Fog is a common atmospheric phenomenon, often described as a cloud resting on the ground. It forms when the air near the surface becomes saturated with water vapor, turning invisible gas into a visible suspension of tiny water droplets. The frequent appearance of fog is governed by physical principles that dictate when and where it occurs, resulting from environmental mechanisms combining to cool the air to its saturation point.
The Fundamental Science of Fog Formation
Fog formation requires three ingredients. First, sufficient moisture (water vapor) must be present near the ground. Second, the air temperature must cool down to its dew point, the temperature at which the air becomes completely saturated (100% relative humidity). When the air temperature and dew point are nearly equal, the air can no longer hold all the water vapor in a gaseous state.
The third ingredient is the presence of microscopic airborne particles known as condensation nuclei. These particles, which can be dust, pollen, or pollution, serve as surfaces for the water vapor to condense upon. Without these nuclei, the air would need to be supersaturated before droplets could form. Cooling the air to its dew point is the primary driver, causing the water vapor to condense onto these abundant nuclei and create the dense visibility reduction known as fog.
Classifying Fog by Environmental Triggers
Fog is classified based on the variety of ways the air can be cooled to its dew point.
Radiation Fog
Radiation fog occurs when the ground rapidly loses heat after sunset, typically on clear, calm nights. This loss of radiation cools the ground, which in turn cools the air directly above it by conduction. This causes condensation to begin near the surface. This type of fog is often shallow and tends to form in low-lying areas like valleys.
Advection Fog
Advection fog forms when warm, moist air moves horizontally over a significantly colder surface, such as a cold ocean current or snow-covered land. The lower layer of the warm air mass is chilled from below, leading to widespread and often very deep fog that can persist for long periods.
Upslope Fog
Upslope fog is generated when wind forces moist, stable air up the side of a mountain or hill. As the air rises, it expands and cools adiabatically, meaning it cools simply because of the reduction in pressure. This causes the air to reach its saturation point and form a layer of fog.
Steam Fog
Steam fog, also known as evaporation fog, forms by adding moisture rather than cooling the air. This occurs when very cold, dry air moves over a much warmer body of water. The warm water evaporates rapidly into the cold air, raising the dew point until the air becomes saturated. The excess moisture then condenses, resulting in wispy columns that resemble steam.
Why Certain Regions Experience High Fog Frequency
Certain geographic locations experience frequent, persistent fog because their features consistently favor one or more of these formation mechanisms. Coastal areas near cold ocean currents, such as the California coast, are prone to dense advection fog. Warm, humid air from the ocean is constantly blown over the cold water, chilling the air mass and causing immediate saturation. This creates sea fog that is then carried inland.
Inland areas, particularly valleys and basins, are susceptible to frequent radiation fog, often prolonged by a temperature inversion. On clear nights, cold, dense air settles in these low-lying areas, and subsequent cooling near the surface leads to fog formation. A layer of warmer air above the valley acts like a lid, trapping the cool, moist air and preventing vertical mixing with drier air. This geographic trapping can lead to dense, long-lasting events, such as the Tule fog in California’s Central Valley.
How Fog Dissipates and Clears
Fog clears when one of the conditions for its formation is reversed. The most common method of dissipation is through solar heating, often referred to as “burning off.” As the sun rises, the ground and the air near it warm, causing the tiny water droplets to evaporate back into invisible water vapor. This process typically lifts the fog from the bottom up until the entire layer evaporates.
Another mechanism is the introduction of drier air, or mixing, usually caused by an increase in wind speed. A stronger wind can entrain drier air from above the fog layer, which lowers the overall humidity of the fog mass, leading to evaporation of the droplets. Finally, fog can dissipate through gravitational settling or light precipitation, where the water droplets within the fog grow large enough to fall slowly to the ground as drizzle, effectively clearing the air.