Advection fog is a type of cloud that forms at ground level, created by the large-scale, horizontal movement of air across a contrasting surface. The term “advection” refers to the transport of atmospheric properties like heat or moisture by the wind. This fog develops when warm, humid air travels over a significantly colder area, causing the air mass to cool rapidly and condense its moisture content.
The Core Mechanism of Advection Fog
The formation of advection fog requires a sustained flow of warm, moist air across a cool, stable surface. As the moving air mass encounters the colder ground or water, the lowest layer of air cools by conduction, the transfer of heat through direct contact. This cooling quickly lowers the air temperature toward its dew point, the temperature at which the air becomes fully saturated.
Once saturation is reached, the excess water vapor condenses into microscopic water droplets, creating a dense layer of fog. This process relies on continuous air movement, allowing advection fog to form even when moderate winds are present. The surface cooling also creates a temperature inversion, trapping moisture near the ground and allowing the fog to thicken and persist.
Geographic Situations Favoring Formation
Advection fog is most commonly observed along coastal regions where warm ocean currents meet cold ones or where prevailing winds carry marine air over colder landmasses. The Grand Banks off Newfoundland are notoriously foggy because warm, moist air from the Gulf Stream flows over the frigid Labrador Current. The California coast also experiences frequent advection fog when warm Pacific air is carried over cooler, upwelled water.
The phenomenon also occurs inland under specific seasonal conditions, particularly in late winter and early spring. In the Midwestern United States, warm, humid air from the Gulf of Mexico may be advected northward over ground that is still snow-covered or frozen. This large temperature difference leads to widespread fog formation, such as when warm air moves over the cold surface of the Great Lakes.
Distinguishing Advection Fog from Radiation Fog
Advection fog differs significantly from radiation fog, which is common inland. The primary difference lies in the mechanism of cooling and the presence of air movement. Radiation fog forms on clear, calm nights when the ground cools by radiating heat into space, chilling the air directly above it, and requires little to no wind. Advection fog, conversely, is driven by the horizontal transport of an air mass, meaning it requires wind to form and sustain itself. Unlike radiation fog, which typically dissipates shortly after sunrise, advection fog can persist at any time of day or night and is often much deeper and more widespread.
Impact on Maritime and Air Travel
Advection fog presents a hazard to both maritime and air transportation. Since it is formed by the movement of an entire air mass, it tends to cover vast areas in thick, persistent sheets, often lasting for days until the wind direction changes or the temperature gradient is eliminated. This persistence leads to prolonged periods of low visibility. For maritime shipping, this fog, often termed “sea fog,” complicates navigation in busy coastal shipping lanes. In aviation, advection fog is a major cause of delays and airport closures, particularly at coastal airfields, necessitating strict adherence to instrument flight rules.