Page:Popular Science Monthly Volume 82.djvu/325

Rh crop-covered surfaces. It is also a well-known fact that a certain portion of the rainfall of continental interiors is supplied from secondary sources not the ocean, such as lakes, rivers, swamps, and to some slight extent even from the forests themselves. But the forests must of course have received the water before they can give it up; they can not supply it by and through themselves. There seems to be no really very good reason for thinking that the rainfall conditions of the interior of North America would be very much changed if all the forests bordering on the coasts were replaced by crops or by grass. It is foolish for us to think that the forests are more important than the ocean in supplying water vapor for rainfall. Without the rainfall supplied by the vapor evaporated from the oceans the existing forests would never have grown at all. The amounts of moisture concerned in the great rain-producing processes of the atmosphere are so large that the local supply from forests can not conceivably play any considerable part. A recent German writer has stated his opinion that

Thus we come to the phase of the discussion which is of much the greatest popular interest. Do forests increase rainfall? Does deforestation result in a decrease of rainfall? It is almost inevitable that the majority of persons should approach these questions with a fairly strong prejudice on the affirmative side. There is the general and universal impression in favor of such an influence, already referred to in the opening paragraph of this paper. In addition, the theoretical considerations above enumerated turn our thoughts in the same direction. By way of a review, then, let us ask, What are our reasons, at this stage of our discussion, for thinking that forests may influence rainfall? First, the barrier and frictional effect, which, by forcing horizontal air currents to rise, should tend to favor condensation, as cloud, and perhaps also as rainfall. The slackening of the air movement above an extended forest ought to increase the thickness of the stratum of moving air, thus giving it a slight, and local, ascending component. This same slackening effect should produce a tendency to light winds and calms, which are often favorable to showers and local thunderstorms, especially if the air is damp. Second, the damper and slightly cooler air in and over a forest may, at least to a slight extent, affect the passing air currents, especially if these are warm and dry, perhaps increasing the tendency to form local fogs, dew, or even light rain over and to leeward of the forest, provided the existing conditions are already favorable. It has even been held by some that when the