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Why this spell of Bay Area fog is different from the usual stuff

SF Gate logo SF Gate 12/12/2018 Amy Graff

a large ship in a body of water: The San Francisco skyline rises above a low fog bank shrouding the bay as seen from the Lawrence Hall of Science in Berkeley, Calif. on Tuesday, Dec. 11, 2018.

The San Francisco skyline rises above a low fog bank shrouding the bay as seen from the Lawrence Hall of Science in Berkeley, Calif. on Tuesday, Dec. 11, 2018.
© Provided by Hearst Newspapers

A thick, drizzly blanket of fog shrouded the San Francisco Bay Area Tuesday and in many areas visibility was less than a quarter-mile, creating dangerous driving conditions. The fog returned Wednesday morning but was much lighter with the worst visibility in the East Bay.

Fog is common in the Bay Area, especially in San Francisco and the coastal areas, but meteorologists say this spell is a little different from what we typically see.

In the more common scenario, the fog is formed by warmer air hovering above the cooler Pacific Ocean and then blows inland, sometimes reaching valleys, often clinging to coastal areas such as San Francisco's Sunset district.

MORE: Dense fog blankets Bay Area with visibility as low as 6 feet in Marin County

The fog on Tuesday and Wednesday is coming from the Central Valley where so-called "tule fog" has formed due to clear skies and cooler temperatures and then drifted into the Bay Area. Adjacent to the Central Valley, the East Bay saw especially thick fog Wednesday while it was patchier in San Francisco.

"This scenario isn't as common, but it can happen," says Spencer Tangen, a meteorologist with the National Weather Service's Bay Area office.

Tangen says the tule fog season runs late fall through early spring. It usually forms after a rain storm passes and the fog pulls in the leftover moisture creating a particularly drizzly layer.


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