INFLUENCE OF FORESTS 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 Temperature 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 
10° 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——trThis 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 evaporate 
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 
