Why is San Francisco so cold in summer?
The ocean off San Francisco is cold all summer, and the hot Central Valley pulls that chilly, foggy air in through the Golden Gate like a giant vacuum. The hotter it gets inland, the stronger the fog and wind in the city.
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Upwelling: cold water from below
In spring and summer, northwest winds along the California coast push surface water offshore, and cold water rises from the deep to replace it. The air above that water chills to its dew point and forms a thick bank of fog just offshore.
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The valley vacuum
Inland, the Central Valley bakes, often near 100°F. Hot air rises and pressure drops, and cool ocean air rushes in to fill the gap. The Golden Gate is the main gap in the coastal hills, so fog and wind pour through it, giving the city summer highs in the 60s °F while towns an hour away swelter.
Why September is warmest
By late summer the upwelling winds ease, the inland heat fades and the ocean warms a little, so the fog machine weakens. San Francisco's warmest, clearest days often come in September and October.
Questions people ask
- Why is one neighborhood foggy and another sunny?
- San Francisco's hills block the fog: western neighborhoods near the ocean stay gray while eastern ones like the Mission are often sunny and several degrees warmer.
- What is Karl the Fog?
- A local nickname for San Francisco's summer fog bank.