376 FUNDAMENTALS OF FRUIT PRODUCTION 



after frost on nitrate-fertilized trees may constitute a splendid testimonial 

 for nitrate fertilization but they do not in themselves indicate that 

 orchard heating without fertilization would have been beneficial. 



Heat Units in the Fuel. — Limits must be recognized to the amount of 

 actual heating any ordinary equipment can secure. 



McAdie^^^ indicates this in some interesting calculations. "At the present 

 time," he states, "with a hundred burners to the acre, using a gallon each of oil, 

 something Hke 15,000,000 British thermal units or 3,760,000 [kilogram] calories 

 would be given off, provided the combustion was perfect, which of course is never 

 true. Now, to raise the temperature of the air 1°F. over an acre to a height of 15 

 feet is practically heating 653,400 cubic feet of air. In practice it is found that to 

 maintain the temperature on a still night 1° above the freezing temperature 

 requires 0.252 calories per hour per cubic foot. Therefore for a period of 7 

 hours, which is about the average duration of a low temperature [McAdie wrote 

 in California], although 10 hours is a safer period, there will be required 1,138,200 

 calories. And if a raise of 5° is required it is evident that more than 5,500,000 

 calories are needed or more than the full number of heat units in the fuel under 

 perfect combustion." 



In practice oil is burned generally at a faster rate than that used in 

 IVIcAdie's calculations, but the pubhshed results of careful experiments 

 indicate that the actual heating achieved rarely exceeds 5° and that 4° 

 is a liberal estimate of what may be expected with ordinarily favorable 

 conditions. A breeze of 6 miles an hour materially lowers the net gain 

 of heat; any movement lowers it somewhat and dead calms are rare. 

 According to Young, in the lard-pail type of heaters only about 40 per 

 cent of the heat in the oil is actually- realized in combustion and even 

 in the high stack type it is doubtful if more than 70 or 80 per cent of 

 its fuel value is attained. A still further loss is caused by the height of 

 the "ceiling layer" of air which, though variable, permits in any case 

 the accumulation of heat at a height above the trees. 



Height of the ''Ceiling Layer." — The holding of heated air within a 

 few feet of the ground appears mysterious unless the inversion of tem- 

 perature be considered. Data introduced previously have shown that 

 the normal adiabatic cooling of the air upward from the earth, charac- 

 teristic of daytime, is modified during radiation nights and that the air 

 only a few feet above the ground is distinctly warmer than that at or 

 near the surface. It is this layer of warm air, acting as a roof or ceiling, 

 that makes possible the warming of the air at the level of the trees. 

 As the warmed air ascends from the heaters it mixes with other some- 

 what cooler air and the mixture finally reaches a layer of the same tem- 

 perature; it then has no impulse to rise further. 



Figure 38, by Humphreys," shows a typical frosty morning tempera- 

 ture gradient and is used by him to indicate how heat may be wasted. 

 He shows that under the given conditions any portion of the surface 



