Definition
Fog that forms when warm, moist air moves horizontally over a cooler surface, cooling the air to its dew point and causing water vapor to condense into suspended droplets near the ground. It commonly occurs in coastal areas where moist marine air flows over cooler land, and unlike radiation fog, it can form and persist with winds up to about 15 knots.
Plain English
Fog created when warm, damp air drifts sideways across cooler ground or water, cools off, and turns into a low cloud at the surface. It often shows up along coasts and can stick around even on breezy days.
Context Anchor
Pilots often encounter advection fog in weather briefings for coastal areas, large lakes, and places where warm moist air moves over cooler land or water.
Derivation
From the Latin advectio, meaning 'a carrying to' or 'bringing.' The word captures the key idea: this fog is carried in by moving air, rather than forming in place. That distinguishes it from fog that forms because the ground itself cools overnight.
Why Pilots Care
Advection fog can cover large areas and persist for hours, sharply reducing visibility for takeoff, landing, and enroute flight.
Grounding Statement
Picture warm, damp air sliding across cold water and becoming a low cloud at the surface.
Intuition Check
Do not assume all fog forms the same way. Advection fog depends on moist air moving over a colder surface; if the air is not being carried across that colder surface, it is not advection fog.
Example Sentence 1
The morning briefing warned of advection fog drifting inland from the coast, with visibility expected to drop below one mile by sunrise.
Example Sentence 2
Advection fog formed overnight when southerly winds carried moist air across the cold lake surface.