Facing limited supplies of water this spring for sprinkler frost protection and the prospect of continued uncertainty regarding availability of water for frost protection in the future, more and more coastal growers are turning their attention to wind machines.

They’re a more expensive alternative to sprinklers. But, they can take advantage of temperature inversions to mix warmer air high above the ground with air closer to the ground and raise temperatures. The ability of these machines to protect vines from frost depends on the strength of the temperature inversion.

 

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Research shows that a conventional-style wind machine can be expected to raise the temperature 5 feet above the ground by about half the difference in temperature between that height and 35 feet above the ground, reports Mark Battany, University of California Cooperative Extension viticulture farm advisor for San Luis Obispo and Santa Barbara Counties.

For example, if the temperature at 35 feet is 4 F warmer than at 5 feet, a conventional wind machine would likely raise the temperature at 5 feet by about 2 F. With a weaker inversion strength of less than 2 F, the wind machine would be expected to raise the temperature at the 5-foot height by less than 1 F.

The stronger the inversion strength, the greater the potential warming that can be achieved. That’s why the ability to measure the local inversion strength is critical when determining where wind machines will provide beneficial protection, Battany notes.

`However, the likelihood and strength of inversion conditions aren’t always apparent by evaluating regional topography. Also, standard near-surface measurements of typical weather stations only provide limited information about inversion conditions, he adds. Even though this general relationship has been understood for decades, relatively little attention has been paid to determining how to measure the inversion strength in a practical manner, he notes.