Nib asked
What causes the fog in San Francisco?
San Francisco's fog is made by the cold ocean and pulled in by the hot inland valley, so the hotter California gets in summer, the foggier the city becomes.
6 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1summer fog
The fog that blankets San Francisco is advection fog: a layer of cloud at ground level. It is most common in summer. The fog season runs roughly from June to September, the very months when the rest of California is hot and dry.
San Francisco's fog is a ground-level cloud that peaks in summer.
2the twist
It sounds backwards, but San Francisco's summer fog depends on heat somewhere else. A cold ocean makes the fog and a hot interior pulls it ashore.
Two ingredients: a cold ocean and a hot inland valley.
a common mix-upSummer fog is not the sign of a cool day everywhere. It is powered by heat inland: a hot Central Valley is what pulls the fog in.
3cold ocean
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.
4fog forms
Air above the Pacific carries a lot of water vapor. When it blows over the cold coastal water, it cools until it can no longer hold that vapor, and the vapor condenses into billions of tiny droplets. A cloud forming right at the surface is what we call fog.
Moist air chilled by cold water condenses into fog.
Why does fog form over cold water?
Show the answer
The water chills the moist air until its vapor condenses into droplets.
5inland pulls it in
In summer the Central Valley can top 100 degrees Fahrenheit (38 Celsius). Hot air rises there, leaving low pressure at the surface. Cooler, heavier air from the ocean flows in to fill the gap, and it carries the fog with it. The Golden Gate, the biggest break in the coastal hills, channels it into the bay and over the city.
Hot inland air rises, and the ocean's cool, foggy air rushes in to replace it.
Why does a hotter inland valley mean more fog on the coast?
Show the answer
Hot air rising lowers the pressure, which pulls the cool foggy air in from the ocean.
6the whole idea
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.