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.

Dollar sense

Several years ago, Battany conducted a study demonstrating how inexpensive, very portable 35-foot tall meteorological towers can be used to assess the suitability of wind machines for frost protection at a specific location in a vineyard. Resembling tall fishing poles, the flexible masts are made of fiberglass and aluminum components purchased at a local hardware store. Data are installed at the top and at the 5-foot height of these towers to measure the temperature difference between the two levels.

 “Given the high value of the wine grape crop at risk and the high costs associated with all active frost protection measures, the potential return on investment for measuring this type of detailed temperature information may be substantial,” Battany says.

He described this project in detail in the December, 2011, issue of his newsletter, Grape Notes. It’s available online at http://ucanr.org/grapenotes.

Other frost protection information available online — http://cesanluisobispo.ucanr.edu/Viticulture/ — includes a brief video, explaining the use of these meteorological towers for assessing inversion conditions, and details on the use of wind machines for protecting grape vines against cold damage.

Battany and his colleagues —  Rhonda Smith, UCCE viticulture farm advisor in Sonoma County, Rick Snyder, UCCE biometeorology specialist — are in the last year of three-year project to document temperature inversions during the spring frost period in vineyard regions throughout Sonoma, San Luis Obispo and Santa Barbara Counties. Each year, from March through May, they’ve been using the 35-foot tall meteorological towers to measure temperature inversion conditions at 60 sites throughout the three counties. Also, they are using this information to estimate the potential effectiveness of wind machines for frost protection.

“This study should significantly improve the information available to grape growers regarding inversion conditions and the suitability of wind machines for frost protection in these general areas,” Battany says.

For growers seeking to assess the suitability of wind machines for their own properties, Battany has written instruction sheets explaining how to construct the inversion towers and how to analyze the temperature data. Numerous vineyard operations are now employing these towers on their own properties to make detailed assessments, he reports.

The number of privately operated towers now exceeds the 60 in use by Battany and his colleagues.

“I've been very happy with the rapid adoption of this method by the vineyard industry,” Battany says.

This project has been funded by grants from the American Vineyard Foundation and the CDFA Specialty Crop Block Grant program.

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