What is the name for fog produced when warm, moist air moves over a colder surface?

Advection fog forms when warm, moist air moves horizontally over a colder surface and cools until it becomes saturated. The resulting tiny water droplets remain suspended near the ground and reduce visibility.

This fog is common along coastlines, where moist air passes over cold ocean currents or chilled land. San Francisco is a well-known example: moist air from the Pacific can move over the cold California Current, producing persistent coastal fog. Advection fog can also form over snow-covered ground, cold lakes, or other surfaces cooler than the incoming air.

The key distinction is movement. In radiation fog, the surface cools the air above it mainly through nighttime heat loss, often under calm skies. In advection fog, wind transports the moist air over a colder surface. A light to moderate breeze can therefore help create and maintain it.

Fog is technically a cloud at the Earth’s surface. Meteorological practice commonly defines fog as visibility below 1 kilometre, while mist is less restrictive.