REPORT FOB 1912 AND 1913 53 



Tlie forest lowers the temperature of the soil in summer and in- 

 creases the temperature of the soil in winter. This influence extends 

 to a depth of at least four feet. 



The relative humidity of the air during the summer is higher in 

 the forest than in the open. 



Forests increase hoth the abundance and frequency of local precipi- 

 tation over the areas they occupy, the excess of precipitation as com- 

 pared with that of adjoining unforested areas amounting in some 

 cases to more than 25 per cent. 



The influence of forests upon local precipitation is more marked in 

 the mountains than in the plains. 



The reason for an increase in the total amount of precipitation 

 over wooded areas as compared with that of barren and deforested 

 ones is due to : 



1. The tendency of moisture-bearing currents to precipitate their 

 moisture more readily above or near the forests than over bare or 

 cultivated fields at the same elevation because of the dampening and 

 chilling effect of the forests upon the atmosphere, which induces a 

 greater condensation of the water vapor. 



2. The air from forests contains a much larger amount of moisture 

 than that over bare or cultivated fields. 



3. The mechanical action of the trees themselves. When a cloud in 

 the mountains passes through a forest, the branches and the leaves of 

 the trees retard its movement. It comes, therefore, into a state when 

 it can no longer retain its moisture in suspension, just as a river 

 carrying sediment deposits part of it as soon as the rapidity of its 

 flow is diminished. The moisture from such clouds is intercepted 

 by the forest in the form of mist or drops of dew or crystals of hoar- 

 frost on the branches and foliage of the trees. 



Forests in broad continental valleys enrich with moisture the pre- 

 vailing air currents that pass over them, and thus enable larger quan- 

 tities of moisture to penetrate into the interior of the continent. The 

 destruction of such forests, especially if followed by weak, herbaceous 

 vegetation or complete baring of the ground, affects the climate, not 



