EFFECT OF FORESTS ON TEMPERATURE. 27 



In order to concentrate the data into the smallest possible 

 space, they have been arranged according to seasons in the 

 following manner : — 



Spring comprises March, April and May. 



Summer ,, June, July and August. 



Autumn „ September, October and November. 



Winter ,, December, January and February. 



The subject of the indirect efl'ects of forests will be dealt 

 with under the following headings :— 



(1.) Effect of forests on the temperature of the air and 

 soil. 



(2.) Effect of forests on the moisture of tlie air and the 

 movement of water in nature. 



(3.) Mechanical effect of forests. 



(4.) Hygienic effect of forests. 



(5.) /Esthetic effect of forests. 



1. Effect of Forests on the Temperature of the Air and Soil, 



a. Tenqm-alure of Hie Air. 



The observations made at the double stations mentioned 

 above have shown that forests, on the whole, slightly reduce 

 the mean annual temperature of the air. The general 

 average of all stations shows that, under the 50'^ northern 

 latitude, the whole effect of a complete forest growth is a 

 reduction of about 1° Fahr. at 5 feet above the ground 

 and '4° in the crowns, as compared with the temperature at 

 5 feet from the ground in the open ; somewhat more than 

 the average in mountainous countries and somewhat less in 

 the plains. Evergreen trees with a dense foliage, such as 

 silver fir and spruce, have the most powerful effect, while 

 deciduous thin-crowned trees exercise little effect. On the 

 whole, it is clear that there is a slight decrease of temperature 

 in forests from the region immediately above the crown 

 towards the ground. 



