Fruit trees like cherry trees need heat for a few days each year when a frost is forecast to prevent ruining a crop.
I was passing a fruit farm today and finally got a chance to look up why farmers put those wind machines in their fruit orchards. I found the attached paper that explains it in lots of detail, but here are the highlights if you're interested. I found the science and economics of it intriguing, considering that the last option they discuss is to fly a helicopter over the orchard.
They're put there for frost protection.
"Wind machines, large propellers on towers which pull vast amounts of warmer air from the thermal inversion above an orchard...provide protection by mixing the air in the lowest parts of the atmosphere to take advantage of the largeamount of heat stored in the air." p12
Only needed for a few days in the spring or winter, they may dispense and disburse hot air, or a fine mist of water. Usually only a few degrees of heating are required. p1
The "critical temperature" for a crop at a given time of year may be well below freezing. The temperature for a week or two preceding a cold event affects the decision to deploy the protection system. p2
Protection against loss of heat by radiation on a cold, still, clear night that creates a temperature inversion, is more often needed than protection against blowing cold wind.p 1-2
An "overtree" system is designed to dispense a mist that freezes on the buds. The freezing of the water releases a tiny amount of heat that protects the crop. (Heat of fusion, p7)
The amount of water required to protect a crop in this manner is about ten times as much water per hour as needed for regular summertime irrigation, stressing the capacity of an irrigation canal system long before spring runoff, p5 forcing farmers to build holding ponds for an adequate early water supply. p6
A wind machine propeller or fan system should never be used in conjunction with an "overtree" system - no explanation given. p13 (I presume because it would favor evaporation over freezing.)
If the sprinkler system fails at any point during a cold night, this stops the continuous heating provided by the freezing process, and evaporation takes over and it becomes a cooling process, ruining the crop. p7
An air heating system with fossil fuel wastes 90% of the heat generated which simply rises and never warms the fruit, p4 and requires moving 30-40 heaters into and out of the orchard per acre. p11
The combination of propane heaters with propellers or fans reduces the fuel usage by up to 90%. p11
An "undertree" system is designed to distribute the heat in oil-heated water to the orchard, and the effect is augmented by fans or propellers that distribute the moist air. About 75% of the heat is wasted and never reaches the plant to protect it. p8 This system can warm up the orchard about 1.7 degrees Celsius, and the fans or propellers can add an equivalent temperature increase. p9
Flying a large helicopter over an orchard is an expensive but viable option, since one helicopter can protect up to 50 acres, flying over signal lights that indicate a temperature risk and "putting out the lights." p13
A more recent blog post explains more:
"Good site selection for frost protection includes air drainage [to prevent a temperature inversion]. The availability of natural heat sources, such as large water bodies and rivers, can help make the site a good choice."
"Heat guns, small gas-powered heaters or even the burning of organic materials such as wood or hay have been used... a heater alone, however, is one of the least effective frost prevention methods."
"Frost fans get air from the inversion layer down into and mixing with the freezing air at ground level... are energy efficient...on average, cover 10 to 13 acres per fan."
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