Why have wind machines proven to be so successful in protecting crops?
Wind machines are an effective tool for protecting a wide variety of crops from the damaging effects of frost in locations around the world. Other systems such as overhead sprinklers, heaters, helicopters, and air drains provide some degree of protection. However, wind machines have a number of advantages, which are not available from these systems.
Wind machines protect crops by raising temperatures. They pull down the warmer air on radiation cooling nights and ventilate the growing area to prevent pooling of cold air. Most importantly, only wind machines provide air movement across the plant surfaces that prevents them from supercooling. On any still, clear night, fans prevent the buds from supercooling up to 4° F (2.2° C) colder than ambient temperature. Even on weak inversion nights, wind machines substantially reduce the chance of supercooling.
Wind machines are a natural form of frost protection that does not affect the growing environment or the crops. Wind machines duplicate nature’s protection – the wind. Most growers have experienced nights with temperatures at or below critical levels and if the wind continues to blow, they see minimal damage. The damage occurs when the wind quits not only because the temperature drops dramatically, but also because the bud will turn 4° (2.2° C) colder than the air temperature.
Wind machines are easy to manage. Wind machines:
- Are available when needed
- Can be started and stopped as conditions change
- Do not affect growing conditions
- Can be moved to new blocks when replanting takes place
- Hold a high market value when selling farms or liquidating equipment
Do wind machines create a “wind chill” effect on crops?
No. Wind machines have the opposite effect by keeping plant surfaces warmer under still, calm conditions. On cold, clear, calm nights, the plant surfaces radiate their heat and actually become colder than ambient air temperature. Wind machines warm those surfaces by moving air across them every five minutes.
At what temperature should the wind machine be started?
Because of the supercooling that takes place when the wind dies, we recommend starting the wind machines 4° F (2.2° C) above the critical temperature for the crop being protected.
What do you mean by the “critical temperature”?
Critical temperature (view critical temperature charts) is the temperature expected to inflict at least 10% mortality in the buds, fruits, canes, or woods, depending on the crop. In tree fruit crops such as apples, pears, peaches or apricots, specific tables have been created after testing that identify the expected 10% kill temperatures. In some areas, the industry provides freezer testing of the wood to determine the temperature limits of the various crops during dormancy periods where the bud charts do not apply.
When should supplemental heat be used with a wind machine?
Supplemental heat is used on nights when the forecasted low is more than 4° F (2.2° C) below the critical temperature. The wind machine can struggle to maintain protection on those nights, especially if there is a cold drift bringing cold, untreated air into the block. The use of supplemental heat can be expected on nights when the sky clears early, the wind dies early and there is a dew point that is 5°+ (2.75°+ C) below the critical temperature. The wind machine will circulate the heat through the block and between machines to prevent the heat from dissipating into the air above the crop.
Where is the use of supplemental heat most important?
Supplemental heat is most needed on the side of the block where cold air is infiltrating the protection zone. Heat in this area will be carried into the block by the drift and will help the wind machine keep the temperature above critical on the edge. Without the supplemental heat, the temperature on the drift edge will decrease during the night and damage will occur as the machine struggles to keep the temperature above critical.
Cold spots develop where cold air pools behind ditch banks, road beds and in low areas. Supplemental heat can be placed in lines between the wind machines so the heat can be circulated back and forth to help improve conditions in the entire block.
What types of fuel are used for supplemental heat?
Many forms of supplemental heat are used, but the most common are under-tree sprinklers, diesel or propane heaters and wood- or coal-fired heaters.
- Under-tree sprinklers – Growers utilize under-tree sprinklers to run large blocks (5+ acres) of water at 40 plus or minus gallons/minute/acre. The heat and humidity generated give an additional 2° (1.1° C) of temperature lift (2 hectares and 375 liters/minute/hectare).
- Heaters – Metal pots heated by diesel, propane, wood, coal, hay, etc. can help the wind machines with an additional 2-4° (1.1-2.2° C) of lift depending on the strength of the system.
Once started, should the wind machine ever be turned off during the night?
The answer is yes and no.
- Yes – If the temperature rises 4° F (2.2° C) above critical due to the arrival of clouds or the wind increases to a steady 10+ MPH (16+ KPH). At that point, the wind is providing the same protection that the wind machine provides.
- No – If the conditions are clear and there is variable wind, you should continue to run the wind machines even if you are seeing temperatures well below the critical temperature. Because the machines are preventing supercooling, you are still benefiting the crop with the wind machine running.