Wind machines

Effectiveness of wind machines for frost protection depends on the strength of the inversion. When the atmosphere 40 to 50 feet above ground is at least 13 F warmer than it is a few feet above the soil surface, the temperature inversion is considered strong. When the temperature difference is less than about 5 F the inversion is considered weak. The temperature response from wind machine operation is often small and depends on the difference between the inversion temperature and the orchard temperature.

An interesting observation made during wind machine research in Chico was that even under ideal conditions, a 36-inch tall cover crop appeared to reduce the wind machine response in the lowest part of the trees canopy. Wind machines can provide economical frost protection, but only in favorable locations; primarily narrow valleys with strong inversions and low ceilings. The weak inversions usually found in orchards on the floor of the Sacramento Valley limit the usefulness of wind machines for frost protection in much of our area.

Helicopters

The helicopter is usually effective for frost protection under the same conditions as those effective for wind machine operation. A warm inversion layer is needed. With the helicopter, the operator has the advantage of being able to choose the level in the inversion where the temperature is most beneficial.

During a radiation frost night an inversion forms and temperature increases with height. At the ceiling height, the temperature reaches a maximum and then begins to decrease. During a strong inversion with a low ceiling, temperatures increase rapidly with height. Under these conditions helicopters can be used successfully for frost protection. Under advection freeze conditions, helicopters are usually ineffective since there may be no inversion at all. If there is a weak inversion, the ceiling is very high and protection with helicopters is difficult.

During the day, pilots should be shown the location of cold spots and hazards must be identified (towers, buildings, etc.). The periphery of the area to be protected should be marked with strobe lights so the pilots know where to fly and the lights can be differentiated from other lights on the ground. If you use helicopters for frost protection, stay in communication with your pilots. Nighttime fatigue of pilots has been identified as a major problem.

Larger helicopters push more air and hence protect a larger area. Adding weight to the helicopter by filling water tanks also increases the thrust and hence the protection afforded. In general, a small helicopter can protect 50 to 100 acres under most mild radiation frost conditions. For colder conditions, one helicopter may be needed for each 40 acres.

The area that needs protection should be covered every 30 minutes. Thermostatically controlled lights can be very useful because they indicate when a pass has been effective and they identify cold spots. Communication with a ground crew can also help identify cold spots.

One method of identifying flying height is to place an electronic thermometer outside the helicopter and fly where the temperature reading is the highest (the ceiling). The other method is to have the ground crew measure the change in temperature at the lower canopy level with passes at various heights until the optimum height for a good temperature response is found.

Helicopter protection can be stopped when the sun rises and the air temperature upwind from the protection site has risen above the melting point, 32 F, or, if the plant tissue is wet, when the wet bulb temperature upwind from the orchard is above the critical damage temperature for the crop.

For a more complete discussion of frost protection see the UC DANR publication #3364, Almond Production Manual, Chapter 23, Frost Protection.