9 



PRECIPITATIONS OR RAINFALL. 



The 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 decision 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 mechanical^ 

 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 a forest) with those of the 

 ordinary meteorological stations consists in the fact 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 the average rainfall of the open country as taken from the 

 ordinary meteorological stations : 



Excess of rainfall in forest station over that of open 

 country, in per cent, of the latter rainfall- 



Between sea level and 328 feet elevation. . . . 1.25 per cent. 



328 and 556 " 14.2 



1,969 " 2,297 19. 



2,297 " 2,625 43. 



Although these figures may not represent the absolute facts of the case, they 

 seem to indicate that in the plains forests have very little effect upon the rain- 

 fall, if any at all, but that their influence becomes considerable with increasing 

 elevation in mountainous countries. 



The results of seven years' observations made at two stations near Nancy 

 show a decided increase of rainfall in the forest. The stations are situated 1,247 

 feet above the sea, one in the middle of an extensive forest five miles to the west 

 of Nancy, the other in an almost woodless country six miles to the north-east of 

 Nancy. 



The results were as follows : 



Increase of rainfall in forest over that in the open in per 

 cent, of the latter- 

 February to April 7 per cent. 



May to July 13 



August to. October 23 



November to January 21 



Mean of year 16 



EVAPORATION. 



Owing to the lower temperature, the greater humidity of the air, and the 

 quieter state of the atmosphere, evaporation must be considerably smaller in 



