INFLUENCE OF FOKESTS ON CLIMATE 3 



headings the various ways in which forests affect climate in 

 temperate regions, as determined by observations taken in 

 France, Germany, and the United States. 



1. The Influence of Forests on TemperaUtrc of the Air and 

 of the Soil. — The mean annual temperature of the air inside 

 a forest is lowered about 1 ° F. on an average. This cooling 

 effect occurs mainly in summer, and is not noticeable in 

 winter. It is owing to the lowered temperature of the air 

 over wooded areas that balloons are observed to sink im- 

 mediately after they begin to move over a forest. The 

 temperature of the air inside a forest is, however, raised at 

 night ; and as a result, spring and autumn frosts are much 

 less frequent and less disastrous in wooded tracts than in 

 the open country. A richly afforested country has a lower 

 temperature in summer than a neighbouring country that 

 is bare of trees in the same latitude. Bosnia, which is 

 covered with forests, is 4° Fahr. cooler in summer than 

 denuded Herzegovina. The soil of the forest is warmer in 

 winter (about 2° F.) and cooler in summer (about 5° to 

 1 0° F.) than agricultural land outside. The relative humidity 

 of the air in the forest is greater than that in the neighbour- 

 ing open country. 



2. The Influence of Forests upon Rainfall. — Tliis is a 

 difficult and complicated subject, as the effect of forests on 

 the total amount of rainfall over a whole country is doubt- 

 ful. In the British Isles and Western Europe, where the 

 evaporation from the Atlantic Ocean plays the most im- 

 portant part in the precipitation over the land, the effect of 

 afforestation in increasing the general rainfall is probably 

 negligible. Where the precipitation over the land comes 

 from the land itself the effect of forests may be considerable. 

 The forest evaporates more water than any other kind of 

 vegetation cover, and much more than free water surfaces, 

 like seas and lakes. " An oak forest at Mariabrunn near 

 Vienna, which was 115 years old, was found to evayorate 

 daily, by transpiration through the leaves, about 2500 

 gallons of water per acre, corresponding to a rainfall of 

 3*5 inches per month, or a rainfall of 17*5 inches during a 



