FORESTS AND CLIMATE 3*1 



we say that there is no hope that we can increase our rainfall really 

 appreciably or effectively by any amount of tree-planting. A whole 

 ocean of water can not give rainfall if the general pressures and tem- 

 peratures and winds are hostile to precipitation. 



As was pointed out at the beginning of this paper, forests are of 

 many different kinds. We can not, therefore, reasonably expect all 

 forests to have the same effects. There may be a difference between 

 tropical and temperate forests, as has already been suggested in the 

 case of Java rainfall, for tropical weather types and rainfall conditions 

 are different from our own, just as tropical forests are different from 

 our own. Tropical rainfalls, as over the great forested Amazon valley, 

 are largely thunderstorm rains, and as forests tend to check air move- 

 ment, and calms are favorable conditions for convectional overturning, 

 it appears as if tropical forests might be expected to influence rainfall 

 more than our own. Furthermore, from the hot and damp tropical 

 forest, and from the leaves of the closely-crowded tropical trees, there 

 must come a large amount of moisture which will increase the vapor 

 content of the ascending air and tend to increase condensation and 

 rainfall. Thus Woeikof, whose emphasis on the case of Java has been 

 referred to, believes that in low latitudes the vast tropical forests do 

 increase the amount of rainfall. Von Hann, the leading authority 

 on climate, holds that we may conclude " with considerable certainty 

 that, at least in the tropics, the forest may increase the amount of 

 rainfall." Hettner, also, in his work in the tropical Cordillera, came 

 to the conclusion that the forests in the Cordillera of Bogota favor the 

 growth of clouds and the production of rain. While this is an inter- 

 esting phase of our discussion, we have as yet no thorough study of 

 tropical conditions by means of the parallel station method. There is 

 also another point. In low latitudes, where the dense tropical forests 

 are found, the rainfall is already so heavy that it is of little or no 

 significance whether there is a good deal more, or a good deal less. 

 In exactly those regions, therefore, where, if anywhere, forests may have 

 a really appreciable influence on rainfall, little or no economic impor- 

 tance attaches to the question. Woeikof believes that rain often begins 

 earlier over tropical forests, and in Mauritius, Walter has called atten- 

 tion to the fact that the number of rainy days seems to be greater over 

 forested areas. 



It need hardly be pointed out that, if rain is already falling, the 

 opportunity for it to reach the earth's surface must be better if it falls 

 through the somewhat cooler and damper air over a forest or a grass- 

 covered surface than through a hotter and drier stratum of air over a 

 desert. In the latter case the loss by evaporation may be so great that 

 the drops do not reach the surface at all. Obviously, the contrasts 

 between these two conditions are greatest in the case of the tropical 

 forests and tropical deserts. It must, however, be observed that this 



