i6— MAN'S INFLUENCE ON CLIMATE. 



By T. R. Sim, F.L.S. 



Man has the power to exert an influence on climate in several' 

 ways. Among these may be mentioned : — 



1. A local climatic change is effected by the production of 

 shelter, whether by a wall or hedge, a clump of trees or a larger 

 plantation, in the same way as a mountain affects the climate on its 

 sheltered side. This is often used for the protection of tender 

 horticultural crops. 



2. A local climatic change is effected by drainage. Drainage 

 is in common use in agriculture, not simply for drying the land, 

 but also on purpose to make the climate more suitable. 



Where levels do not allow mechanical drainage the use of quick- 

 growing trees may answer the same purpose by absorbing the moisture 

 and passing it into the air. 



3. A local atmospheric change is produced in towns, and in 

 the neighbourhood of factories, foundries, mines, etc., by the 

 emission into the atmosphere of smoke in sufficient quantity to 

 produce haze and fog, or of gases having a deleterious effect on 

 vegetable as well as on animal life. A smoke " smudge "" is used 

 in horticulture to prevent damage from frost. 



4. But a much more important change than either of these is 

 effected by the production of abundant foliage, especially that of 

 trees ; and by the maintenance of a cool moist canopy thereby. 



The result in all cases is the effect of a natural law under which 

 at a high temperature the atmosphere can carry a larger amount of 

 moisture than it can at a low temperature. Consequently, when a 

 warm air-current passing over the sea landw'ard absorbs from the 

 sea its full proportion of moisture, and afterwards comes in contact 

 with a colder surface or a colder air-current, it is no longer able 

 to maintain the moisture it contains, and precipitation as rain follows. 



A warm moisture-laden cloud striking against a cold mountain 

 or cool forest has its own temperature reduced thereby, and gives 

 off part of its moisture as rain, mist or dew, while in the forest the 

 natural tendency where there is a mass of foliage o\'erhead and a 

 more or less open space below is to produce under this canopy a 

 considerable mass of an atmosphere much colder and more per- 

 manently cold than that existing where the canopy is absent. This 

 coldness is further augmented where the humus produced by decaying 

 leaves forms a deep hygroscopic mass in which every drop of rain 

 that falls is retained instead of running off at once into some 

 stream. 



E\ery degree of cold added means an additional precipitation 

 nf rain, dew or mist. Man's action, therefore, in maintaining and 

 in producing forests of a satisfactory class, has a most beneficial 

 climatic effect where, as in South Africa, more moisture is desired, 

 while the opposite is the case in very cold and forest-clad countries. 



When, as in South Africa, the total forest area is small, and 

 part is thinned out without such protection being afforded as allows 



