Forest Influences 315 



In Switzerland the change of temperature from the 

 normal, experienced under the influence of the Foeher, 

 or warm wind, has been noted as 28° to 31° F., and a re- 

 duction in relative humidity of 58 per cent. A Foeher 

 of twelve hours' duration has been known to "eat up" 

 entirely a snow cover of two and one-half feet. In Denver, 

 a Chinook has been known to induce a rise in temperature 

 of 57° F. in twenty-four hours (of which 36° in five minutes) , 

 while the relative humidity sank from 100 to 21 per cent. 



Anything that lessens the velocity of the wind lessens 

 evaporation : hence the influence of the forest. Any one 

 who has been in the heart of a dense forest knows that the 

 severest wind is not felt there. This is because the trees 

 impede the velocity of the wind, and consequently evapora- 

 tion in the forest is less. As one approaches the windward 

 side of the forest, one feels the wind more and more. 

 This shows that the degree of influence that the forest 

 exerts on the wind is in direct proportion to the size and 

 density of the forest. But even the narrow windbreaks 

 and shelter belts exert a very appreciable influence. 



King has found in experiments. made in Wisconsin that 

 the influence of even a thin stand of woodland on the rate 

 of evaporation was considerable. In one experiment made 

 in the month of May, the instruments were so placed as 

 to measure the evaporation to the leeward of a scant 

 hedgerow 6 to 8 feet high, containing a few trees 12 

 feet high and many open gaps. It was found that 

 at 300 feet from the hedge the evaporation was 30.1 per 

 cent greater than at 20 feet, and at 150 feet it was 7.2 

 per cent less than at 300 feet. The experiment was made 

 during a moist north wind. It is sufficiently evident, 



