EFFECT OF VEGETATION ON THE RAINFALL OF S.A. 145 



writer is of the opinion that the vegetation of a country has a 

 very considerable effect on the number, extent, and intensity of 

 thunderstorms in summer. In the first place, it should be clearly 

 realized that vegetation puts a very considerable amount of 

 moisture into the atmosphere. It has been calculated that an 

 area covered with vegetation puts 25 per cent, more moisture 

 into the atmosphere than a free water surface in the same con- 

 ditions of wind velocity and temperature. Much depends on 

 the amount of vegetation and on the kind of vegetation. 



Forest trees give an enormous amount, so much so that 

 trees are often planted to dry up marshy districts, which defy 

 every other means. Now let us consider the case of the inland 

 plateau of South Africa. The air which reaches there, travel- 

 ling from the ocean, must go over fairly large tracts of territory, 

 some from the north, some from the south-east. If this sur- 

 roimding tract of territor}^ is very arid, then this air will .y-ain 

 in temperature, but not in moisture content ; consequently it will 

 only be in exceptional circumstances that you will get a con- 

 vectional rise sufficient to give rain. If, on the other hand, 

 the surrounding tract is well covered with vegetation, then the 

 air going over it will not be so much heated, because the tem- 

 perature of the land will be less, and, what is more important, 

 it will gain a considerable amount of moisture, but not neces- 

 sarily an increase in relative humidity. When the inland plateau 

 is reached, the causes which give rise to convection need onh 

 be present to a feeble extent to produce rain, consequently wv 

 should expect that the num'ber of rainy days would be increased. 

 and this is confirmed by experience. Now it is the universal 

 experience that the rainfall is greatest near the coasts, which 

 first receive the wind, and that it gradually decreases as you go 

 inland. The effect of a country well covered with vegetation 

 is to make the decrease very gradual in deed — that is to say, 

 to bring oceanic conditions much nearer to inland places. 



Consider the regions suitable for forest growth. The pre- 

 sence of forests allows the land to absorb readily the rainfall 

 which falls on it, and prevent the rapid run off to the sea 

 almost as soon as it has fallen. The forests afterwards slowly 

 transpire the moisture in the air, whence it is carried inland, 

 as a general rule, by the prevailing wind, and may again be 

 precipitated as rain there. 



In conclusion, the writer urges that all suitable areas shoul.l 

 be planted with trees, and that the wholesale denudation of tht- 

 land, whether of trees, bush, or grass, should be discouraged 

 as much as possible. This policy would enable the forest dis- 

 tricts to make use of what rainfall the}- had, and to confer at 

 the same time a benefit to those inland. Not only would those 

 inland get a larger rainfall, it would come in a much more useful 

 manner, and would be more evenly distributed over the rainy 

 sea.son. 



(Read, July 5, 191 7.) 



