Injury from Cold During Growing Period. 135 



213. 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 

 these fully cleared, because the remaining forests ob- 

 struct 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 out- 

 let through which the colder air may flow to lower re- 

 gions 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. 



214. Thermal Belts. In some elevated districts of 

 mountainous regions, localities of greater or less extent 

 are found in which damaging frosts are almost unknown. 

 These have been called thermal belts, and their freedom 

 from frost is explained by the merging of the warm air, 

 that rises somewhat rarified by heat from the lower val- 

 leys, with the atmosphere of the more elevated region 

 that is rarified to an equal extent by the high altitude. 

 Thus the warm air ceases to rise, but lends its heat to 

 temper the climate of the adjacent mountains. 



215. Liability to Damaging Frost Depends Com- 

 paratively Little upon Latitude. Within the tropics 

 are areas where frost is unknown because the temper- 



