Frost and the Prevention of Damage by It. 7 



The drainage of cold air down a valley floor is usually interfered 

 with considerably by outside influences. As soon as the flow begins 

 there is more or less mixing of the cold lower air with the warmer 

 upper air and inequalities in barometric pressure over whole re- 

 gions may temporarily prevent the flow or even reverse its direction 

 for short periods. Local winds of slight velocity and covering a very 

 limited area often cause a mixing of the air that causes the surface 

 temperature to rise suddenly from 5 to 10. It is not often possible 

 to know in advance that the drift of the air on a valley floor will 

 continue from one direction during a cold night, though there may 

 be one particular direction from which it very seldom comes. 



Effect of Water Vapor on Rate of Cooling. Water vapor is the 

 most effective of the various gases present in the atmosphere in ob- 

 structing radiation of heat from the earth. Therefore, the amount 

 of water vapor present in the atmosphere above a given locality has 

 considerable influence on the rate of fall in temperature at that place 

 during the night ; the temperature falls more slowly when the humid- 

 ity is high than when it is low, other conditions being the same. 



The amount of invisible water vapor in the atmosphere varies 

 greatly at different times. At a given temperature only a certain 

 maximum amount can be present. If, when the maximum amount is 

 present, the temperature is lowered, a certain portion of the water 

 vapor is changed to liquid or frozen water, as the amount of water 

 vapor which can be present in the air is greater when its temperature 

 is high than when it is low. No matter how dry the air under nat- 

 ural conditions may be, if its temperature be lowered sufficiently, a 

 point will be reached where the invisible vapor will begin to appear 

 in a liquid or frozen form. The temperature at which this condensa- 

 tion begins is called the dew point. The drops of moisture which 

 appear on the outside of a pitcher of ice water on a warm day are 

 formed through the chilling of the air in contact with the pitcher. 

 These droplets begin to appear on the pitcher as soon as its tempera- 

 ture has reached the dew point. 



The actual amount of water vapor in the air expressed in terms of 

 weight per given volume of air, is called absolute humidity. With a 

 given dew point the absolute humidity is always the same ; therefore 

 to determine the absolute humidity it is only necessary to find out 

 what is the temperature of the dew point. 



As a general rule the temperature of exposed objects falls more or 

 less steadily after sunset until it reaches the dew point, at which 

 time the invisible water vapor in the atmosphere begins to be de- 

 posited on them. If the dew point is above 32 F., the freezing 

 point of water, dew is formed; if it is 32 F. or lower, frost 

 forms. Since dew or frost does not begin to form until the temper- 

 ature of the ground or other object reaches the dew point, it is ap- 



