THE OCCURRENCE OF FROST 341 



mometer doubtless registers closer to the plant temperature than the 

 sheltered thermometer, it must be remembered that predictions 

 are based on and apply to sheltered thermometer readings. The dif- 

 ferences between the three temperatures maj^ not be great but they are at 

 times great enough to vitiate conclusions drawn from observations and 

 they may conceivably become at times great enough to have material 

 effects. 



Dewpoint and Its Relation to Frost. — Air is commonly known to 

 contain more or less water vapor. Other things equal, the higher its 

 temperature the more vapor it can contain and conversely the lower its 

 temperature the less moisture it can hold. If, therefore, any sample of air 

 be cooled enough it will reach the point where it can no longer hold as 

 vapor all the moisture it contains and some of it is deposited. Obviously 

 the drier the air at a given temperature the farther must its temperature 

 fall before the moisture is condensed. The dewpoint, or temperature 

 at which condensation occurs, varies, then, with the absolute amount of 

 moisture present in the air. 



As radiation proceeds from soil, vegetation and other substances 

 it has been shown that the temperature of the air in the immediate 

 neighborhood of these substances falls. In a calm this cooling frequently 

 proceeds to the point at which moisture is condensed; if this point is above 

 the freezing point dew is formed; if below, frost is formed directly. It 

 should be observed that frost is but an index of a low temperature and is 

 not of itself injurious. It should be observed further that radiating sub- 

 stances, particularly in a dry atmosphere, may cool the air several 

 degrees below the freezing point without any deposit of frost. This is 

 the black or dry frost. It is possible, too, for cooling to be extremely 

 localized so that frost forms when the free air temperature is several 

 degrees above freezing; frosts have occurred with a free air temperature of 

 40°F. 



The condensation of moisture from the air sets free a certain amount 

 of heat and retards the further fall in temperature. To that extent dew 

 or even frost formation is beneficial as compared with low temperature 

 without moisture condensation. It was formerly assumed that the 

 liberation of heat from condensation would check any further tempera- 

 ture fall and that because of this the dewpoint as determined the previous 

 evening would forecast the minimum temperature of the night. Later 

 investigations have shown this view to be unwarranted. 



Relation of Clouds and Wind to Frost Occurrence. — Evidently con- 

 ditions favoring loss of heat by radiation and a calm condition of the air 

 combine to produce dew or frost. Clouds reflect the heat lost by radiation 

 and even radiate some of their own heat so that the passage of a cloud may 

 for a short time raise the temperature a degree or two. Therefore cloudy 

 nights, though still, are not very likely to be frosty. A fair breeze does 



