By Scott Alumbaugh
Published: September, 2007
Throughout the summer and into fall, you can just about set your clock by the winds in the central San Francisco Bay. They will be calm in the morning, the glassy shallow flats reflecting a cool gray sky. By 9 a.m. or so, there will be a stirring of breeze from the south. By 11 a.m., the breeze will stiffen to 12 to 15 knots. It will also clock west to come in straight through the Golden Gate and head for the Berkeley Hills. By 2 p.m., the Bay’s got a bone in her teeth. Winds are blowing a full 15 to 20 knots, or more. The water from Fort Point to the Olympic Circle will be riven with white caps, streaked with foam. Wind chop will have built up to three feet, or more. By 6 p.m., the winds have mellowed some, but the sea has not. By sunset, things are quiet. And after 11 p.m., you might have to start your motor to make it home.
What makes the winds so predictable? Northern California geography.
Every summer, a high pressure zone sets up in the North Pacific. High pressure winds circulate clockwise in the Northern Hemisphere due to the earth’s rotation—this also accounts for the prevailing northwesterlies along the California coast. These warm winds pass over the water along the Northern California coast. The winds move the topmost layer of water, which causes the upwelling of colder water from below. The warm air passes over the cold water, and creates our summer fog. That’s what keeps San Francisco Bay so cool in summer.
Meanwhile, as San Francisco is cooling, the Central Valley is heating up. One-hundred degree weather in Sacramento is common this time of year. Hot air rises. The rising air creates a vacuum of low pressure relative to the Bay. The air from the Bay moves toward Sacramento to equalize the pressure. The Golden Gate is the only sea level passage for offshore air. It’s like a door thrown open, and the air rushes in. The bigger the temperature difference between Sacramento and San Francisco, the stronger the winds in the Bay. At night, when temperatures aren’t quite so disparate, the winds calm.
The Sacramento vacuum sucks cool air from the Bay a little further inland every day. After about ten days or so, it cools the Valley enough to decrease the temperature differential, which tempers the Bay winds. But with the winds not blowing so hard, the inland air heats up again, and the wind machine kicks back into gear.
So, yeah, Sacramento sucks in the summer. And Bay sailors are grateful for that.
Scott Alumbaugh
is a US SAILING certified, Coastal Passagemaking instructor. He holds a 100 Ton Masters license, has worked as a delivery and charter skipper in the United States, Mexico and in the Caribbean, and is a sailing instructor at OCSC Sailing in Berkeley Marina.
