INFLUENCE UPON CLIMATE OF SURROUNDING COUNTRY. 17 



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wlien placed under the trees, the bahince rep resen tinker that which 

 passes through the foliage and drii»s to the ground or runs down along 

 the trunks of trees, or else is intercepted and evaporated. The x)er- 

 centage withheld by the trees and which either evaporates from their 

 surfiice or trickles along the trunk to the ground is somewhat greater 

 in the leafy season, though the difference is not great. Deciduous and 

 evergreen trees show hut slight differences in this respect. More rain 

 is usually caught by gauges at a given height above the forest crown 

 than at the same height in open lields, but it still remains doubtful 

 whether the rainfall itself is really larger over the forests, since the 

 recorded catch of the rain-gauge still requires a correction for the in- 

 fluence of the force of the wind at the gauge. 



In such cases where over a large area deforestation and reforestation 

 have seemingly gone hand in hand with decrease and increase of rain- 

 fall, the possible secular change in rainfall must also be considered. 

 Yet the experience of increased rainfall over the station at Lintzel 

 (p. 113), with increase of forest area, points strongly toward a i)ossible 

 interdependence under given conditions. 



By condensing dew, hoar frost, and ice on their branches, trees add 

 thereby a little to the precipitation which reaches the ground, and by 

 preventing the rai>id melting of snow more water remains available 

 under forest cover. 



The question as to the march of destructive hailstorms with reference 

 to forest areas, which seems settled for some regions in France, re- 

 mains in doubt for other, especially mountain, regions. 



From these statements we would expect as a con secpicnce of defor- 

 estation an effect on the climate of the deforested area in three direc- 

 tions, namely : («.) extremes of temi)erature of air as well as soil are 

 aggravated; (b) the average humidity of the air is lessened, and possi- 

 bly (c) the distribution of precipitation throughout the year, if not its 

 quantity, is changed. 



INFLUENCE OF FORESTS UPON THE CLIMATE OF THE SURROUND- 

 ING COUNTRY. 



(1) An influence of the forest upon the climate of its surroundings 

 can only take place by means of diffusion of the vapor which is tran- 

 spired and evaporated by tlic crowns and by means of air currents pass- 

 ing through and above the Ibrests being modified in temperature and 

 moisture conditions; the mechanical effect upon such air currents by 

 which they are retarded in their progress may also be effective in 

 changing their climatic value. 



(2) Local air currents are set up by the diflxjrence in temperature of 

 the air within and without the forest, analogously to tliose of a lake or 

 pond, cooler currents coming from the forest during the day in the 

 lower strata and warmer currents during the night in tlic upper strata. 



12444— Xo. 7 2 



