Injury from Cold During Growing Period. 131 



up their heat gradually and thus temper the surrounding 

 atmosphere. 



214. The Localities Most Subject to Untimely Frosts are 



narrow and deep valleys inclosed on all sides, and in- 

 clined 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 atmos- 

 phere in the vicinity by exposing a large radiating sur- 

 face 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. 



215. 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. 



216. Liability to Damaging Frost Depends Comparatively 

 Little upon Latitude. Within the tropics are areas where 

 frost is unknown because the temperature never falls to 



