126 PRINCIPLES OP PLANT CULTURE 



211. Factors tending to avert frost. — Wind tends to 

 avert frost because it prevents the settling of the colder 

 air and thus keeps the temperature of the lower strata 

 of the atmosphere nearly uniform. 



Clouds, haze and smoke tend to avert frost because 

 they act to some extent like a blanket in preventing the 

 radiation of heat from the earth, and thus check the fall 

 in temperature (216). 



212. Influence of a body of water. — The proximity of 

 a body of water tends to avert frost because the water 

 cools slower than the air and thus checks the fall in tem- 

 perature of the atmosphere in the vicinity; also because 

 it keeps the neighboring atmosphere moist, thereby raising 

 the temperature of its dew point (205). The proximity 

 of buildings and trees tends to avert frost, probably because 

 these objects give up their heat gradually and thus temper 

 the surrounding atmosphere. 



213. Localities subject to frosts. — The localities most 

 subject to untimely frosts are narrow and deep valleys 

 inclosed on all sides, and inclined valleys that serve as 

 channels through which cold air flows to lower levels. 

 Partially-cleared districts usually suffer more from frosts 

 than those fully cleared, because the remaining forests 

 obstruct air drainage. 



Marsh areas are subject to frost, because, in addition 

 to their low situation as compared with the surrounding 

 land, their luxuriant vegetation tends to cool the at- 

 mosphere in the vicinity by exposing a large radiating 

 surface and promoting abundant evaporation. 



Valleys surrounding elevated lakes that have an outlet 

 through which the colder air may flow to lower regions 

 are particularly free from damaging frosts. The valley 

 of Keuka Lake, in west-central New York, so famous for 

 its vineyards, is of this class. 



