'Regular' Ol' Bay Area Fog or Tule Fog?

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Cartoon: "Regular" fog or tule fog? Summer fog flowing over Sutro tower. Text reads, "most bay area fog forms when hot inland temps bring cool moisture onshore." Second panel: fog flows from the valley towards the ocean. "Tule fog forms in colder inland valleys and sometimes gets pulled to the warmer coast."

Here in the Bay Area we're used to fog.

Summertime tends to be the foggiest time along the coast — just ask those freezing tourists at Fisherman's Wharf in July.

Put simply, our summer marine layer happens when inland valleys heat up, drawing cool condensed moisture (fog) from over the Pacific Ocean inland through the Golden Gate and other gaps in the coastal hills.

But there's more than one way to freeze in the fog.

Winter is the time for tule fog, a low radiation fog that forms from the ground up and is named after the tule reeds in the marshes of the Central Valley.

Every so often in winter we see dense tule fog drift west, as far as the Golden Gate.

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The air pressure and temperature relationships are flipped in wintertime, when it is usually much warmer at the coast than in the inland valleys — so the fog flows from the valleys to the sea.

Amy Whitcomb, writing in Bay Nature, likens it to a big inhale in the summer, when fog is drawn inland — followed by an exhale in winter, when tule fog moves toward the coast.

Tule fog over the bay is relatively unusual, but it is a common fact of winter life in the Central Valley, where the fog sometimes reduces visibility to zero.

So even though the Bay Area's summertime fog and the Central Valley's tule fog are very different ... they're both cold and wet.

Dress in layers and know which way the wind blows.