Weather Watch

Cooler Central Coast seawater may bring more fog this spring

Three sea lions lounge on a PG&E navigation buoy near Montaña de Oro State Park with Valencia Peak in the background. The buoy has an automatic fog horn. With colder seawater temperatures this year, more coastal low clouds are expected.
Three sea lions lounge on a PG&E navigation buoy near Montaña de Oro State Park with Valencia Peak in the background. The buoy has an automatic fog horn. With colder seawater temperatures this year, more coastal low clouds are expected.

Over the past few years, many Central Coast residents who live along the shoreline have commented on the reduced amount of low-level, hazy coastal stratus clouds and the fog and drizzle they can create.

Their observations are correct. Median cloud cover data from both San Luis Obispo County Regional Airport, and Santa Maria Public Airport has shown a decrease in the marine layer over the past few years.

Numerous factors affect coastal stratus clouds, such as storms and wind direction and speed. Over the past week, we’ve seen plenty of Santa Lucia (offshore) winds that warmed and dried the atmosphere and pushed the marine layer out to sea.

But another underlying condition that’s affected coastal cloud development has been the above-average seawater temperatures over the past few years. Since 1976, PG&E’s Diablo Canyon nuclear power plant has maintained temperature recorders housed in steel canisters and placed at fixed locations in the intertidal and subtidal zones along the Pecho Coast, which lies between Point San Luis and Point Buchon. These calibrated instruments have recorded millions of temperatures over the decades.

Over the past few years, seawater temperatures from these recorders have been well above typical. The average seawater temperature in September is 57 degrees, but in September 2015, the water averaged above 63 degrees — with a few days reaching nearly 67 degrees. Many surfers on those days went without wetsuits.

Normally, the March water temperature is about 53 degrees. However, in March 2015 and March 2016, seawater temperatures reached above 55 degrees.

Changes are underway this year, however. So far this March, the seawater temperature has plummeted to 52 degrees. (By the way, the coldest March on record occurred in 1977, when seawater temperatures averaged a numbing 50 degrees.) These colder seawater temperatures will probably contribute to a more typical coastal stratus year.

Here’s why: As we move into spring, the stronger northwesterly (onshore) winds push moist marine air over the Pacific Ocean’s surface toward the California coast. This air near the ocean’s surface is cooled from underneath by the colder seawater and produces an inversion layer, which means the air at the surface of the ocean is colder than the air above. The cold air interacts with the warm air, and condensation occurs, producing clouds.

Think of a cold glass of iced tea on a hot day. The water vapor in the air condenses on the outside of the cold drink. The same process occurs along our coastline, except the water vapor condenses on microscopic dust or salt particles near the inversion layer, producing marine stratus clouds. The colder the seawater temperature, the more likely condensation will occur.

Inversion layers can occur anywhere from a few feet above the ocean surface up to thousands of feet in altitude. When stratus clouds lower to the ocean surface, that’s what we call fog.

If the inversion layer is high enough, the marine stratus can surge into San Luis Obispo’s coastal valleys. As it further increases in elevation, it can travel through the Templeton gap or over the Cuesta Grade into the interior, providing relief from the summer heat.

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Just as a reminder: Daylight saving time starts at 2 a.m. Sunday, so be sure to “spring forward” one hour. PG&E encourages its customers to check the batteries in your smoke and carbon monoxide detectors, which could save lives.

Save the date: Carolyn Berg and I will be speaking at the League of Women Voters “Lunch & Learn with the League” on Saturday, March 25, from 11:30 a.m. to 2 p.m. at the Holiday Inn, 9006 W. Front Road, Atascadero. The topic will be the effects of climate change on our environment. Please contact Mardi Geredes via email at trekrmardi@aol.com to RSVP.

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 or follow him on Twitter: @PGE_John.

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