EVAPORATION. 63 



cubic foot of air is generally the same in both places, 

 but the forest air is cooler, and therefore its relative 

 humidity is greater. Relative humidity is the amount 

 of vapor actually in the air, expressed as so much per 

 cent of all it could hold at the same temperature. The 

 amount of water that the air can hold changes when 

 the temperature changes, but in such a way that air 

 cooled until it is onl} 7 half as warm as before can hold 

 much less than half as much vapor. If a hot and a 

 cold stream of air, both saturated with water vapor, 

 meet and mix, the mixture can no longer hold as much 

 vapor as the two streams separatery, and a part is con- 

 densed, usually in the form of rain or snow. German 

 and Swiss observations have shown that the average 

 humidity is greater in the forest by from 8 to 10 per 

 cent. This difference increases with the altitude above 

 sea level and the density of the forest cover. The in- 

 crease of humidity explains why dew is more frequent 

 in the neighborhood of the forest than at a distance. 



EVAPORATION. 



The Avater which falls to the earth from the atmos- 

 phere had first to be eA r aporated, so that year by year 

 the quantity of water which the air takes from the sur- 

 face of the globe by evaporation is the same as that 

 which falls upon it in the shape of rain, hail, snow, and 

 dew. The effect of the forest on this great movement 

 of water is to detain more of it on those portions of the 

 earth which are sheltered by trees. It does this partly 

 by tending to increase the rainfall, but its effect in less- 

 ening the loss of water through evaporation is prob- 

 ably much more important. The colder and moister 

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