What Causes Rainfall?


Now if we could find that forests increase rainfall, as has frequently been asserted, we'd have an automatic and endless chain reforesting service for a fact! More trees to make more rain to make more trees to make more rain! It wouldn't take long to restore all the lost millions of acres of goodly trees that we now need so badly. But, unfortunately for the larger claims in this direction, we know, from the pages in the front part of our geographies, that the causes of rain are, for the most part, wholly beyond the influence of the forest. These causes are the great currents of warm and cold water in the ocean, the presence or absence of mountain ranges, and the direction of the prevailing winds.

Nevertheless, careful observation and records show that forests do affect rainfall to some extent, owing to their colder and moister air and the resistance which they offer to the movement of the winds. Forest air, when sufficiently moist and sufficiently cool, will precipitate its moisture as rain, or coming in contact with warmer, moisture-laden winds will cause this moisture to fall as rain. The effect of a forest in checking moisture-laden winds is something like that of mountain ranges, although, in the case of the mountain ranges, there is an additional element. Warm, moist winds striking a mountain are forced up the incline and, owing to the fact that air expands as it rises, are thereby cooled and so cause rainfall.


But when we come to speak of weather in broader terms, that is when we speak of the climate of a given region, the case for the trees is much stronger. What we call "climate" is the average condition of the weather; and this depends, not only on the distance of the region from the equator, its elevation above the sea, the distribution of land and water, the relief of the land -- as flat or hilly or mountainous -- and the direction of the prevailing winds, but, to an important degree, to the extent of its woodlands. We have already seen the several ways in which the woods not only cool the air within their own borders but actually create breezes. Woods also, as we saw in the March chapter, ward off bad weather when it comes riding in on the winds -- the biting blasts of winter and the hot, dry blasts of summer. The trees are all the more successful in this because they are such skilful wrestlers. They don't oppose the winds too stiffly, but "give in" a little, and "sidestep" their onrushes. They do this by the elastic swaying of the twigs and branches and, in the case of younger trees, the swaying of the trunks also.

Whatever limitations there are in the abilities of the trees to produce rain, there are none when it comes to taking care of it after it has fallen. What with the leaves and the mould and the undergrowth that make up the forest floor, forests have been known to hold a rainfall of five inches; and most of this rainfall, after meeting the "current expenses" of the trees and the requirements of a conservative "reserve system," in the little water reservoir cells, is returned to the springs and "the brooks that babble by," and so passes on to the rivers and the sea, where it again being its endless round of service.

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