EFFECT OF FORESTS ON MOISTURE. 37 



beech. At the same time, the greatest effect was recorded in 

 beech woods during summer, due to the dense leaf canopy 

 which exists in such woods at that season. 



The detailed figures show that the mean annual excess in 

 forests ranged from 3 to 10 per cent., which explains why dry 

 air currents striking through forests become in a short time 

 relatively moist, so that precipitations may be caused. At 

 any rate there can be no doubt that the formation of dew is 

 much greater in the vicinity of forests than on open ground 

 away from woodlands. 



h. Prpcipitafions, or liainfall. 



Tlie question, whether, and in how far, forests affect the 

 rainfall, is one which has been actively discussed for many 

 years past, but so far no final conclusion has been possible. 

 That forests can affect precipitations follows from the facts 

 that forest air is relatively moister than air in the open, and 

 that the trees mechanically affect the movement of the air ; 

 but, on the other hand, the rainfall depends chiefly on other 

 much more powerful agencies in comparison with which the 

 effect of forests is small. Numerous comparative observations 

 have been made, but only a certain portion has so far been 

 published, and unfortunately those which seem to indicate a 

 decided effect of forests on the rainfall are not always very 

 reliable. The great difficulty in comparing the results of 

 observations at forest stations (that is to say, stations situated 

 inside forests) with those of the ordinary meteorological 

 stations is that elevation above the sea affects the rainfall 

 most powerfully, because air cools on rising and precipitations 

 become more frequent with elevation. 



Although further observations are necessary before a final 

 conclusion can be arrived at, the following data may prove 

 interesting : — 



In the Prussian system the forest stations have shown the 

 subjoined increase of rainfall over ilie average rainfall of the 



