Frost and the Prevention of Damage by It. 



11 



It was also found that the temperature often fell several degrees 

 lower at night over wet ground than over dry ground, because 

 of the heat expended in evaporating moisture from the wet ground 

 during the day. By covering the bogs with coarse sand, the mois- 

 ture is prevented from rising fo the surface from below, and cool- 

 ing by evaporation is checked. By keeping the bog free from 

 weeds, draining, and sanding, damage by frost may be greatly 

 lessened. 



PROTECTION FROM FROST 



Since a crop which represents the results of the labor and care of 

 an entire season may be destroyed by frost in a single night, various 

 methods of protection against frost have been practiced for cen- 

 turies in different parts of the world. 



FIG. 3. Average dates of first killing frost in fall. 



The three general principles used in frost protection devices in the 

 United States are: 



(1) Conserving heat, (2) Mixing or stirring the air, and (3) Add- 

 ing heat. 



CONSERVING HEAT 



The most important method by which the ground cools during the 

 night is radiation. If it is possible to prevent radiation or reduce it 

 sufficiently, the ground heat will be conserved and there will be no 

 damage. This may be partially accomplished by covering the 

 ground or plants with various materials. 



Covering with Glass. Glass is one of the best materials known 

 for screening plants and preventing frost damage, since it is almost 

 150830 20 Bull. 1096 3 



