378 FUNDAMENTALS OF FRUIT PRODUCTION 



than the heat from the small fires, being thus rendered ineffective in 

 warming the air at lower levels. Large fires of course emit a considerable 

 amount of radiation heat which warms the surfaces exposed, but since 

 the intensity of heating by radiation diminishes as the square of the 

 distance from the source of heat it soon becomes ineffective. In addi- 

 tion the current set up above the large fire draws in the colder surface 

 air to replace the warmed air driven high aloft and it is easy to conceive 

 that it may disturb ceiling layers considerably. 



Effect of Wind. — Winds, besides carrying heat away directly, break 

 up the "ceiling layer" of warm air and unless the heated areas are very 

 large and the wind such that the warmed air is "blown down," they 

 make heating efforts of little avail. Windbreaks, therefore, though at 

 times they may invite frost conditions, may render heating more effec- 

 tive, though they cannot preserve tjie ceiling layer which is necessary 

 for full realization of its possibilities. 



Humphreys, ^^ assuming a radiation per minute per square centimeter of 

 0.1 calorie and evidently basing his calculations on soil surface area alone, 

 disregarding vegetative surfaces, concluded that for each plot of ground 10 

 meters by 10 meters there would be needed per hour 6,000,000 calories, which, 

 assigning a value of 8,500 calories per gram of petroleum, indicates the need of 

 approximately a pint and a half of oil per hour to offset radiation or to hold the 

 temperature from falling. If a moderate air movement occur, new air must be 

 warmed constantly. Humphreys, assuming the dewpoint below 32°, land sur- 

 face horizontal, temperature of air 32° and a wind of 2K miles per hour (approxi- 

 mately 1 meter per second), with air weight 1,290 grams per cubic meter, makes 

 an interesting calculation of the amount of heat necessarj'- to warm the entering 

 air 2°C. to an elevation of 12 meters. He states: "Now the specific heat of the 

 atmosphere is very approximately 0.24. Hence to warm 1 cubic meter of the 

 given air 1°C. requires about 310 calories. Hence, to warm the air 2°C. to an 

 elevation of 12 meters, as it enters the given area with the given velocity of 1 

 meter per second, will require, per linear meter at right angles to its direction, 

 approximately 2 X 12 X 310 X 7,440 calories per second, or the consumption of, 

 roughly, 3.7 liters or 6.5 pints of oil per hour." 



A considerable amount of the heat imparted to the air as it enters is retained 

 while the air drifts through the orchard; therefore, though radiation must be 

 fought equally at all points the raising of air temperature itself is moit jt^roperly 

 done on the windward edge. With the somewhat idealized conditions enumer- 

 ated above, assuming an orchard 1 kilometer square (about 247 acres) with the 

 breeze at right angles to one side the oil requirements are stated by Humphreys : 

 to counteract radiation 8,600 liters; to warm the entering air 3,700 liters. A rec- 

 tangular orchard might require more or less oil to warm the entering air, accord- 

 ing to the direction of the breeze and if the breeze is quartering two sides must be 

 warmed, but the amount to offset radiation alone is constant. In other words 

 the oil necessary to offset radiation is determined by area alone ; the amount nec- 

 essary to warm entering air is determined by the outline of the orchard and by 

 the direction and velocity of the wind. 



