14 LOCALITY AND FOREST VEGETATION. 



and those between north-east and north-west the coldest. The 

 de«:,'ree of the gradient further niodilies this effect, which also 

 depends on the latitude. 



The aspect atfects the temperature also in exposing a 

 locality to air currents, or protecting it against them. This 

 effect may be favourable or the reverse according to the nature 

 of the air currents. 



(I. Pn-scncf or Absence of Forest Vegelaiioii. 



Localities, which are bare of vegetation, are struck In- the 

 full force of the sun's rays, causing the temperature at the 

 surface of the soil to rise to the highest possible degree. At the 

 same time, air currents sweep unimpeded over such localities, 

 causing a rapid change of the atmosphere. 



On localities covered with a full crop of forest vegetation 

 the sun's rays strike the crowns of the trees ; the heat 

 absorbed by the air at some height above the ground, 

 penetrates only slowly through the leaf canopy to the layer 

 of air belo^Y it and thence to the soil. During the night again 

 the leaf canopy prevents, or at any rate reduces, radiation. 

 It follows that the air in forests is cooler during the day and 

 warmer during the night, than the air on bare localities. This 

 effect is intensified by the fact that the foliage of the trees 

 impedes the force of air currents. 



Direct observations have established the following facts : — 



(1.) The climate of wooded countries is more ecjuuble than 

 that of open countries. 



(2.) The mean temperature of soil and air in wooded 

 countries is somewhat lower than that of soil and air in 

 bare countries. This reduction of temperature would 

 ordinarily act beneficially in warm southern countries, while 

 it may become injurious in cold northern countries where 

 the temperature is already lower than is desirable. 



(3.) The greatest difference occurs in summer, next in 

 spring, then in autumn, and it is very small in winter. It 

 follows that in forests the commencement of vegetation in 



