Having learned that fog often forms when wet air hits cold land, I’m now wondering why San Francisco sees so much more fog than Seattle does.

They’re both Marine West Coast, with moist, westerly Pacific Ocean winds, and both have water on both their east and west sides.

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    $\begingroup$ As far as I am aware, the coastal fog along the California coast is caused by a cold oceanic current just off the Pacific coast called the California Current which starts around the latitude of Vancouver and extends all the way to the tip of Baja California (in Mexico). Hypothesis: This stream flows very close to the coast at San Francisco, while Seattle is separated from the ocean proper by a strip of land some 60 miles wide (Olympic Peninsula). So presumably there is lots of fog along the Pacific coast of the peninsula. $\endgroup$
    – njuffa
    Apr 2, 2017 at 22:57
  • $\begingroup$ You might compare Seattle to Oakland, rather than San Francisco. $\endgroup$
    – jamesqf
    Apr 3, 2017 at 4:28

1 Answer 1


As njuffa alludes to there is a current just offshore from the California coast. This is a deep, cold ocean current. As it hits the continental shelf off the coast it is deflected upwards towards the ocean's surface. But here's where it gets different: a thermal low that forms in the Central Valley of California during the summer draws in the air from the Pacific through the delta and bay waters. As that air meets the cold upwelling water that was deflected upward by the continental shelf, it draws moisture from it. That cold water also cools the air which then forms fog and gets drawn into San Francisco. As that moist air gets drawn further inland, it warms sufficiently for the fog to return to vapor. This actually cools the Central Valley to where this onshore breeze (we call it the delta breeze) gets shut off, the valley warms again, and the whole cycle repeats. Of course, other weather factors (position of the Eastern Pacific High, for instance) can override this process, but it is pretty much the standard summertime pattern.

If you need citations, I'm not sure I can provide them, as this is something I learned in a physical geography class in college many years ago. I've also seen some other explanations that are just plain wrong.


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