PROTECTION AGAINST FROST 



377 



air warmed from 32 to 34°F. would rise to about 30 feet only, as shown 

 by the adiabatic curve from 34°, until it would reach the layer of air 

 having a temperature equal to its own. If it were warmed to 40°, 

 however, it must rise over 250 feet, cooling somewhat by diminished 

 pressure, until it reaches air with an equal temperature. Thus in one 

 case the ceiling is about 30 feet high, in the other it is 250 feet high. When 

 the gradient begins at, say, 24° instead of 32°, in other words when the 



900 



.£ 500 

 c 

 o 



P 400 



300 



200 



100 



ZZ 33 34 



35 



36 2.T 



Temp era+u re,d eg.fohr. 



Fig. 38. — Illustrating the physical possibility of protecting outdoors from frost by artificial 

 heating. (After Humphreys^'') 



outside unheated surface air is at 24°, whether or not the gradient is 

 affected at 500 feet, the ceiling above the 34° mark is raised, meaning 

 that not only must the air now be heated from 24 to 34°, 10 degrees 

 instead of 2, but a greater amount of air must be heated. The increasing 

 difficulty of heating toward morning is due evidently to other factors 

 besides the heaters themselves. 



If a few large fires are employed the body of warmed air rising from 

 them is so great that it does not become mixed readily and rises farther 



