188 REPORT OS THE FORESTRY COMMITTEE 



Stance produced, then an acre will evaporate during the vegetative period about 

 3,600 tons of water. This example shows the extent to which ordinary crops can 

 contribute to the moisture content of the air and the cooling which accompanies 

 this evaporation. Forests, being the most highly developed form of vegetable 

 life, exert this influence in the greatest degree. 



The cooling effect of forests upon the temperature of the air has been proven 

 by long series of observations continued for a long number of years in France, 

 Germany, Austria, Switzerland, and now in the United States. All these ob- 

 servations, while they may differ as to the absolute figures, confirm each other. 

 The yearly mean temperature at equal elevations and in the same locality has in- 

 variably been found to be less inside than outside the forest. In a level country 

 this difference is about 0.9 degree F. It increases, however, with latitude, and at 

 an elevation of about 3,000 feet is 1.8 degrees F. 



The monthly mean temperature is less in the forest than in the open for each 

 month of the year, but the difference is greatest during the summer months, 

 when it may reach 3.6 degrees F., while in winter it does not often exceed 0.1 de- 

 gree F. 



The daily mean temperature shows the same condition, but to a greater de- 

 gree. During the hottest days the air inside the forests was more than 5 degrees 

 F., cooler than that outside, while for the coldest days of the year the difference 

 was only 1.8 degrees F. 



The temperature of the air within the forests is therefore not only lower, 

 but also subject to less fluctuation, than in the open. 



In tropical and subtropical regions the influence of the forest upon the tem- 

 perature of the air is the greatest. The remarkably moderate temperature of the 

 Amazon basin, whose geographical position near the Equator over 600 miles 

 away from the Atlantic Ocean and isolated by high mountains from the Pacific 

 Ocean, should have made it one of the hottest places in the globe, is^ attributed to 

 the cooling effect of the enormous forest areas found there. 



TEMPERATURE OF THE SOIL. 



Forests influence the temperature of the soil in almost the same way as they 

 do that of the air. The forest soil is warmer in winter by 1.8 degrees F. and cool- 

 er in summer by from 5.4 degrees to 9 degrees F. than soil without forest cover, 

 and this holds true at a depth of at least 4 feet. In the spring, and especially in 

 the summer, the forest soil is cooler than that of open land. In the fall and 

 winter, however, it is warmer, but the degree of difference is always less than in 

 summer. 



RELATIVE HUMIDITY. 



The relative humidity of the air is higher in the summer in the forest than in 

 the open. This difference is usually between 4 and 10 per cent, but in some places 

 m?y be as much as 12 per cent. In regions of heavy snow there is practically no 

 difference in the relative humidity during the spring when the snow melts The 

 reason for the higher relative humidity of the air in the forest is because the 



