126 Principles of Plant Culture. 
Marsh areas are subject to frost, because, in addition to 
their low situation, as compared with the surrounding land, 
their luxuriant vegetation, by exposing a large radiating 
surface, and promoting abundant evaporation, tends to cool 
the atmosphere in the vicinity. 
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 
from the lower vallies, and is somewhat rarified by heat, 
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 Com= 
paratively Little upon Latitude. Within the tropics are 
areas where frost is unknown because the temperature never 
falls to the freezing point. But in localities subject to frost, 
the liability of damage to vegetation from this cause is gov- 
erned more by cold air drainage (210) and proximity to water 
than by latitude. It is as important to select locations for 
peach growing with reference to spring frosts, in the Caro- 
linas as in the peach belt of Michigan, and favorable loca- 
tions for the apple in Wisconsin sometimes escape damage 
from spring frosts in seasons when the apple crop is cut off 
from extensive regions of the southern states from this cause. 
