THE WATER OF THE H.\BITAT 



25 



available for the purposes of the plant, while the whole humidity 

 of the air is the stimulus that controls the water loss of a leaf. 

 The actual amount of water present in the air is called the abso- 

 lute humidity, and is expressed in milligrams per cubic centi- 

 meter. The relative humidity is the relation between the amount 

 of moisture in the air and the amount necessary to saturate the 

 air under given conditions. It is expressed in percents, and is 

 in common use as the expression of humidity. 



33. Modifying influence of temperature and wind. Humidity 

 is affected by temperature, wind, pressure, altitude, exposure. 



Fig. 6. Dwarf spruces at timber line on Pike's Peak, produced by the 

 drying and the mechanical action of almost constant winds. 



cover, and water content. High temperatures increase the capacity 

 of the air for moisture and low temperatures diminish it; (he 

 former lower the percent of relative humidity, the latter raise it. 

 Of two regions, or two habitats with the same rainfall, the warmer 

 is the drier. During the day the relative humidity falls as the 

 temperature rises, and rises in the evening as the air grows cooler. 

 Wind has also a powerful effect upon humidity in that dry winds 

 lower the amount of air moisture by mixture or removal, while 



