FROST 18 



A quiet air is favorable for frost. Radiation proceeds more rapidly 

 from the surface than from the air above the surface. This is shown 

 by the fact that a thermometer placed in the grass on a quiet, clear 

 night will read 10° or even 15° below one suspended three or four feet 

 above the surface. If there is much wind, this difference will not occur, 

 because the wind mixes the colder air at the surface ^vith the warmer 

 air above, thus giving a more imiform temperature. 



A moderately dry atmosphere is favorable for frost, because when the 

 air is humid only a slight fall of temperature will occur before the 

 temperature at which dew begins to form {dew-point) is reached, and 

 when the vapor in the air begins to change into water (dew), the heat 

 that was used originally to change the water into vapor is no longer 

 required and is said to be liberated, and tends to raise the temperature 

 of the air, or at least to retard the fall. 



The effect of the liberation of heat in the process of the formation of 

 dew may be appreciated when it is said that the heat added to the air 

 in the formation of a pint of dew is sufficient to raise the temperature 

 of more than five pints of water from the freezing to the boiling point. 



Under ordinary conditions, when the dew-point is 10° or more 

 above the frost-point, 32°, a frost is not likely to occur, but if the 

 dew-point approaches 32°, frost is likely to occur. 



In a cranberry marsh near Mather, Wis., during the season of 1906, 

 Cox found that the minimum temperature averaged 8.2° below the 

 temperature of the dew-point as observed the previous evening, and 

 in extreme cases the difference was as much as 20° and 22°. On 

 a marsh near Berlin, Wis., on the night of September 27, 1906, at 

 11 P.M. the dew-point was found to be 43°, yet frost began to form 

 in parts of the marsh at 1 p.m. when the temperature had fallen to 

 28°; frost became general at 2 a.m., and the following morning a 

 minimum temperature of 24.4° was observed. 



The dew-point of the previous evening cannot, therefore, be regarded 

 as a safe guide for the minimum temperature of the following night. 



The chief value of dew-point observations of the previous evening 

 appears to be in the fact that they indicate the temperature at which 

 the heat from the condensing vapor will begin to be poured into the 

 air, and if this temperature is much above the frost-point, this addition 

 of heat may be reasonably expected to check the fall of temperature 

 and thus ward off a frost. 



