Weather Watch

Warmer weather means less frost on the Central Coast this year

Frost is seen on a rooftop in Baywood Park.
Frost is seen on a rooftop in Baywood Park.

The whirling noise of wind machines in the vineyards late at night and early in the morning hours often kept me awake in Sonoma County, where I grew up.

Needless to say, the mere mention of frost at this time of the year can cause many to look for cardboard boxes and old sheets to cover their newly sprouted plants.

Viticulturists closely monitor local weather stations throughout the night for indications of possible frost. Because of the well-above-normal temperatures this year, many of our vineyards are reporting bud break, and the young shoots are vulnerable to frost damage.

Here’s why: The nights allow more of the atmosphere’s heat at the surface to radiate out into space, especially during clear, dry and calm conditions. When conditions are calm, denser cold air flows downward along our mountain slopes and accumulates in the valleys. Even when the land is only gently contoured, the cold air will accumulate in the low-lying areas.

Often, our valley floors will be much colder than the surrounding hillsides. This condition can lead to below-freezing temperatures.

Wind machines mix the cold air at the surface with the warmer air above, which can prevent frost from developing.

Helicopters can provide the same function. A friend in the Navy Reserve who owned a helicopter company got a contract to fly helicopters up and down the rows of vineyards during the dark hours of late winter and early spring. Helicopters hovering above the vineyard can mix out the temperature inversion layer, thus raising the temperature near the vines.

Another method is to use water sprinklers; the resulting ice protects the grapes by holding the temperature of the vine at 32 degrees.

To save water, John Salisbury discs the rows between his vines.

“The resulting dark soil absorbs the heat from the sun during the day and releases it at night. Often, this is all the difference we need because this can give us an extra 2 degrees of protection,” he said.

Once, I took a job in high school lighting kerosene-fueled orchard heaters when the temperature dropped near freezing. Suffice it to say, I didn’t get a lot of sleep that March.

Here’s the scary part: Frost can develop when the air temperature is as high as 38 degrees Fahrenheit. This happens because many things cool faster than the air that surrounds them, such as blades of grass, vines, car windshields and roofs on buildings. When these objects reach 32 degrees, water vapor in the atmosphere will accumulate as ice crystals through a process called “deposition.”

According to the UC Cooperative Extension in San Luis Obispo County, the last frost of the season in Paso Robles historically occurs April 7 — with the last hard frost by March 5. A hard frost happens when the temperature drops below 28 degrees.

Closer to the ocean, San Luis Obispo will usually have its last frost by Feb. 15. These dates hold true at our beaches as well. In the far-eastern regions of San Luis Obispo County, Cuyama has to wait until April 20 for the last frost of the season.

However, like spinning a roulette wheel, these are only average dates. It’s not unusual for locations such as Cuyama or Windrose Farm near Creston to see Jack Frost as late as early May.

To get a sense of the actual probabilities of dropping below the freezing level before you plant your garden, the Western Regional Climate Center has information at www.wrcc.dri.edu/ summary.

It provides climate data for communities throughout the United States. Just click on California, and a vast list of locations in alphabetical order will appear on the left-hand side of your screen. For example, I selected “San Luis Obispo Polytech.” When you do, another list of options will appear in the left column. Scroll down until you reach “Spring ‘Freeze’ Probabilities.” Click on this link, and a temperature graph will appear.

At Cal Poly, there is approximately an 86 percent probability the temperatures will drop to 36 degrees until Feb. 1, and about 72 percent probability by March 1. This probability quickly drops to 43 percent by April 1. On the really rare occasion, there is actually a 10 percent probability that San Luis Obispo could see 36-degree temperatures until May 1.

Historically in January through March, the average low temperature in San Luis Obispo is 42.4 degrees. So far this year, the average is 47.7 degrees. In Paso Robles, the normal average low temperature is 37.7 degrees for the first quarter of the year. So far this year, it’s 40.6 degrees.

These temperatures that are much warmer than average are most likely being caused by El Niño combined with global warming. NOAA’s Climate Prediction Center is predicting a 50 percent probability of above-normal temperatures for most of California through May.

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Earth Day event: Join PG&E employees April 16 to celebrate Earth Day at Montaña de Oro State Park. The event is one of a number of service projects sponsored by PG&E and the California State Parks Foundation. Please register at the State Parks website, www.calparks.org/ help/earth-day/earth-day -registration.html. Rangers will provide tools and supervise.

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A correction to last week’s Weather Watch column. I wrote that skunks are related to the other mustelids. However, Scott Ellis wrote to say, “This was considered the case until fairly recently, (late 1990s) when DNA testing indicated that skunks are sufficiently distinct from others in this family to warrant a separate group, the Mephitidae family, which is composed of skunks and ‘stink badgers.’ 

John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at pgeweather@pge.com.

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