Predicting Frost 



253 



sideration, because they may be obtained early in the 

 day, before it is possible to secure- any reliable indica- 

 tions from local observations as to the probability ' of 

 frost. But when the warnings issued by the Weather 

 Bureau cannot be obtained, and the farmer must rely on 

 himself, there are no instrumental 

 readings that will take the place of 

 a careful observation of the condi- 

 tion of the sky, the direction and 

 force of the wind, and the trend of 

 the temperature." 



"In the day, plants usually 

 receive more heat from the sun than 

 they give off (radiate), and conse- 

 quently become warmer; but at 

 night the process is reversed, and 

 they radiate more heat than they 

 receive and thus grow colder. When 

 the surface of a plant has lost 

 (radiated) sufficient heat to cause 

 its temperature to fall to 32 or 

 below, frost forms. Any condition 

 that causes increased radiation will 

 increase the liability of frost, and 

 conversely, whatever checks radia- 

 tion or supplies additional heat to 

 the air will tend to ward off frost." Favorable con- 

 ditions for frost are a clear night, a quiet or still night, 

 a moderately dry atmosphere. The surface air is likely to 

 be colder than that above, so that a wind may warm the 

 surface air by mixing it with the warmer strata. 



Wilson makes the following observations on weather 

 conditions as indicating frost : 



FIG. 93. One form of 

 a wet-and-dry bulb hy- 

 grometer. 



