At about this time of year, I often receive e-mails from gardeners who ask when the last day of frost will occur.
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.
At the first signs of freezing, many of these grape growers will use wind machines and helicopters to mix the cold air at the surface with the warmer air above to prevent damage to their vines.
You see, frost can develop when the air temperature is as high as 36 degrees or even 38 degrees. 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 reached 32 degrees, water vapor in the atmosphere will accumulate as ice crystals through a process called “deposition.”
According to the University of California Agriculture and Natural Resources corroborative extension in San Luis Obispo County, the last frost of the season in Paso Robles historically occurs on April 7— with the last hard frost by March 5. A hard frost happens when the temperature drops below 28 degrees Fahrenheit.
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 a website 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 Polytechnic.” When you do, another list of options will appear in the left column. Scroll downward until you reach “Spring ‘Freeze' Probabilities.” Click on this link, and a temperature graph will appear.
At Cal Poly, home of climatology for San Luis Obispo, 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.
So far this year, we’ve seen well above freezing temperatures in January, February and the first half of March. In fact, both Paso Robles and San Luis Obispo experienced their warmest January on record. This past February was the fifth warmest on record at Cal Poly with an average temperature of 58.4 degrees. So far this March, we’ve seen an average of nearly 61 degrees at the San Luis Obispo Airport.
At this time, NOAA’s Climate Prediction Center is predicting a 50 percent probability of above normal temperatures for most of California through May.
Land stewardship is important to PG&E. The company manages 12,820 acres that surround Diablo Canyon Power Plant. This has allowed for coastal hiking trails open to public use, including the Pecho Coast Trail that leads to the restored Point San Luis Lighthouse and the Point Buchon Trial which leads to Windy Point. To learn more about these preserved lands, visit www.pge.com.