Fog
Fog is a visible aerosol consisting of tiny water droplets or ice crystals held in the air near the Earth's surface. 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.
Fog appears when water vapor 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. 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%. 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. 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.
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 often 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. 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.
File:San francisco in fog with rays.jpg|thumb|upright|Advection fog layer in San Francisco with the Golden Gate Bridge and skyline in the background
File:Sydney Harbour Fog - panoramio.jpg|thumb|upright|Advection fog over Sydney Harbour and the Sydney Opera House, Australia
Advection fog occurs when moist air passes over a cool surface by advection 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".File:Aerial View of freezing fog in the Okanagan Highlands.webm|thumb|Aerial video of freezing fog in the Okanagan Highlands
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.
Frozen fog is any kind of fog where the droplets have frozen into extremely tiny crystals of ice in midair. Generally, this requires temperatures at or below, making it common only in and near the Arctic and Antarctic regions. It is most often seen in urban areas where it is created by the freezing of water vapor present in automobile exhaust and combustion products from heating and power generation. Urban ice fog can become extremely dense and will persist day and night until the temperature rises. It can be associated with the diamond dust form of precipitation, in which very small crystals of ice form and slowly fall. This often occurs during blue sky conditions, which can cause many types of halos and other results of refraction of sunlight by the airborne crystals. Ice fog often leads to the visual phenomenon of light pillars.