Fog is a meteorological phenomenon often described as a stratus cloud that rests at or near the Earth’s surface. It forms when the air’s temperature and its dew point are very close, causing the air to become saturated with water vapor. This saturation causes water vapor to condense into millions of tiny liquid water droplets suspended in the air. Advection fog is one of the most common and often densest types of fog, and its formation is directly tied to the movement of air across different temperature zones. The resulting reduction in visibility can be significant, posing hazards to transportation across land, sea, and air.
The Mechanism of Formation
The creation of advection fog is driven by advection, the horizontal transfer of heat or moisture by the movement of air. This fog requires a mass of warm, moist air to move across a significantly cooler surface, which can be either land or water. As the warm air travels over this cold surface, the layer of air closest to the ground is cooled rapidly by conduction. This process quickly lowers the air temperature while the moisture content remains high.
The air continues to cool until its temperature drops to the dew point, reaching saturation. Once saturated, the excess water vapor condenses around microscopic particles, such as dust or salt, which act as condensation nuclei. This forms the visible fog droplets characteristic of advection fog. The movement of the air mass sustains the fog, constantly feeding the condensation process with more warm, moist air. For the fog to be sustained, the wind speed is usually light, typically remaining between three and nine knots.
Typical Locations and Persistence
Advection fog is most frequently observed in coastal environments, particularly where warm ocean air encounters cold ocean currents. A prominent example is the West Coast of the United States, where moist air from the Pacific Ocean is blown over the cold, upwelling waters near the shore. The fog bank can then be pushed inland by sea breezes, blanketing coastal cities like San Francisco. Another common location is the Grand Banks off Newfoundland, where the warm, moist air of the Gulf Stream meets the cold Labrador Current.
The phenomenon is not exclusive to coastlines; it can also form inland during the winter months. This occurs when mild, moist air moves over a cold landmass or a surface covered in snow or frost. Because the fog is continuously generated by the movement of the air mass, it tends to be persistent and long-lasting, sometimes persisting for several days. This contrasts with other fog types that dissipate quickly when the sun warms the ground.
Distinguishing Advection Fog from Radiation Fog
Advection fog is fundamentally defined by the horizontal movement of air, which is the “advection” component of its name. Unlike other types, it can form under cloudy skies and with moderate winds because the cooling mechanism comes from the underlying surface rather than the air’s own cooling. This horizontal transport allows advection fog to occur at any time of day or night.
In contrast, radiation fog is characterized by stillness and vertical cooling. Radiation fog forms when the ground cools rapidly overnight through the emission of long-wave radiation, which then cools the air immediately above it. This process requires clear skies and very light winds to prevent mixing. As a result, radiation fog typically forms at night and dissipates quickly after sunrise as the sun warms the ground.