Why does San Diego have dense, cool fog every day? – San Diego Union-Tribune

Strange is a word rarely used to describe the weather in San Diego — especially in October, when warm Santa Ana winds help push the marine layer offshore.

But the city's coastal areas are in the midst and fog of an unusual period of dense, changing fog that has lasted more than a week and will continue for several more days, the National Weather Service said.

The fog was so thick Saturday that an 814-foot Japanese aircraft carrier was nearly invisible during a port call in San Diego.

The region is experiencing a meteorological chain reaction. A massive high-pressure system developed along the entire West Coast, causing temperatures to rise in places like Campo, reaching 102 degrees on Monday. Around the same time, sea surface temperatures off San Diego County dropped from the mid-70s to the low 60s. The cool, moist air collided with the heat dome over the land, creating fog.

“A lot of people think it's unusually cool along the coast right now, but that's actually not the case,” said Alex Tardy, a weather service meteorologist. “If you were to float 1,500 feet above your head, you would encounter air with a temperature of about 85 degrees. You have to think three-dimensionally to imagine it, and that’s not easy.”

As surfers will tell you, it's also unexpected. Sea temperatures fluctuate. But such a sharp drop is not all that common this time of year.

Tardy said the shift was likely caused by upwelling – a phenomenon in which onshore winds strike the coast at an angle, pushing the warm upper ocean layer away from the shore and causing cooler water to rise to the surface.

Originally published: October 8, 2024 at 3:33 p.m