Fog is a cloud forming at or near the Earth’s surface, made of tiny liquid water droplets or, in very cold conditions, ice crystals. These particles scatter light, causing reduced visibility. Understanding fog formation explains when this phenomenon appears.
The Fundamental Process of Fog Formation
Fog forms when water vapor cools to its dew point, the temperature at which it becomes saturated. Once saturated, excess water vapor transforms into tiny liquid droplets.
Water droplets require microscopic particles, known as condensation nuclei, to form. These nuclei, such as dust, pollen, or pollutants, are always present. Without them, water vapor struggles to condense into visible droplets, even at saturation. Nuclei presence influences fog formation.
Atmospheric Conditions for Fog
Several atmospheric conditions align for fog to develop. A primary factor is air cooling to its dew point, which reduces its capacity to hold water vapor and leads to saturation.
High humidity, meaning significant moisture, is another important condition. When air temperature and dew point are very close, fog is more likely. Light winds are conducive to fog, allowing air to cool and moisture to accumulate without dispersal. Clear skies, especially at night, facilitate cooling by allowing heat to radiate from the ground.
Understanding Different Fog Types
Different fog types arise from distinct combinations of these atmospheric conditions.
Radiation Fog
Radiation fog, a common type, forms on clear, calm nights when the ground rapidly loses heat through radiation. The cooling ground chills the air above it to its dew point, causing fog to develop, often in low-lying areas. This fog typically dissipates after sunrise as the sun warms the ground and air.
Advection Fog
Advection fog occurs when warm, moist air moves over a cooler surface, such as cold land or water. As warm, humid air passes over the colder surface, it cools to its dew point, leading to widespread condensation. Coastal regions are prone to advection fog when warm, moist ocean air encounters cooler land. This fog can persist for extended periods and cover large areas.
Upslope Fog
Upslope fog develops when moist air rises along elevated terrain, such as hills or mountains. As the air ascends, it undergoes adiabatic cooling, expanding and cooling due to lower atmospheric pressure. This cooling brings the air to its dew point, causing widespread condensation and fog on the slopes.
Evaporation Fog
Evaporation fog, also known as steam fog, forms when cold air moves over warmer water. Warm water evaporates into the cold air, rapidly increasing its moisture. This added moisture quickly saturates the colder air, leading to condensation and fog resembling steam rising from the water.
Why Fog Doesn’t Always Form
Even when conditions seem favorable, fog does not always materialize. Strong winds prevent fog formation by mixing moist air near the surface with drier air from aloft. This turbulent mixing prevents the air from cooling sufficiently or becoming saturated. If the wind is too weak, dew might form on surfaces instead of fog.
Low humidity inhibits fog, as there is insufficient moisture to reach saturation, even with significant cooling. Extensive cloud cover can prevent the ground from radiating heat effectively, limiting the cooling necessary for some fog types, particularly radiation fog. These factors demonstrate the precise balance of conditions required for fog.