CLIMATIC. 79 



Vapors afloat in the air, when chilled down so as to coalesce, form mist, 

 snow, rain, hail, just as the aerial condition necessitates. 



"Mountains act as condensers," says Prof. Tyndall ("Heat as a Mode of 

 Motion," p. 384), "partly by the coldness of their own masses, which they 

 owe to their elevation." Cold wind may produce the same result. If the 

 wind is comparatively warm and laden with moisture, it (the moisture), 

 says Huxley, "will be readily precipitated on exposure to refrigerating in- 

 fluences." 



FOREST TEMPERATURE. 



Ordinarily the temperature of the air under and just ov^r a forest is 

 considerably lower than that over an open field; if a vapor-laden wind 

 blows over the forest there is a tendency to precipitation. The trees with 

 their living foliage and carpet of dead leaves, and the extra moisture thus 

 economized, make larger surfaces for heat absorption, preserved in latent 

 state. Being thus shielded, the forest is cooler in summer and warmer in 

 winter than the open field. When the leaves fall in autumn, they are 

 retained on the ground, and over them is spread an even mantle of snow, 

 thus preventing the penetration of frost to any great extent, and rescuing 

 the roots of the trees and any tender flora from injury. 



Prof. Tyndall shows that the heat-absorbing power of the air is almost 

 entirely dependent on the presence of water vapor. As the dense forest is 

 more.moist than the open field the former being better shielded and having 

 more wet ground protected from the direct action of the wind a thicker 

 and wider spread curtain of vapors hangs over the forest, absorbing the 

 sun's heat, intercepting its drying tendencies and reactions to damaging 

 frosts, and so balancing the temperature, the coolness or warmth of the 

 forest air is kept steady and salubrious. A cold wind or chill of the air in 

 a vaporous strata of the forest generally causes precipitation, but not 

 always. The relative conditions of the air determine this. If a cold air 

 wave, charged with moisture, meets a warmer, drier wave, the tendency to 

 precipitation is neutralized, the moisture being then scattered and lifted as 

 vapors to higher altitude. 



EVAPORATION. 



Evaporation is the result of the ever active agency of heat and electricity. 

 It is one of the processes by which plants grow. Without it all life would 

 cease upon the earth. It is also a cooling process, self-inductive to precip- 

 itation. 



It occurs at surfaces. The greater the water or leaf surface (thickness 

 a factor), the greater the rapidity of evaporation. 



Cold lessens the capacity of the air to hold vapor; heat increases the 

 capacity. 



The drier the. air, the greater is the rapidity of evaporation. 



Hence, in the Minnesota climate, the absolute necessity of great water 

 and leaf supply to produce rapid saturation of the aerial fluid for the 

 needed precipitation. 



