Frost and the Prevention of Damage by It. 5 



ever, as soon as a small portion of this layer becomes warmer than 

 the air above and around it, its density is lessened and it is forced up- 

 ward and replaced by the cooler and denser air near by or above. 

 This air is also warmed in turn by conduction from the ground and 

 rises to make room for more cool air. The 1 heated air continues to 

 rise until it reaches a point where its temperature is the same as that 

 of the air surrounding it. This process continues until, near sun- 

 down, the temperature of the air is highest near the ground and de- 

 creases at a more or less uniform rate with increased distance above 

 the ground up to a height of a thousand feet or more. 



After the sun goes down the ground cools rapidly through radia- 

 tion, and its temperature soon falls below that of the layer of air in 

 contact with it. As soon as this occurs the surface air begins to 

 lose heat to the ground by conduction. The air near the ground now 

 becomes cooler than the air above and its density becomes increas- 

 ingly greater. Instead of rising, as did the surface air during the 

 day, its increased density tends to keep it in contact with the ground. 

 Thus over a level plain on a clear, calm night we find a relatively 

 thin layer of cold air near the ground with an increase in tempera- 

 ture up to an altitude of between 300 and 800 feet. 



Air Drainage. Over gently sloping ground the force of gravity 

 tends to cause this thin surface layer of cold air to move down the 

 slope and to gather in depressions in somewhat the same manner as 

 water. The similarity between the flow of water and of air down 

 a slope is inexact, however, because of the difference between the 

 physical characteristics of air and water. Water is a practically 

 incompressible liquid; therefore neither its tolume nor its density 

 is much affected by a change in pressure. Air is a compressible gas 

 and its physical condition is influenced greatly by such a change. 

 The atmosphere exerts a pressure at sea level of about 15 pounds 

 to the square inch. Because of its compressibility, the density of the 

 air decreases rapidly with increase in altitude. In accordance with 

 a law governing the behavior of gases, a decrease in density is ac- 

 companied by a decrease in temperature and an increase in density 

 causes an increase in temperature. When air moves downward 

 along a gentle slope or a steep hillside, its altitude is constantly 

 decreasing and its pressure and density are constantly increasing. 

 The increase in density causes an increase in temperature (heating 

 by compression) at the rate of about 1.6 F. for every 300 feet de- 

 crease in elevation and an increase in elevation causes a decrease 

 in temperature (cooling by expansion) at the same rate, provided 

 there has been no loss or gain of heat from other sources. 



Over a gently sloping plain or valley floor it is possible for the 

 cold surface air to drain down the slope in much the same manner 



