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Fog is a visible aerosol consisting of tiny water droplets or ice crystals suspended in the air at or near the Earth's surface. [1] [2] Fog can be considered a type of low-lying cloud usually resembling stratus and is heavily influenced by nearby bodies of water, topography, and wind conditions.
Radiational cooling occurs due to the emission of infrared radiation, either by the air or by the surface underneath. [20] This type of cooling is common during the night when the sky is clear. Evaporative cooling happens when moisture is added to the air through evaporation, which forces the air temperature to cool to its wet-bulb temperature ...
Tule fog is a radiation fog, which condenses when there is a high relative humidity (typically after a heavy rain), calm winds, and rapid cooling during the night. The nights are longer in the winter months, which allows an extended period of ground cooling, and thereby a pronounced temperature inversion at a low altitude.
With sufficient humidity in the cooler layer, fog is typically present below the inversion cap. An inversion is also produced whenever radiation from the surface of the earth exceeds the amount of radiation received from the sun, which commonly occurs at night, or during the winter when the sun is very low in the sky.
The fog season is usually based in the cooler months (late autumn, winter and early spring). An example is found in the San Joaquin Valley and Sacramento Valley areas of California's Great Central Valley, where a thick ground fog, known as Tule fog, may form, in particular in the months from November through March. [1]
Thus, fog and mist are not precipitation; their water vapor does not condense sufficiently to precipitate, so fog and mist do not fall. (Such a non-precipitating combination is a colloid.) Two processes, possibly acting together, can lead to air becoming saturated with water vapor: cooling the air or adding water vapor to the air.
A virga, also called a dry storm, is an observable streak or shaft of precipitation that evaporates or sublimates before reaching the ground. [1] A shaft of precipitation that does not evaporate before reaching the ground is known in meteorology as a precipitation shaft.
The decline in fog is generally attributed to climate change, and is concerning for the local ecology, for example the redwood trees. [12] Climate change contributes to the warming of our oceans, directly resulting in less fog as ocean water is not cold enough to mix with hot, moist air currents to create fog. [13]