FORESTS AND CLIMATE 3 21 



crop-covered surfaces. It is also a well-known fact that a certain por- 

 tion 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 for- 

 ests 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 sup- 

 plied 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 



It is beyond any question that a forest can not increase the moisture-content 

 of the atmosphere as a whole. On the contrary, it takes from the air a large 

 amount of moisture which has been brought from the ocean by warm ascending 

 currents. Indeed, under certain weather conditions extended forests even favor 

 a decrease of cloudiness by producing a descending current of air, in contrast 

 with the ascending current produced over an easily-warmed open field. 



Influence of Forests upon Eainfall: Why do We Expect It? 

 Thus we come to the phase of the discussion which is of much the 

 greatest popular interest. Do forests increase rainfall? Does defor- 

 estation 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 direc- 

 tion. 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 move- 

 ment 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 fo/est, provided the existing conditions 

 are already favorable. It has even been held by some that when the 



