14 BULLETIN NO. 161 



bering that with 100 heaters to an acre we get a rise in tem- 

 perature of approximately 4 degrees Fahrenheit, it figures out 

 that these smudge pots develop 6 watts per square foot in rais- 

 ing the temperature of the air in the orchard lo Fahrenheit. 

 This value is only very approximate. (See page 17 this bulle- 

 tin). With the electric heaters in the box mentioned earlier 

 it required 6*^ watts per square foot per degree and in heating 

 in the open with about 100 H. P- it required 14 watts per degree 

 per square foot. This larger value for the electric heaters than 

 for the smudge pots is probably due to four causes. The electric 

 heaters give out no smudge (colorless flames in the case of any 

 organic combustion give out carbon dioxide and water vapor, 

 an effective smudge). In the experiments with the electric 

 heaters the rise in temperature was from 20 F. to 40o F- above 

 that of the surrounding air while in the case of the smudge pots 

 the rise was only about 4o F. The greater the difference in 

 temperature the greater the rate of loss of heat and thus the 

 more power would have to be supplied to get it lo warmer. 

 The electric heaters were not placed in the orchard because the 

 electric power was not available there. The foliage on the 

 trees in the orchard would make it easier for the pots to warm it 

 up. The tests with the smudge pots were made in the orchard 

 at night when as a result of slow cooling the temperature 

 gradient in the air was negative, while the experiments with 

 the electric heaters were performed either in the late afternoon 

 or in the evening just before midnight when this was not true. 

 It is largely because of this temperature inversion in the early 

 morning, that it is possible to satisfactorily or sufficiently heat 

 the orchard. 



TEMPERATURE INVERSION AND EXPLANATION 

 OF EFFECTIVE HEATING 



Normally the higher in the air one goes the colder the air 

 becomes, the temperature falling 1 Fahrenheit for about 350 

 feet rise. At a height of about six miles, the temperature is 

 about 50o Fahrenheit below zero. At night after the sun goes 

 down, the air and the ground begin to cool but the ground cools 

 much faster than the air, so that by early morning it is found 

 that the soil temperature is lower than the air and hence as you 

 rise from the ground, the air gets warmer to a maximum at a 

 height of about 300 feet depending on the night, and then as you 

 go still higher it begins to get colder- This warm air at this 

 height should not rise even though it is lighter than that above 



