Fog is a familiar atmospheric phenomenon defined simply as a cloud resting on the Earth’s surface. This visible aerosol is composed of tiny water droplets or ice crystals suspended in the air. Fog significantly reduces visibility to less than one kilometer, which distinguishes it from mist. The various types of fog are determined by the specific meteorological process that causes the air to become saturated, leading to condensation.
The Mechanics of Fog Formation
Fog formation requires the air to reach saturation, a state where it can hold no more water vapor. This saturation is achieved when the air temperature cools down to the dew point, the temperature at which water vapor begins to condense into liquid water. Fog is more likely to form when the air temperature and the dew point are close, often when the difference is less than 2.5 degrees Celsius.
Condensation requires microscopic airborne particles known as condensation nuclei. These particles, such as dust, salt, or pollutants, provide a surface for water vapor molecules to cluster and form droplets. The density of the fog, and the resulting reduction in visibility, depends on the number and size of these droplets.
Fog Types Defined by Surface Cooling
Many common types of fog occur when an air mass is cooled by contact with a colder surface, causing the temperature to drop to the dew point. Radiation fog forms predominantly over land on clear nights with light winds. The ground rapidly loses heat by radiating it into space, which cools the layer of air immediately above it through conduction.
This process often creates a temperature inversion, leading to saturation and condensation near the surface. Radiation fog, sometimes called ground fog, is often shallow and tends to be thickest around sunrise. As the sun warms the ground, the heated air causes turbulence and evaporation, leading to the fog dissipating.
Advection fog relies on the horizontal movement of air (advection) rather than static cooling. This type forms when warm, moist air blows over a significantly colder surface, such as a cold ocean current or snow-covered ground. The lower layer of the moving air mass cools rapidly by conduction with the cold surface, causing the water vapor to condense.
For example, persistent summer fog in San Francisco occurs when warm Pacific air moves over cold, upwelling ocean water. Because advection fog involves the movement of an entire air mass over a large cold area, it is often more widespread and persistent than radiation fog. The fog layer can be deep, often confined beneath a strong temperature inversion that prevents vertical mixing.
Fog Types Defined by Air Movement and Moisture Addition
Other fog types are produced by dynamic air movement or the active addition of water vapor. Upslope fog, also known as orographic fog, forms when moist, stable air is forced to rise along the slope of a mountain or hill. As the air mass gains altitude, it expands due to lower atmospheric pressure and cools adiabatically.
This cooling lowers the air’s temperature to its dew point, leading to saturation and the formation of a fog layer that appears to hug the terrain. This mechanism is similar to how clouds form, but it occurs at ground level. Continuous forced lifting allows upslope fog to persist for hours or days if the wind flow remains steady.
Steam fog, also called evaporation fog or “Arctic sea smoke,” occurs when cold, dry air moves over a much warmer body of water, such as a lake or river. The warm water rapidly evaporates into the cold air immediately above it, adding moisture and quickly raising the dew point. When this newly warmed, moist air mixes with the much colder air above it, the mixture saturates and condenses. This results in wispy plumes of fog rising off the water’s surface, resembling steam.
Precipitation fog, also known as frontal fog, is formed by the addition of moisture through evaporation. This fog typically forms ahead of a warm front when warm rain falls into a layer of colder, drier air near the surface. As the raindrops fall, they evaporate into the cold air, increasing its water vapor content until saturation is reached. The resulting fog can be dense and widespread, often occurring when strong winds are absent.
Specialized Cold Weather Fog
Under extremely cold conditions, the physical state of the water in the fog changes, creating specialized types of cold weather fog. Freezing fog occurs when the air temperature is below freezing, but the fog droplets remain liquid in a supercooled state. These liquid droplets can exist down to temperatures as low as -20 degrees Celsius.
The defining characteristic is that the supercooled droplets freeze instantly upon contact with any solid surface, such as trees or vehicles. This freezing process deposits a white, feathery coating of ice known as rime ice.
Ice fog, by contrast, forms only in extremely cold environments, typically below -30 degrees Celsius. At these low temperatures, water vapor sublimates directly into minute ice crystals rather than liquid droplets. The fog is composed entirely of these suspended ice crystals and is commonly found in polar regions or near sources of concentrated moisture in very cold air.