

 tion lies in the more continuous cloudy period associated with the season of summer 

 rain. On the harsher temperate fringe the growth of occasional spring vegetation de- 

 pends markedly on the duration of the less frequent cloud periods after rain. 



There is a general tendency too for the winds of dry deserts to blow across them 

 towards the tropics. This may affect the methods adopted by specialised plants to 

 maintain themselves within their desert habitat by seed transportation. Where the wind 

 is very uni- directional as in the Libyan Desert one notices that on the fringe nearest 

 the temperate zone the desertward wind is made use of and plants of the 'tumbleweed* 

 type abound, whereas on the tropical leeward fringe the seeds or even whole trans- 

 portable plants tend to be barbed, to enable nomad fauna to carry them against the 

 wind. 



A more important wind effect is the carriage of loess- forming dust from the wind- 

 eroded desert core outwards to and far beyond the fringe. The quantity so transported 

 must be enormous. Good evidence exists^^) of a desert surface being lowered 23 metres 

 since mid- palaeolithic times — say 4cm. per century. It is interesting to speculate on 

 how much less fertile the surrounding lands would be without the benefit of the desert. 



The Biological Limit 



In most desert regions the biological limit is never reached. In the Libyan Desert 

 trees may live on purely local catchment in places specially favoured by shade and 

 underground storage where it is said to rain only once in 15 years, (mean effective an- 

 nual rainfall about Im/m perhaps). Jerboa have been found where no other local life 

 is apparent, but they seem limited to within say 50 km. downwind of seeding plants. 

 Maybe they get their moisture from dew. The most extreme ecology I know of is that 

 of the few hawks and snakes who live in utterly lifeless country where there is no lo- 

 cally produced nourishment at all. Their ecology must be based wholly on casualties 

 from trans -desert bird migrations. But this in a way is cheating. 



(1) G.W.Murray, 1951, Geogr. J.. 117 (4), All- A'iA. 



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