UNFAVORABLE TEMPERATURE 127 



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 rarefied by heat from the lower valleys, 

 with the atmosphere of the more elevated region that is 

 rarefied 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 frost. Liability to damaging frost 

 depends comparatively little upon latitude. Within the 

 tropics are areas where frost is unknown because the tem- 

 perature never falls to the freezing point. But in localities 

 subject to frost, the liability of damage to vegetation from 

 this cause is governed more by cold-air drainage (209) 

 and proximity to water than by latitude. It is as impor- 

 tant to select locations for peach growing with reference to 

 spring frosts in the Carolinas as in tlje peach belt of Michi- 

 gan, and favorable locations for the apple in Wisconsin 

 sometimes escape damage from spring frosts in seasons 

 when the apple crop is cut off by frost from extensive 

 regions of the southern states. 



216. Methods of preventing injury by frost. Any non- 

 conducting material lying between the earth and space, 

 whether spread directly upon the earth or at a considerable 

 height above it, acts as a blanket to intercept the radiating 

 heat, and thus prevents in a measure the cooling of objects 

 beneath it. For this reason, straw, muslin or other non- 

 conducting material, spread over plants, usually protect 

 them from frost. 



While it is easy to protect a few plants from frost by 

 covering them directly, it is much more difficult to protect 



