Fog is a visible aerosol consisting of tiny water droplets or ice crystals held in the air near the Earth's surface. Reprint from 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. In turn, fog affects many human activities, such as shipping, travel, and warfare. Recent atmospheric research indicates that fog also functions as an active aquatic ecosystem. Fog droplets harbor microbiomes where bacteria actively grow, divide, and interact with atmospheric chemistry.
Fog appears when water vapor (water in its gaseous form) condenses. During condensation, molecules of water vapor combine to make tiny water droplets that hang in the air. Sea fog, which shows up near bodies of saline water, is formed as water vapor condenses on bits of salt. Fog is similar to, but less transparent than, mist.
Definition The term fog is typically distinguished from the more generic term cloud in that fog is low-lying, and the moisture in the fog is often generated locally (such as from a nearby body of water, like a lake or ocean, or from nearby moist ground or marshes).Use of the term "fog" to mean any cloud that is at or near the Earth's surface can result in ambiguity as when, for example, a stratocumulus cloud covers a mountaintop. An observer on the mountain may say that he or she is in a fog, however, to outside observers a cloud is covering the mountain. () In fact, some people commonly mistake mist for fog. These two are a little bit different as mist is thinner than fog. () Further distinguishing the terms, fog rarely results in rain, while clouds are the common source of rain. By definition, fog reduces visibility to less than , whereas mist causes lesser impairment of visibility.
Formation
Fog forms when the difference between air temperature and dew point is less than . Fog begins to form when water vapor condenses into tiny water droplets that are suspended in the air. Some examples of ways that water vapor is condensed include wind convergence into areas of upward motion; precipitation or virga falling from above; daytime heating evaporating water from the surface of oceans, water bodies, or wet land; transpiration from plants; cool or dry air moving over warmer water; and lifting air over mountains. Water vapor normally begins to condense on condensation nuclei such as dust, ice, and salt in order to form clouds. Fog, like its elevated cousin stratus, is a stable cloud deck which tends to form when a cool, stable air mass is trapped underneath a warm air mass.
Fog normally occurs at a relative humidity near 100%.Gleissman, Stephe (2007). Agroecology: the ecology of sustainable food systems. CRC Press. p. 73. . This occurs from either added moisture in the air, or falling ambient air temperature. However, fog can form at lower humidities and can sometimes fail to form with relative humidity at 100%. At 100% relative humidity, the air cannot hold additional moisture, thus the air will become supersaturated if additional moisture is added.
Fog commonly produces precipitation in the form of drizzle or very light snow. Drizzle occurs when the humidity attains 100% and the minute cloud droplets begin to coalesce into larger droplets.Allred, Lance (2009). Enchanted Rock: A Natural and Human History. University of Texas Press. p. 99. . This can occur when the fog layer is lifted and cooled sufficiently, or when it is forcibly compressed from above by descending air. Drizzle becomes freezing drizzle when the temperature at the surface drops below the freezing point.
The thickness of a fog layer is largely determined by the altitude of the inversion boundary, which in coastal or oceanic locales is also the top of the marine layer, above which the air mass is warmer and drier. The inversion boundary varies its altitude primarily in response to the weight of the air above it, which is measured in terms of atmospheric pressure. The marine layer, and any fog-bank it may contain, will be "squashed" when the pressure is high and conversely may expand upwards when the pressure above it is lowering.
Fog microbiome and ecology While the atmosphere has long been known to transport microorganisms, research on overland radiation fogs demonstrates that fog droplets serve as an active microhabitat. Fog water contains bacterial concentrations similar to those in continental or marine bodies of water.
The composition of the fog microbiome is different from the dry aerosol microbiome. It is enriched with specific bacterial taxa, most notably by pink-pigmented, photoheterotrophic Methylobacterium species. The fog water bacteria have also been found to increase in cell volume and exhibit higher rates of active cell division compared to those in dry air, indicating in situ biological growth. Following the dissipation of fog, the concentration of these bacteria in the local aerobiome can increase by an average of 45%.
Types Fogs are classified into several types based on the process that formed them.
Radiation fog is formed by the cooling of Earth's surface and lower atmosphere after sunset, in clear and calm conditions. The ground reradiates heat absorbed during the day, cooling due to heat loss. The cooling ground then cools adjacent air by conduction, causing the air temperature to fall and be cooler than the air immediately above it, a condition known as an inversion. When the air underneath the inversion layer is sufficiently humid and reaches the dew point, fog will form. Moderate to strong wind tends to prevent fog formation by circulating the air and preventing the cooling effect. Extensive fog is most likely to form in a slight breeze, as turbulence can spread the cooled surface air. This can produce a layer of fog several hundred feet in depth, above which the air remains warm. Low thermal conductivity of soil also contributes to the formation of radiation fog by limiting the upward flow of heat to the surface and allowing the radiative cooling process to dominate. High soil moisture can increase the humidity just above the ground, contributing to fog formation. Radiation fog most often occurs over swampy terrain and in valleys where cold air flows in and collects at the bottom. It usually covers a wide area, however can not form over a water surface. Radiation fog forms mostly at night but formation often begins in the late afternoon. Radiation fog is thickest shortly after sunrise, when the sun's energy increases air turbulence and therefore thicker fog, before it is strong enough to evaporate fog droplets into warmed air. During summer, longer days mean that the Earth's surface is heated to a greater degree and there is less opportunity for radiative cooling to occur. Additionally, there is less moisture in the surface of the soil. Radiation fog usually occurs in late autumn and winter, in colder conditions with longer nights. Radiation fog can persist all day in the winter months especially in areas bounded by high ground. An example of radiation fog is tule fog.
Ground fog is fog that obscures less than 60% of the sky and does not extend to the base of any overhead clouds.Climate education update: News and information about climate change for teachers and students . Atmospheric Radiation Measurement. Climate Research Facility. U.S. Department of Energy. education.arm.gov However, the term is usually a synonym for shallow radiation fog; in some cases the depth of the fog is on the order of tens of centimetres over certain kinds of terrain with the absence of wind.
Advection fog occurs when moist air passes over a cool surface by advection (wind) and is cooled. It is common as a warm front passes over an area with significant snow-pack. It is most common at sea when moist air encounters cooler waters, including areas of cold water upwelling, such as along the California coast. A strong enough temperature difference over water or bare ground can also cause advection fog.
Although strong winds often mix the air and can disperse, fragment, or prevent many kinds of fog, markedly warmer and humid air blowing over a snowpack can continue to generate advection fog at elevated velocities up to or more – this fog will be in a turbulent, rapidly moving, and comparatively shallow layer, observed as a few centimetres/inches in depth over flat farm fields, flat urban terrain and the like, and/or form more complex forms where the terrain is different such as rotating areas in the lee of hills or large buildings and so on.
Fog formed by advection along the California coastline is propelled onto land by one of several processes. A cold front can push the marine layer coast-ward, an occurrence most typical in the spring or late fall. During the summer months, a low-pressure trough produced by intense heating inland creates a strong pressure gradient, drawing in the dense marine layer. Also, during the summer, strong high pressure aloft over the desert southwest, usually in connection with the summer monsoon, produces a south to southeasterly flow which can drive the offshore marine layer up the coastline; a phenomenon known as a "southerly surge", typically following a coastal heat spell. However, if the monsoonal flow is sufficiently turbulent, it might instead break up the marine layer and any fog it may contain. Moderate turbulence will typically transform a fog bank, lifting it and breaking it up into shallow convective clouds called stratocumulus.
Frontal fog forms in much the same way as stratus cloud near a front when raindrops, falling from relatively warm air above a frontal surface, evaporate into cooler air close to the Earth's surface and cause it to become saturated. The water vapor cools and at the dewpoint it condenses and fog forms. This type of fog can be the result of a very low frontal stratus cloud subsiding to surface level in the absence of any lifting agent after the front passes.
Hail fog sometimes occurs in the vicinity of significant hail accumulations due to decreased temperature and increased moisture leading to saturation in a very shallow layer near the surface. It most often occurs when there is a warm, humid layer atop the hail and when wind is light. This ground fog tends to be localized but can be extremely dense and abrupt. It may form shortly after the hail falls; when the hail has had time to cool the air and as it absorbs heat when melting and evaporating.
Freezing conditions
occurs when liquid fog droplets freeze to surfaces, forming white soft or hard rime ice. This is very common on mountain tops which are exposed to low clouds. It is equivalent to freezing rain and essentially the same as the ice that forms inside a freezer which is not of the "frostless" or "frost-free" type. The term "freezing fog" may also refer to fog where water vapor is super-cooled, filling the air with small ice crystals similar to very light snow. It seems to make the fog "tangible", as if one could "grab a handful".
In the western United States, freezing fog may be referred to as pogonip. It occurs commonly during cold winter spells, usually in deep mountain valleys. The word pogonip is derived from the Shoshone word paγi̵nappi̵h, which means "cloud". In The Old Farmer's Almanac, in the calendar for December, the phrase "Beware the Pogonip" regularly appears. In his anthology Smoke Bellew, Jack London describes a pogonip which surrounded the main characters, killing one of them.
The phenomenon is common in the inland areas of the Pacific Northwest, with temperatures in the range. The Columbia Plateau experiences this phenomenon most years during temperature inversions, sometimes lasting for as long as three weeks. The fog typically begins forming around the area of the Columbia River and expands, sometimes covering the land to distances as far away as La Pine, Oregon, almost due south of the river and into south central Washington.