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what causes the fog in san francisco
San Francisco's fog is a cloud resting on the ground, and it's thickest in summer, when you'd expect sunshine.
Here's the twist: the hotter the inland valleys get, the foggier the city becomes. Fog needs two things, a cold ocean and a hot valley.
First, the water is cold. Winds along the coast push surface water out to sea, and icy water rises to replace it. Around eleven to fourteen Celsius, even in summer.
Warm, moist air drifting over that cold water gets chilled until its water vapor turns into tiny droplets. That is fog, a cloud born at sea level.
The hot Central Valley, over a hundred degrees, sends air rising. Pressure drops, so fog flows in through the Golden Gate, kept low by a warm lid.
So, the whole story: a cold ocean makes the fog, and a hot valley pulls it ashore. That is why San Francisco is chilly while California bakes.
San Francisco's fog is the ocean's chill, pulled in by the valley's heat.