Frost and the Prevention of Damage by It. 



15 



turc of the air within this 40-foot layer. In other words, the warmer 

 upper air acts as a roof which stops the ascent of the heated air. 

 (See fig. 4.) 



It is plain that the degree of temperature inversion near the 

 ground, that is, the rate at which the temperature increases with 

 increase in altitude, determines the thickness of the layer of air that 

 must be warmed to obtain a definite increase in temperature at the 

 ground. If the inversion is strong the surface temperature can be 

 raised several degrees more than when the inversion is slight, the 

 amount of fuel consumed being the same in both instances. 



FIG. 4. Continuous records of the temperature 5 feet and 35 feet above ground on a 

 tower in a pear orchard. Note the large difference in temperature at the two levels 

 before the orchard heaters were lighted at 4 a. m. By 5 a. m. the temperature was 

 practically the same at the two levels, showing that the heat from the burning oil had 

 been nearly all expended in raising the temperature of the air within 35 feet of the 

 ground. This point is further illustrated by the fact that at 5 a. m. when most of the 

 heaters were extinguished, the temperature at the 5-foot level fell rapidly, while it 

 remained practically stationary at the 35-foet level. 



The amount of this temperature inversion varies greatly from 

 night to night, and in different localities. It is mainly determined 

 by the amount of fall in temperature from afternoon to early morn- 

 ing. If the afternoon temperature is high and the temperature falls 

 to freezing on the following morning the inversion in temperature is 

 likely to be great. 



A large number of small fires will be found to be more efficient in 

 raising the temperature than a small number of large fires, especially 

 in localities where the temperature inversion is relatively slight. 

 The heated gases leave the large fires at a high temperature and tend 

 to rise some distance above the ground, while the gases from a num- 

 ber of small fires are mixed with the surrounding cooler air until the 



