28 Forests — their Influence upon Climate and Rainfall. [January, 



The demonstrable variation in temperature of the moist air of 

 the woods and the currents outside, and the mingling of these, 

 doubtless reduce the temperature sufficiently to cause local pre- 

 cipitation. At first sight it might appear impossible that this 

 could result in anything so tangible as rain, but we must examine 

 this question carefully. During the warm season the temperature 

 of the air in the forest is lower than that of the air outside, which 

 is due in a measure to the trees intercepting the rays of the sun, 

 causing shade, which has a cooling effect, and partly also as 

 pointed out by Pettenkofer (Pop. Sci. Monthly for Feb., 1878), to 

 'the slight draught which is always caused by shade in the open 

 air. Every one who has ever passed from the open air on a 

 hot mid-summer day to within the borders of a forest, must 

 have experienced with a relish the refreshing influence of the 

 shade. Again, the temperature of the trees of a forest, and even 

 their tops, is found to be lower than the air in the forest. This fact 

 is easily explained : the rapid evaporation of watery vapor from 

 the leaves, as shown by our researches, renders the action of the 

 solar rays neutral, and their temperature is somewhat reduced.^ 

 The observation has been made (according to Pettenkofer) that 

 the trunks of trees breast high, even at the hottest time of day, 

 are 5 Centigrade cooler than the air of the forest. Ebermayer 

 speaks of the temperature of the trees in a forest as being always 

 lower than the air of the forest. 



As already indicated by the present researches, forests moisten 

 the air over, in and to some extent around themselves. Now in 

 the light of these facts may we not be pardoned for concluding 

 that warm currents sweeping over a country and striking the 

 cool, moist air in and above the forest, and mingling with it would 

 have a portion, at least, of the contained moisture condensed into 

 gentle showers, extending their beneficent influence to neighbor- 

 ing fields? Again, let some stray current come along of a lower 

 temperature than the air of the forest, and the moist air over the 

 forest would readily be condensed, since it is a well-known fact 

 that a moist air discharges its vapor more readily in the form of 

 rain than a dryer atmosphere. We have now seen how trees can 

 cause local rains ; it will also be observed that the rain is formed 

 chiefly above the forest, though it may be through the influence 

 of winds that it falls to the earth for some distance around. By 

 increasing the frequency of light rains, forests tend to obviate 



