How Does Fog Form?

Fog is a cloud of small water droplets near ground level. It is formed by the condensation of water vapor on tiny particles that are always present in natural air; this occurs when the air becomes saturated with moisture. This increase of moisture can by caused by three processes:

• when the air cools by adiabatic expansion—as air rises, it encounters a decrease in pressure and expands, cooling as it does so
• when two humid airstreams with different temperatures mix—for example, when air that has been in contact with a wet ground or water surface has a different temperature from the air above and combines with this air
• when the air is cooled by radiation—this happens because the ground continuously radiates heat, but after the sun sets, the ground loses more heat than it absorbs, which cools the ground and, in certain weather conditions, chills the air above it

There are different types of fog. For example, upslope fog occurs when humid air is forced up the sides of hills or mountains. Advection fog forms when warm, moist air moves slowly over a colder surface, such as when warm air from the Gulf Stream blows over the cold Labrador Current, leading to dense fogs off the Grand Banks of Newfoundland. Steam fog forms when cold air moves over a warm, wet surface, causing moisture to evaporate into the air, which then becomes saturated. This type of fog is common in Arctic regions when cold air moves over open water or newly formed openings in pack ice. Frontal fog forms near weather fronts when raindrops from warmer air above evaporate into cooler air near the Earth’s surface, increasing its moisture content and leading to saturation. Radiation fog forms over land on calm, clear nights when the ground loses heat through radiation and cools the air above it to below the dew-point temperature. Ice fog occurs in extremely cold temperatures, when the water droplets that make up fog freeze into ice crystals.