Advection Fog

What is Advection Fog?

Advection fog is a type of fog that develops when warm, moist air is transported by wind over a surface that is colder than the air itself. As the air cools to its dew point, condensation occurs, forming a thick layer of fog. This process is called advection—the horizontal movement of air masses.

Formation Mechanism

• Warm, moist air moves (advects) over a colder surface (land, sea, or snow).
• The air cools to or below its dew point due to contact with the cold surface.
• Condensation occurs, resulting in fog formation.
• A moderate wind (typically around 15 KT) is necessary to maintain the movement of the air mass and sustain the fog.

Where and When Does It Occur?

• Common over coastal areas, especially where warm and cold ocean currents meet (e.g., Grand Banks, North Sea, California coast).
• Can form over land, especially when warm air flows over snow-covered or frozen ground (sometimes called 'thaw fog').
• Occurs both day and night, unlike radiation fog which is mostly nocturnal.

Characteristics and Persistence

• Can cover large areas and persist for long periods, especially if the temperature difference between the air and surface remains.
• May be blown inland from the sea, affecting airports and coastal operations.
• Stronger winds can lift advection fog into low stratus clouds or disperse it, but if the surface remains much colder than the air, the fog can persist even with winds up to 30–40 KT.

Dissipation

• Advection fog typically dissipates when the surface warms (e.g., daytime heating) or if the wind changes direction, bringing in drier or cooler air.
• However, if the underlying temperature contrast remains and moist air continues to flow, the fog may reform quickly.

Advection vs. Radiation Fog

• Advection fog requires horizontal air movement and a cold surface, while radiation fog forms due to nocturnal cooling of the ground under clear skies and light winds.
• Advection fog can occur over sea or land, at any time, and is often more persistent and widespread than radiation fog.

Aviation Impacts

• Advection fog can severely reduce visibility at airports, especially in coastal regions, and is a major operational hazard for flight planning and approach/landing phases.