Since I wake up to the early morning sounds of the wind machines I have time to look around the internet. Here’s what I found on frost protection.
Orchard heaters have been used for centuries to protect orchards. Most heaters are designed to burn oil and can be placed as freestanding units or supplied by a pipeline network throughout the orchard. Propane, liquid petroleum and natural gas have also been used as fuels. The initial cost is generally lower than for other systems, but the cost of the fuel makes this system the most expensive in terms of operating cost.
Wind machines can be effective during a radiation frost. Their purpose is to circulate warmer air down to orchard level. A single wind machine can protect up to 10 acres. A typical wind machine is a large fan about 16 feet in diameter mounted on a 30 foot steel tower. The fan is typically powered by an industrial engine delivering 85 to 100 horsepower. Helicopters have been used as wind machines. They hover in one spot until the temperature increases, then they move to the next area. Repeated visits to the same area are usually necessary during a typical frost.
Overhead irrigation is probably the most commonly used means of frost/freeze protection in the southeastern United States. Heat lost from the plant part to its environment is replaced by heat released as the applied water changes to ice. As long as water is supplied at an adequate rate the temperature of the plant will remain at or near 32° F. Advantages of overhead irrigation include lower operating cost, convenient to operate and multiple uses including drought prevention, heat suppression, fertilizer application and possible limb breakage from heavy ice loads.