Frost and the Prevention of Damage by It. 9 



The important requirements for the occurrence of frost, a clear 

 sky and little wind, are present during the passage of an area of 

 high barometer. As the first-mentioned type of disturbance, the 

 area of low barometer with overcast skies and rain, nearly always 

 precedes the area of high barometer, the saying in many sections 

 of the country, " Three days rain and then a frost," has some 

 basis. 



During the passage of a well-defined area of low barometer the 

 radiation from the sun is more or less completely cut off by heavy 

 clouds and the ground is not warmed much during the day. If rain 

 has fallen, the evaporation from the wet ground uses up a great deal 

 of heat and this also tends to keep the temperature low during the 

 day. Therefore, on the first clear night after a rain during the frost 

 season the temperature at sunset is likely to be within 15 or 20 

 of the freezing point and not much cooling by radiation is necessary 

 to form frost, although frost is not expected in many cases on the 

 first night on account of wind conditions. 



Though the moisture in the ground after a rain tends to prevent 

 warming of the ground during the day, it also tends to prevent a 

 large fall in temperature during the night. The water vapor 

 taken up by the atmosphere from the wet ground diminishes radia- 

 tion. When the dew point is reached the latent heat given up checks 

 the rate of cooling still more, and when the freezing point is reached 

 the conversion of the ground moisture into ice also liberates heat and 

 aids in preventing a further fall in temperature. 



Before the second night after the rain the surface of the ground 

 has usually dried out considerably. The dew point is likely to be 

 lower and a more damaging frost is likely to occur at that time. 

 Before the third night the day temperature has usually risen high 

 enough to make unlikely the occurrence of a heavy frost on the 

 Pacific coast, while in the central and eastern part of the United 

 States frost may occur as late as the fourth night if the high pressure 

 area is well defined and moves slowly. 



Large bodies of water exert a modifying influence on the climate 

 of localities to the leeward and such localities do not often suffer 

 much damage from frost. A light wind blowing from a large body 

 of water is generally more or less laden with water vapor, which 

 cuts down the rate of radiation ; and as the temperature of the water 

 is usually considerably above freezing, the temperature of the air 

 passing from it to the land is high enough to prevent the formation 

 of frost. 



Rivers often give up a large amount of moisture to the surface 

 air so that when the temperature falls to the dew point a fog 

 forms which covers a part or all of the lower land in the valley, 

 cutting off radiation and preventing a further fall in temperature. 



