11.4: Fog

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Where is the most famous fog in the U.S.?

San Francisco, the City by the Bay, is known for its fog. There's an old joke: "I spent the coldest winter of my life in San Francisco one summer." That's because of the fog! There is a big difference in temperature between the two sides of the city. On the Pacific side, where there is fog, temperatures are low. On the bay side, where there isn't fog, temperatures are much higher.

Fog

Fog (Figure below) is a cloud on the ground. Fog forms when humid air near the ground cools below its dew point. Each type of fog forms in a different way.

Radiation fog forms at night. Skies are clear, and the relative humidity is high. The ground cools as temperature goes down. Air near the ground cools below its dew point. Tule fog is an extreme form of radiation fog. This type of fog strikes the Central Valley of California in winter.
Advection fog is famous in San Francisco, California. Warm, moist air comes in from the Pacific Ocean. The air blows over the cold California current. When the air cools below its dew point, fog forms. Sea breezes bring the fog onshore.
Steam fog appears in autumn when cool air moves over a warm lake. Water evaporates from the lake surface. As the water vapor cools, it condenses. The fog appears like steam.
Warm humid air travels up a hillside and cools below its dew point to create upslope fog.

Pictures of tule, advection, steam, and upslope fog

(a) Tule fog in the Central Valley of California. (b) Advection fog in San Francisco. (c) Steam fog over a lake in Algonquin Park, Canada. (d) Upslope fog around the peaks of the Sanqing Mountains in China.

Fog levels are declining along the California coast as climate warms. The change in fog may have big ecological changes for the state.