Fog forms in stable air with light winds, high relative humidity and conditions which bring the air to its dew point. The resulting layer of fog is normally only a few hundred feet thick. Radiation fog is common over land in late fall and winter. Advection Fog forms when wind moves warm, moist air over a cooler surface. The cold surface cools the warmer air and lowers it to its dew point.
Condensation occurs, resulting in the formation of a layer of fog.
The layer of fog will deepen as wind speed increases up to about 15 knots. With winds stronger than 15 kts the fog will lift as stratus cloud. This type of fog persists unless there is a change in the air mass or the wind direction. Advection fog is common along the Pacific Coast since surface water along the coast is much colder than the surface water farther offshore. As warm, moist air from the Pacific Ocean moves over the colder coastal waters it is chilled from below , lowering it to its dew point forming fog.