92 OUTLINES OF FORESTRY. 



Consequently, any increase in the temperature 

 of air permits it to hold a greater quantity of 

 vapor. Conversely, any decrease in the tempera- 

 ture of air causes its ability to hold moisture as 

 vapor to decrease. 



If, therefore, the temperature of the air be suffi- 

 ciently decreased, a part of the vapor it contains 

 will appear in some visible form. 



3. On the quantity of vapor already in the air. 

 When a given bulk of air has as much vapor in 



it as it can hold, all evaporation ceases. Conse- 

 quently, the drier the air over a water surface, the 

 greater is the rapidity of evaporation. 



4. On the velocity of the wind. 



The wind brings fresh and drier air to the 

 water surfaces, and at the same time removes the 

 air into which such surfaces were discharging 

 their vapor. An increase in the velocity of the 

 wind, therefore, increases the rapidity of evapora- 

 tion. 



5. On the pressure of the air. 



The greater pressure the air exerts on a water 

 surface, the slower the rapidity of evaporation. A 

 low barometer permits a water surface to throw off 

 its vapor with much greater rapidity than a high 

 barometer. 



