On these rough assumptions, if R is the mean annual rain as measured by a gauges 

 E is the effective mean, and T is the mean number of years or fractions of a year be- 

 tween effective storms, we have E = c/T, and R = 3E = 3c/T, where c is the effective 

 storm constant which I will take as 18m/m for the Libyan Desert. For the neighbour- 

 hood of Cairo, where R = 40m/m, we get T = 1.5 years, and we should therefore expect 

 patches of blown sand, away from the concentration in wadis, to become green most 

 but not every year, which is about right. The assumed constant for an effective storm 

 and the ratio of effective to total rain are of course very tentative, and need investi- 

 gation. But the general idea may prove useful in default of any other means of esti- 

 mating infrequent rain. There are I believe no permanently inhabited places in the 

 world where an effective storm has not occurred in living memory. And the experience 

 of travellers in the reriote interior of the Libyan Desert suggests that this applies 

 even here too. Odd bits of local information from this desert seem to indicate a general 

 figure for T between 30 and 50 years, reduced to 4 to 10 years for the few isolated bits 

 of high ground. Taking therefore a general figure of 35 years for T for the Libyan 

 Desert as a whole, we get a mean effective annual rainfall at the present day of half a 

 millimetre and a quite unmeasurable gauge figure of perhaps three times this. 



Nomadic Life 



Rain over a great desert region does not fall everywhere at the same time, or in 

 the same year. Nomadism depends on this fact. It enables a whole tribe to live per- 

 manently in an area where effective rain falls at any one place only once in two or 

 more years. An extreme case is that of the indigenous Libyan Desert Tibu who till 

 recently wandered in small groups across hundreds of kilometres of lifeless 30- 50 

 year country from one favoured hill spot of 4 to 10 year rain to another, with a few 

 sheep or goats and even with a cow. Wild nomad fauna such as addax antelope seem 

 to roam over the same rainfall range. We also have the semi -nomad, based on the 

 desert fringe, who in certain years migrates desertwards with his cattle, but without 

 water, for the grazing to be had off 3 to 5 year areas, and himself drinks nothing but 

 his cattle's milk for six months or more. 



Civilisation seems to have overlooked the nomad way of life, even though it ex- 

 ports meat. Surely no other way could be persuaded to produce anything at all from 

 large areas of the world. But for some reason one never hears it suggested that nomad- 

 ism might be encouraged and may- be modernised. Better varieties of the specialised 

 herbage might be introduced gradually, better control of grazing, radio for the more 

 rapid.spread of news of rain elsewhere. Why not, if we wish seriously to improve the 

 productivity of deserts? As things are, nomadism tends to be discouraged as a politi- 

 cal nuisance. If traditional nomadism is allowed to die, as it is rapidly doing, for ex- 

 ample where oil - fields are being developed, the chances of re-creating this way of 

 life seem remote. Vast areas which can now produce and export at least some food 

 will then be permanently unproductive. 



Effect of Small Long- Period Rainfall Changes 



In extreme cases of aridity where the remembered rainless period approaches the 

 span of human life it is of course iirpossible to get at the real mean period. This 



11 



