ORCHARD HEATING 15 



it unless the temperature gradient just above the point of 

 maximum temperature is larger than the adiabatic rate, which is 

 approximately 1 degree Fahrenheit to every 183 feet rise, 

 assuming no condensation to take place because of low humidity. 



When this condition exists and the smudge pots are lighted, 

 the hot air from them instead of rising to the very top of the 

 atmosphere because of its lightness is heated slightly and being 

 lighter than the cold air near the cold ground begins to rise and 

 yet cooling as it rises becomes colder and hence heavier than 

 the air at the above mentioned height of 300 feet hence does not 

 rise to this point but stops. Suppose the air near the ground 

 to be at a temperature of 28o and that at a height of 300 feet it 

 is 38. Then if the air about the smudge pots is warmer 2o or 3 

 to 31o or 32o it will rise to a height of about 50 or 60 feet at 

 which height will be found air of the same temperature and 

 density as that which is rising, so that if a moderate heat is 

 supplied, not even the air up to the 300 level will need to be 

 warmed. Instead of having to warm up all of the atmosphere 

 above the heaters, we need simply to warm up a small part of 

 the air below this level- This also explains how it is possible 

 to warm up "all outdoors" with the small fires, why it is better 

 to use many small fires instead of very hot large ones which 

 would upset and remove this sort of ceiling and also why it is 

 so very difficult to heat when there is even a slight breeze. 



Considering the heating of the orchard through convection, 

 the heat would rise to the artificial ceiling and stop and then 

 this warmed region would gradually extend downward and after 

 all the air from the tops of the trees to this ceiling had been 

 warmed, then the trees would begin to warm up. 



It is also interesting to observe that at the end of four hours, 

 assuming no wind and diffusion as very slight, there would be 

 over the orchard twenty four times as much smudge as there 

 is at the end of ten minutes and yet it is the experience of in- 

 vestigators that it takes only a few minutes for an equilibrium 

 condition to be established with a constant difference in temper- 

 ature between the heated and unheated area. 



