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Wind Machines

Wind machines are one of the most cost effective methods for frost protection under radiation freeze conditions.
The principle underlying their value is that during the day, the sun warms the soil and plants and then at night, this lighter warm air rises, leaving a colder mass of air near the earth’s surface.  This higher, warmer air is called a temperature inversion.
Wind machines capitalize on the inversion development in a radiation frost and work with nature to pull the warmer air down into the orchard or growing field to raise temperatures and save crops. The effect is essentially that of a large fan which mixes the air within and above the orchard so that the average air temperature near the ground is raised. 
They became more widely accepted during the 1940s and 1950s and are used on a wide variety of crops including grapevines, deciduous trees and citrus. 
Growers typically install wind machines when trees are 7 to 8 years old and can use them over 30 years.
Wind machines can be powered by electric motors, gasoline engines, liquefied petroleum gas (LPG) powered engines, or diesel engines. 


Wind machines generally consist of a steel tower with a large rotating fan near the top. There is usually a two- or four-blade fan with a diameter typically varying from 3 to 6 m. The typical height for fans is about 10-11 m above ground level. However, lower heights are used for lower canopies.
Power to operate the fan usually comes from an engine mounted at the base of the tower.
The principle of the system is to move heavy cold air to prevent stratification, and allow the warmer inversion layer air to replace the colder air near the ground. To be most effective, wind machines should be started before the temperature drops below freezing. Depending upon the field layout and contour, a single large tower machine will effectively protect up to about 10 acres with a 10o temperature inversion differential at 50 feet. As the inversion strength (differential) weakens, the area of protection becomes less.

There are three types of wind machines available on the market:  
1. Tower machines are the most common type. With a blade that is positioned at a slight tilt from perpendicular to the ground, they are designed to draw warm air from above down and mix it with the colder air at the surface as the head rotates around the tower.  
2. Tower-less machines that are designed to blow the cold air near the ground upward to mix with the warmer air above and circulate back.  
3. Ground-level mobile machines (frost fans) that blow cold air out of the site and permit warmer air from above to replace it can be effective depending on the strength of the inversion and contour of the site. These units are positioned near the highest point on a site in a natural air drainage pathway to improve the cold air flow.

An effectively installed wind machine can even provide more even sufficient frost protection when used in conjunction with uniformly scattered orchard heaters.
The combination provides adequate protection for even the colder locations. The wind machine will protect the grove for some of the nights by itself, but for the very cold nights, the heaters are available to add heat and thus provide positive protection. Usually the heaters are lighted whenever the wind machines cannot maintain the temperatures above the danger point. This combination method has the advantage over heaters used alone, in that it is cheaper, while providing as complete protection.
When these two methods are combined, the required number of heaters per acre is reduced by about half.

Helicopters have also been used as wind machines. They hover in one spot until the temperature has been increased enough and then they move to the next area. Repeated visits to the same location are usually required.


  • More economical than orchard heaters - Wind machines use only about 5% to 10% of the energy per hour required by heaters. 
  • Low labor requirements for operation
  • Adequate protection against local radiation frosts when temperatures go only 2 or 3 degrees below the damaging point
  • Useful in increasing air movement in groves where dead air occurs.
  • Increased effectiveness of heaters.