Frost and the Prevention of Damage by It. 



AYe have seen that a relatively large amount of heat is required to 

 change liquid water to water vapor. Evaporation is going on at all 

 times, even when the air is saturated, although when saturation has 



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FIG. 17. Continuous records of the air temperature inside standard instrument shelters 

 in the two orange groves for which the comparative yields are given above. The 

 grove at station C was not protected. The grove at station II was partially protected 

 with coal heaters, set 50 to the acre. Station H was located in a row which contained 

 no heaters and the temperature was probably somewhat higher in the rows containing 

 heaters while firing was in progress. Note that the temperature .fell as low as 23 for 

 a short time, without any permanent injury to the fruit. As the weather had been 

 cold for some time before these low temperatures occurred, however, the trees and 

 fruit were considerably more resistant to cold than usual. 



been reached, the condensation of vapor balances the evaporation. 

 Evaporation also takes place directly from ice. When a thermometer 

 is covered with a film of water or ice, or contains frost on the bulb, 



