50 THE FUTURE OF ARID LANDS 
cycle and the controlling factor in the hydrologic cycle. Some of 
the rain soaks into the ground, some runs off in streams, some is 
evaporated, and some is transpired by vegetation. That which is 
evaporated or transpired by plants, like stream waters, may 
travel great distances, but ultimately it again falls somewhere as 
rain, snow, hail, fog, or dew. In tropical areas rainfall may actually 
be beyond the capacity of man to use, as with the Amazon, but 
in arid and semi-arid areas, rainfall is low, and every effort must 
be made to conserve usage to the best ends. This applies even 
when an arid area is using imported precipitation as in Egypt, 
where the Nile valley is dependent in the main for its supply of 
water on the rainfall in the Abyssinian highlands. 
One of the first problems in the hydrologic cycle of arid areas 
is that of accurately measuring precipitation. Because rainfall 
is generally low, it can be very local and extremely sporadic, and 
this erratic distribution both geographically and in time makes 
for considerable discrepancies in accurate measurement. 
While on the subject of rainfall, thought naturally turns to 
the fascinating possibility of rain-making, and a preliminary 
report by the World Meteorological Organization entitled, 
“Artificial inducement of precipitation with special reference to 
the arid and semi-arid regions of the world,” prepared by the 
Technical Division of WMO Secretariat from reports received 
(4) should be noted: 
From the consideration of the regional reports quoted in this paper, 
the following conclusions might be drawn: 
1. Operations which have so far been carried out have produced re- 
sults that could be termed, at best, inconclusive; neither the complete 
failure of the methods employed nor the cértainty of getting substantial 
increases of rainfall have been demonstrated. 
2. The most favourable meteorological conditions for the artificial 
inducement of precipitation are to be sought in regions and during sea- 
sons where natural precipitation is most likely. 
3. Present day techniques, either “cold” or “warm” cloud seeding, 
have very little value, if any, in augmenting the precipitation in areas 
of very low rainfall or during dry periods in regions of normally medium 
rainfall. 
These tentative conclusions should not be taken as the expression of 
a negative attitude towards studies and experiments on the artificial 
