14 Farmers' Bulletin 1096. 



Good results have been obtained by turning fairly warm irrigation 

 water into fields or orchards on moderately cold nights. This method 

 will not afford protection against damage by heavy frosts. Frequent 

 irrigation of citrus trees during the winter months may start new 

 growth and render the trees much less resistant to cold. 



STIRRING THE AIR 



The temperature of the air 40 feet above the ground is often from 

 7 to 10 higher than that 1 foot from the ground. It is obvious that 

 if the air within this distance from the ground could be mixed, a 

 damaging frost would not be likely to occur in most cases. Attempts 

 have been made to do this with large power-driven fans, but it was 

 found that the expense was far too great for the plan to be consid- 

 ered from a practical standpoint. 



ADDING HEAT 



The third principle of frost protection is concerned with the addi- 

 tion of heat to the lower air to replace that which is lost by radiation 

 and conduction. This is generally accomplished by lighting a large 

 number of small fires throughout the area to be protected. Oil, 

 wood, coal, oil-soaked shavings, tree prunings, and carbon briquets, 

 or a combination of two or more of these fuels is used. 



Persons unfamiliar with temperature conditions in the lower air 

 on frosty nights sometimes speak of the fallacy of attempting to 

 " warm up all out-of-doors." It is well known that warm air is less 

 dense and therefore lighter than cold air. This fact is exemplified in 

 many ways in everyday life; the hot gases from a stove or furnace 

 rising through a flue and the lifting power of the old hot-air bal- 

 loons are good illustrations. As a matter of fact, roughly speaking, 

 the warmed air continues to rise and cool until it reaches a point 

 where it has the same temperature as the air surrounding it. On first 

 thought it might be supposed that the air warmed by fires in or- 

 chards or fields would pass upward to a considerable altitude and be 

 replaced by cold air from outside the heated area so rapidly that the 

 effect on the air temperature in the heated area would be very slight. 



However, the factor of temperature inversion on frosty nights 

 (the relatively thin layer of cold air near the ground with warmer 

 air overlying it) completely alters the situation, in that the heated 

 air does not rise far before it finds itself surrounded by air of the 

 same temperature as itself. As the hot gases leave the fires, they mix 

 rapidly with the surrounding colder air, so that the resulting tem- 

 perature of the whole mass is not very high. When the air 40 feet 

 above the ground is 10 warmer than that a foot from the ground, 

 the heat from the fires is nearly all expended in raising the tempera- 



