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Fog is a visible aerosol consisting of tiny water droplets or ice crystals suspended in the air 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. In turn, fog affects many human activities, such as shipping, travel, and warfare.

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

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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).[3] By definition, fog reduces visibility to less than 1 km (0.62 mi), whereas mist causes lesser impairment of visibility.[4][5]

Formation

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See also: Cloud physics
Minute droplets of water constitute this after-dark radiation fog, with an ambient temperature of −2 °C (28 °F). Their motion trails are captured as streaks.
A close-up view of water droplets forming fog. Those outside the camera lens's depth of field appear as orbs.

Fog forms when the difference between air temperature and dew point is less than 2.5 °C (4.5 °F).[6][7] 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;[8] precipitation or virga falling from above;[9] daytime heating evaporating water from the surface of oceans, water bodies, or wet land;[10] transpiration from plants;[11] cool or dry air moving over warmer water;[12] and lifting air over mountains.[13] Water vapor normally begins to condense on condensation nuclei such as dust, ice, and salt in order to form clouds.[14][15] 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.[16]

Fog normally occurs at a relative humidity near 100%.[17] This occurs from either added moisture in the air, or falling ambient air temperature.[17] 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.[18] 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

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Fog can form multiple ways, depending on how the cooling occurred that caused the condensation.

Radiation fog is formed by the cooling of land after sunset by infrared thermal radiation in calm conditions with a clear sky. The cooling ground then cools adjacent air by conduction, causing the air temperature to fall and reach the dew point, forming fog. In perfect calm, the fog layer can be less than a meter thick, but turbulence can promote a thicker layer. Radiation fog occurs at night and usually does not last long after sunrise, but it can persist all day in the winter months especially in areas bounded by high ground. Radiation fog is most common in autumn and early winter. Examples of this phenomenon include tule fog.[19]

Ground fog is fog that obscures less than 60% of the sky and does not extend to the base of any overhead clouds.[20] 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 layer in San Francisco with the Golden Gate Bridge and skyline in the background
Advection fog over Sydney Harbour and the Sydney Opera House, Australia

Advection fog occurs when moist air passes over a cool surface by advection (wind) and is cooled.[21] 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 80 km/h (50 mph) 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.[22]

Freezing conditions

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Freezing fog occurs when liquid fog droplets freeze to surfaces, forming white soft or hard rime ice.[23] 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".

Aerial video of freezing fog in the Okanagan Highlands

In the western United States, freezing fog may be referred to as pogonip.[24] 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".[24][25] 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 10 to 30 °F (−12 to −1 °C) 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 150 miles (240 km) due south of the river and into south central Washington.

Frozen fog (also known as ice 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 −35 °C (−31 °F), making it common only in and near the Arctic and Antarctic regions.[26] 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.

Topographical influences

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Fog over the Pedra do Sino (Bell Rock; left) and Dedo de Deus (God's Finger; right) peaks in the Serra dos Órgãos National Park, Rio de Janeiro state, Brazil

Up-slope fog or hill fog forms when winds blow air up a slope (called orographic lift), adiabatically cooling it as it rises and causing the moisture in it to condense. This often causes freezing fog on mountaintops, where the cloud ceiling would not otherwise be low enough.

Valley fog forms in mountain valleys, often during winter. It is essentially a radiation fog confined by local topography and can last for several days in calm conditions. In California's Central Valley, valley fog is often referred to as tule fog.

Yucca Valley, California tule fog

Sea and coastal areas

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Sea fog (also known as haar or fret) is heavily influenced by the presence of sea spray and microscopic airborne salt crystals. Clouds of all types require minute hygroscopic particles upon which water vapor can condense. Over the ocean surface, the most common particles are salt from salt spray produced by breaking waves. Except in areas of storminess, the most common areas of breaking waves are located near coastlines, hence the greatest densities of airborne salt particles are there.

Condensation on salt particles has been observed to occur at humidities as low as 70%, thus fog can occur even in relatively dry air in suitable locations such as the California coast. Typically, such lower humidity fog is preceded by a transparent mistiness along the coastline as condensation competes with evaporation, a phenomenon that is typically noticeable by beachgoers in the afternoon. Another recently discovered source of condensation nuclei for coastal fog is kelp seaweed. Researchers have found that under stress (intense sunlight, strong evaporation, etc.), kelp releases particles of iodine which in turn become nuclei for condensation of water vapor, causing fog that diffuses direct sunlight.[27]

Sea smoke, also called steam fog or evaporation fog, is created by cold air passing over warmer water or moist land.[28] It may cause freezing fog or sometimes hoar frost. This situation can also lead to the formation of steam devils, which look like their dust counterparts.[29] Lake-effect fog is of this type, sometimes in combination with other causes like radiation fog. It tends to differ from most advective fog formed over land in that it is (like lake-effect snow) a convective phenomenon, resulting in fog that can be very dense and deep and looks fluffy from above. Arctic sea smoke is similar to sea smoke but occurs when the air is very cold. Instead of condensing into water droplets, columns of freezing, rising, and condensing water vapor is formed. The water vapor produces the sea smoke fog and is usually misty and smoke-like.[30]

Garúa fog near the coast of Chile and Peru[31] occurs when typical fog produced by the sea travels inland but suddenly meets an area of hot air. This causes the water particles of fog to shrink by evaporation, producing a "transparent mist". Garua fog is nearly invisible, yet it still forces drivers to use windshield wipers because of condensation onto cooler hard surfaces. Camanchaca is a similar dense fog.

Effects

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Visibility

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Fog in Serbia
Light fog reduces visibility on a suburban street, rendering the cyclist very hazy at about 200 m (220 yd). The limit of visibility is about 400 m (440 yd), which is before the end of the street.

Depending on the concentration of the droplets, visibility in fog can range from the appearance of haze to almost zero visibility. Many lives are lost each year worldwide from accidents involving fog conditions on the highways, including multiple-vehicle collisions.

The aviation travel industry is affected by the severity of fog conditions. Even though modern auto-landing computers can put an aircraft down without the aid of a pilot, personnel manning an airport control tower must be able to see if aircraft are sitting on the runway awaiting takeoff. Safe operations are difficult in thick fog, and civilian airports may forbid takeoffs and landings until conditions improve.

A solution for landing returning military aircraft developed in World War II was called Fog Investigation and Dispersal Operation (FIDO). It involved burning enormous amounts of fuel alongside runways to evaporate fog, allowing returning fighter and bomber pilots sufficient visual cues to safely land their aircraft. The high energy demands of this method discourage its use for routine operations.

Shadows

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Sutro Tower casts a 3-dimensional fog shadow

Shadows are cast through fog in three dimensions. The fog is dense enough to be illuminated by light that passes through gaps in a structure or tree, but thin enough to let a large quantity of that light pass through to illuminate points further on. As a result, object shadows appear as "beams" oriented in a direction parallel to the light source. These voluminous shadows are created the same way as crepuscular rays, which are the shadows of clouds. In fog, it is solid objects that cast shadows.

Sound propagation and acoustics

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See also: Acoustic location, Acoustic shadow, and Foghorn

Sound typically travels fastest and farthest through solids, then liquids, then gases such as the atmosphere. Sound is affected during fog conditions due to the small distances between water droplets, and air temperature differences.

Though fog is essentially liquid water, the many droplets are separated by small air gaps. High-pitched sounds have a high frequency, which in turn means they have a short wavelength. To transmit a high frequency wave, air must move back and forth very quickly. Short-wavelength high-pitched sound waves are reflected and refracted by many separated water droplets, partially cancelling and dissipating their energy (a process called "damping"). In contrast, low pitched notes, with a low frequency and a long wavelength, move the air less rapidly and less often, and lose less energy to interactions with small water droplets. Low-pitched notes are less affected by fog and travel further, which is why foghorns use a low-pitched tone.[32]

A fog can be caused by a temperature inversion where cold air is pooled at the surface which helped to create the fog, while warmer air sits above it. The inverted boundary between cold air and warm air reflects sound waves back toward the ground, allowing sound that would normally radiate out escaping into the upper atmosphere to instead bounce back and travel near the surface. A temperature inversion increases the distance that lower frequency sounds can travel, by reflecting the sound between the ground and the inversion layer.[33]

Record extremes

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Particularly foggy places include[citation needed] Hamilton, New Zealand and Grand Banks off the coast of Newfoundland (the meeting place of the cold Labrador Current from the north and the much warmer Gulf Stream from the south). Some very foggy land areas in the world include Argentia (Newfoundland) and Point Reyes (California), each with over 200 foggy days per year.[citation needed] Even in generally warmer southern Europe, thick fog and localized fog are often found in lowlands and valleys, such as the lower part of the Po Valley and the Arno and Tiber valleys in Italy; Ebro Valley in northeastern Spain; as well as on the Swiss plateau, especially in the Seeland area, in late autumn and winter.[citation needed] Other notably foggy areas include coastal Chile (in the south); coastal Namibia; Nord, Greenland; and the Severnaya Zemlya islands.[citation needed]

As a water source

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Redwood forests in California receive approximately 30–40% of their moisture from coastal fog by way of fog drip. Change in climate patterns could result in relative drought in these areas.[34] Along the coast of California, fog is the only source of water for plants and animals for up to 7 months of the year.[35] Some animals, including insects, depend on wet fog as a principal source of water, particularly in otherwise arid climates like in many African coastal areas. Some coastal communities use fog nets to extract moisture from the atmosphere where groundwater pumping and rainwater collection are insufficient.

Artificial fog

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An artificial opacifying fog triggered remotely to scare off burglars.

Artificial fog is man-made fog that is usually created by vaporizing a water- and glycol- or glycerine-based fluid. The fluid is injected into a heated metal block which evaporates quickly. The resulting pressure forces the vapor out of a vent. Upon coming into contact with cool outside air, the vapor condenses in microscopic droplets and appears as fog.[36] Such fog machines are commonly used for entertainment applications.[37]

Historical references

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See also

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Technology

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Weather

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References

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Further reading

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Fog

View on Grokipedia
from Grokipedia
Fog is a meteorological phenomenon characterized as a suspension of numerous tiny water droplets or ice crystals in the air near or at the Earth's surface, forming a type of cloud that touches the ground and reduces horizontal visibility to less than 1 kilometer (0.62 miles). It occurs when moist air cools to its dew point, causing water vapor to condense into these microscopic particles, typically with droplet diameters ranging from 5 to 50 micrometers. Dense fog, defined by the National Weather Service as reducing visibility to less than 0.25 miles (402 meters), poses particular challenges due to its opacity. Fog forms through various processes influenced by local topography, wind patterns, and humidity, including radiative cooling, advection of moist air, and orographic lift; it is most prevalent in autumn and winter in mid-latitudes. The presence of fog significantly impacts transportation and aviation by reducing visibility, often causing delays, diversions, and increased accident risks. In aviation, fog contributes to weather-related accidents, with approximately 100 fatalities annually in the United States as of 2023-2025 data. On roadways and maritime routes, it heightens collision risks, leading to economic losses such as up to $200,000 per airport closure event. Advanced forecasting and low-visibility procedures help mitigate these effects, though fog remains a key challenge in meteorology and safety.

Fundamentals

Definition

Fog is defined as a visible aerosol consisting of tiny water droplets or ice crystals suspended in the air near the Earth's surface, reducing horizontal visibility to less than 1 kilometer (0.62 miles). This phenomenon occurs when these microscopic particles scatter light sufficiently to impair sight at ground level. The World Meteorological Organization (WMO) establishes the key criterion for fog as the suspension of water droplets or ice particles that limits visibility to below 1 km at the observer's height, distinguishing it from similar atmospheric obscurations. In contrast, mist involves the same type of droplets but with visibility ranging from 1 to 10 km, while haze comprises dry particles such as dust or smoke that reduce visibility without high humidity or wetness. Fog is often regarded as a low-lying cloud in direct contact with the ground, forming a deck that envelops the surface rather than floating above it. As a , fog represents a dispersion of the phase ( droplets) within a gaseous medium (air), where the particles remain stably airborne due to their small size, typically preventing immediate . In colder conditions, ice fog substitutes ice crystals for droplets, maintaining the same -impairing properties.

Physical Properties

Fog is composed primarily of tiny droplets suspended in the atmosphere, with typical diameters ranging from 5 to 50 micrometers. These droplets are sufficiently small to remain aloft for extended periods without significant , distinguishing fog from . In cases of freezing fog, the particles consist of ice crystals rather than droplets, with sizes generally around 10 to 20 micrometers. The (LWC) of fog, which quantifies the mass of water per unit volume of air, typically ranges from 0.01 to 0.5 g/m³. This low —about an less than in clouds—contributes to fog's relatively sparse structure while still allowing it to persist under stable atmospheric conditions. Variations in LWC influence the fog's overall and , with higher values (up to 0.5 g/m³ in dense fog) correlating with greater opacity and slower dissipation. Optically, fog exhibits a high coefficient for visible light, arising from interactions between the incident s (approximately 0.4 to 0.7 micrometers) and the droplet sizes. This process efficiently redirects light in multiple directions, reducing transmission and creating the hazy appearance that impairs visibility. The efficiency peaks when droplet diameters are comparable to the light , enhancing fog's attenuating effect on solar and artificial illumination. Thermally, the formation of fog droplets involves the release of during vapor , which warms the surrounding air and helps sustain near-adiabatic in the surface layer. This heat liberation moderates cooling rates, often aligning the environmental with the moist adiabatic value (around 6°C/km), thereby promoting stability and inhibiting vertical mixing near the ground. Fog droplets also possess electrical conductivity due to their to net charges from atmospheric ions or collisions, which can accelerate coalescence in the presence of . Charged droplets experience enhanced attraction, increasing collision probabilities and potentially hastening droplet growth or in electrified environments.

Formation

Processes

Fog formation begins with thermodynamic processes that cool moist air to its , leading to saturation and subsequent necessary for droplet . The primary mechanisms involve , where longwave radiation emitted from the Earth's surface cools the overlying air layer, or adiabatic expansion, as air rises and expands, reducing its temperature without heat exchange. When the air temperature reaches the , relative humidity approaches 100%, and slight further cooling creates , where water vapor exceeds the equilibrium amount over a flat surface. Nucleation of fog droplets occurs through heterogeneous condensation on hygroscopic aerosol particles, such as or , which act as (CCN). This process follows Köhler theory, which describes the equilibrium over a curved droplet surface containing soluble material, balancing the effect (increased vapor pressure due to curvature) and the solute effect (decreased vapor pressure due to dissolved ions). The theory predicts a critical supersaturation for each nucleus size, beyond which the droplet grows unstably; for typical atmospheric CCN, this critical radius is on the order of micrometers, enabling activation at supersaturations of 0.1–1%. Once nucleated, droplets grow primarily by diffusion of from the supersaturated environment to the droplet surface. The mass growth rate is governed by Fick's law of diffusion, expressed as: dmdt=4πrD(ρv−ρs)\frac{dm}{dt} = 4\pi r D (\rho_v - \rho_s) where mm is the droplet mass, tt is time, rr is the droplet radius, DD is the diffusion coefficient of water vapor in air (approximately 2.5 × 10^{-5} m²/s at standard conditions), ρv\rho_v is the ambient vapor density, and ρs\rho_s is the over the droplet surface. This process dominates in early fog stages, with growth rates of about 1–10 μm/hour depending on supersaturation levels./07%3A_Precipitation_Processes/7.04%3A_Liquid_Droplet_Growth_by_Diffusion) In denser fogs, where droplet concentrations exceed 100 cm⁻³, further growth occurs via coalescence, in which droplets collide and merge. Collision kernels are driven by for small droplets (<10 μm), relative velocities from turbulence in the atmospheric , or differential for larger ones, leading to a broadening of the droplet size distribution. reverses these growth processes when ambient relative falls below 100%, causing to diffuse away from droplets, shrinking them until they may fully dissipate if subsaturation persists./13%3A_Temperature_Kinetic_Theory_and_the_Gas_Laws/13.06%3A_Humidity_Evaporation_and_Boiling) This mechanism contributes to fog dissipation, particularly under mixing with drier air aloft.

Meteorological Conditions

Fog development is favored by large-scale atmospheric conditions that promote high relative humidity near the surface, typically approaching 100%, which allows air to reach saturation with minimal additional cooling or moisture addition. This high humidity often occurs under calm wind conditions, with speeds generally less than 3 m/s (approximately 6 knots), as stronger winds would enhance vertical mixing and prevent the air from becoming stably stratified near the ground. Such environments minimize the dispersion of moisture and facilitate the accumulation of water vapor close to the surface. Temperature inversions play a critical role by trapping moist air layers near the surface, inhibiting their upward mixing and allowing to proceed efficiently. These inversions commonly form on clear nights when the ground loses heat rapidly through longwave radiation, cooling the adjacent air and creating a layer where temperature increases with height. This stable configuration is particularly conducive to in regions with sufficient low-level moisture. Synoptic-scale patterns significantly influence fog predisposition by establishing the broader moisture and stability regimes. High-pressure systems, or anticyclones, often promote clear skies that enhance nocturnal while light winds maintain stability, creating ideal setups for over land. Conversely, the approach of frontal boundaries can introduce abundant from converging air masses, elevating near-surface even in less stable conditions. Local surface factors, such as and cover, further amplify these atmospheric setups by contributing to elevated through enhanced . Wet soils and damp release into the overlying air, accelerating the convergence of and and thereby hastening saturation. This effect is pronounced in areas with recent or high . Fog occurrences exhibit distinct seasonal variations, being more frequent during fall and winter in mid-latitudes due to longer nights that permit extended periods of and cooler surface temperatures that maintain high relative . In these seasons, the reduced daylight shortens the time available for daytime heating to dissipate near-surface moisture, prolonging persistence.

Types

Radiation Fog

Radiation fog forms primarily at night under clear skies and calm winds, when the ground radiates more heat to than it receives from incoming solar radiation, leading to rapid cooling of the near-surface air layer until it reaches saturation at the . This process, a specific case of to the , is most effective in areas with high moisture availability and minimal turbulent mixing. This type of fog is particularly prevalent in humid continental climates during winter, where cold, moist air masses support frequent occurrences, often forming in low-lying valleys and flatlands such as California's Central Valley or Italy's . In these regions, radiation fog can develop nightly under stable atmospheric conditions, typically lasting from 2 to 12 hours before dissipating as solar heating warms the ground and mixes the air after sunrise. Its vertical extent is generally shallow, often less than 100 meters, though cold air drainage into valleys can pool cooler air and enhance both and thickness up to several hundred meters.

Advection Fog

Advection fog forms through the horizontal transport of warm, moist air over a cooler underlying surface, such as chilled land or sea, which rapidly cools the air from below until it reaches its and begins. This mechanism relies on surface cooling to drive the process, often in conjunction with light winds that maintain the stability of the without excessive . Unlike radiation fog, which develops from vertical of stationary air near the ground, advection fog emphasizes the role of air movement in bringing warmer layers into contact with colder surfaces. This type of fog is particularly common in maritime climates, where ocean currents create persistent temperature contrasts. For instance, in during summer, warm continental air advects over the cold , resulting in frequent advection fog that blankets the city and nearby coastal areas. Another prominent example occurs at the Grand Banks off Newfoundland, where warm air from the moves over the cold , generating extensive sea fog that affects maritime navigation. Advection fog often persists for days due to steady patterns, with typical thicknesses of 100 to 300 meters, and forms under moderate speeds of 4 to 7 m/s that facilitate air transport while limiting vertical mixing. The Grand Banks hold the record for the longest sea-level fogs worldwide, averaging more than 120 foggy days per year historically. In urban environments, the effect interacts with fog by elevating local temperatures and lowering relative , which delays its onset and reduces the likelihood of formation compared to surrounding rural areas. A notable example of persistent fog occurred in the U.S. Midwest in January 2024, where dense fog blanketed the region throughout much of the month, affecting visibility across multiple states due to warm, moist air advected northward over cold land surfaces under stable conditions. This event highlighted how extended periods of light winds and high can sustain fog, leading to widespread disruptions.

Orographic and Upslope Fog

Orographic fog, also known as upslope or hill fog, forms when moist air is forced upward over elevated by , leading to adiabatic cooling and of into fog droplets. This process occurs primarily on windward slopes where the induces orographic uplift, causing the air to cool at the dry adiabatic of approximately 9.8°C per kilometer when unsaturated, or the moist adiabatic of about 6°C per kilometer once saturation is reached and begins. The resulting fog often envelops hills or mountains, reducing along slopes and summits. This type of fog is prevalent in regions with significant topographic relief, such as the in the , where it commonly develops during winter and spring on the eastern flanks due to prevailing westerly winds. Similarly, in the , upslope fog frequently forms over the rugged terrain, particularly on windward hills during periods of moist onshore flow. The fog's persistence depends on the continuity of the upslope winds and stable atmospheric conditions, typically lasting from several hours to multiple days until the wind direction shifts or the air mass dries out. A related variant is valley fog, which arises from cold air drainage where denser, cooled air flows downslope into basins or low-lying areas, pooling and enhancing effects at night. This drainage promotes inversion layers that trap moisture, leading to fog formation in valleys, often amplifying radiation fog processes. In , such valley fog is particularly frequent during winter, contributing to over 50 foggy days per year in some valleys due to the region's glacial topography and cold air pooling.

Frontal Fog

Frontal fog develops primarily at the boundaries of fronts, where warm, moist air is lifted over cooler air masses, causing adiabatic cooling and subsequent into fog droplets. This process is common along both warm fronts, where the warm air overrides colder surface air, and cold fronts, where mixing occurs post-passage. Additionally, it can form during post-frontal conditions involving , as the stable air layer beneath traps moisture near the surface. A specific subtype, known as precipitation-induced frontal fog, arises when rain from overlying warm clouds evaporates into a drier, cooler air layer below the , increasing the relative until saturation is reached and fog forms. This mechanism is distinct from pure evaporation fog but shares the role of vapor addition in achieving . The fog often blends seamlessly with low stratus clouds, creating a layer that extends from the surface upward. Frontal fog is prevalent in mid-latitude cyclones, where it typically persists for only a few hours due to the dynamic movement of fronts, though it can cover large areas synoptically. These events are tied to baroclinic zones—regions of sharp contrasts—characterized by strong vertical that enhances frontal lifting. The fog layer remains shallow, usually less than 200 thick, limiting its vertical extent while allowing widespread horizontal coverage.

Evaporation Fog

Evaporation fog, also known as mixing fog, arises when from evaporating surfaces is incorporated into cooler, drier overlying air, leading to saturation and condensation into visible droplets. This process requires a significant contrast between the warm evaporative source and the cold air, which limits its occurrence to specific environmental conditions. A prominent subtype is steam fog, which develops when cold air flows over relatively warm water bodies, such as lakes in autumn. The warm water rapidly evaporates, saturating the cold air and forming wispy, smoke-like clouds that rise turbulently from the surface. This phenomenon is particularly evident in regions with large inland lakes, like the , where cold continental air masses in fall and early winter interact with unfrozen waters, producing frequent episodes that enhance local moisture and visibility hazards. In polar regions, evaporation fog manifests as arctic smoke, or sea smoke, over open leads in sea ice or warmer coastal waters. Extremely cold air, often below -20°C, passes over water temperatures several degrees warmer, causing intense evaporation that condenses into turbulent, patchy fog plumes resembling smoke. These formations typically occur in unstable, low-level convection driven by the temperature gradient, persisting for minutes to hours before dissipating as the air mixes or the source cools. Evaporation fog can also form from the drying of recent or into nearby unsaturated air pockets, where moisture from wet surfaces evaporates into colder, drier layers without broader precipitation systems. This variant is common over damp ground or roads after light events, adding vapor that quickly reaches saturation in stable conditions. Such fog is generally confined to locales with pronounced surface-air temperature differences, including the during transitional seasons, where dozens of steam fog outbreaks occur annually, impacting downwind areas. Its horizontal extent is typically small, often less than 1 km in diameter for individual patches, though intense opacity can reduce visibility to near zero within these zones.

Freezing Fog

Freezing fog forms when air temperatures drop below 0°C (32°F), creating a suspension of tiny supercooled liquid droplets that remain unfrozen despite the subfreezing conditions. Upon contact with exposed surfaces such as aircraft wings, roadways, or , these droplets instantly freeze, depositing a layer of or glaze that can accumulate rapidly. This phenomenon typically arises under meteorological conditions similar to those producing other types, such as on clear nights or of moist air over cold surfaces, but requires subfreezing temperatures to maintain the supercooled state. in freezing fog is often reduced to less than 1 km (0.62 miles), qualifying it as dense fog and posing immediate risks to . A related variant, ice fog, occurs in extremely cold environments below -30°C (-22°F), where directly sublimes into crystals rather than forming liquid droplets, eliminating the process. This type is prevalent in continental polar regions like and during winter, where high from human activities or natural sources combines with intense cold to generate the crystals. Ice fog similarly impairs visibility to below 0.5 km and can persist in urban areas with pollution sources that provide sites, though particles are not strictly necessary for formation. Unlike standard freezing fog, ice fog does not produce rime on contact but contributes to hoarfrost buildup over time. The primary hazards of freezing fog stem from ice accumulation and reduced visibility, endangering and ground transportation. In , supercooled droplets can induce rapid icing on surfaces, altering and potentially leading to stalls if not addressed by de-icing systems; this has prompted strict operational restrictions at during such events. On roadways, the freezing deposits create slick, opaque layers that contribute to skids and multi-vehicle collisions, particularly in regions with frequent winter occurrences. Freezing fog episodes are generally short-lived, lasting only a few hours as daytime heating or disperses the fog, but they recur often in continental winter climates, amplifying cumulative risks over the season.

Effects

Visibility and Transportation

Fog impairs visibility primarily through the scattering of light by water droplets suspended in the air, reducing the distance over which objects can be discerned. This process follows Koschmieder's law, which quantifies meteorological visibility VV (in kilometers) as V=3.91βV = \frac{3.91}{\beta}, where β\beta (in km⁻¹) is the atmospheric extinction coefficient representing the combined effects of scattering and absorption by fog particles. In dense fog, β\beta can exceed 3 km⁻¹, limiting visibility to less than 1 km and severely restricting sightlines for transportation users. In , fog frequently causes flight delays and cancellations by dropping below safe thresholds for , with overall accounting for about 75% of U.S. air traffic delays. For instance, dense fog at delayed over 800 inbound and outbound flights across three days in December 2024. On roadways, fog contributes to over 38,700 crashes annually , resulting in more than 600 fatalities and 16,300 injuries, as low increases speed variance and collision risks. Crash rates rise substantially in under 200 meters, with studies indicating a 2- to 3-fold increase compared to clear conditions due to drivers' reduced ability to perceive hazards. For example, in early January 2026, the India Meteorological Department issued dense fog advisories for the Indo-Gangetic Plains in northern India, from Punjab to West Bengal, where visibility was expected to drop to less than 200 meters overnight into morning hours due to clear skies, cooling temperatures, and surface moisture—processes characteristic of radiation fog. Warnings urged drivers to use low beams or fog lights, reduce speeds, and allow extra travel time to ensure safety. Similarly, during the same period, the National Weather Service issued numerous dense fog advisories across multiple regions in the United States, including west central and southwest Florida, Middle Tennessee, central North Carolina, central and southeast Illinois, New Jersey, New York City, and the Tampa Bay region, with visibilities dropping to one quarter mile or less in many locations. Advisories warned motorists to slow down, increase following distances, use low-beam headlights, and allow extra travel time due to hazardous morning commute conditions. Some regions, such as Queen Anne’s and Caroline counties in Maryland, reported school delays potentially linked to these weather conditions. Drivers should not use hazard lights while driving in fog. Hazard lights are intended for emergencies, such as when the vehicle is stopped or disabled on the roadway. Using them while moving in fog can confuse other drivers, who may mistake the vehicle for being stationary or in distress, and can obscure brake lights and turn signals. Instead, use low-beam headlights (high beams reflect off fog and reduce visibility), fog lights if equipped, slow down significantly, increase following distance, and pull over safely if visibility is too low to continue. Maritime navigation faces heightened collision risks in fog, prompting the use of audible signals since the , when steam-powered foghorns were first mandated internationally to warn vessels of hazards in zero-visibility conditions. Modern technologies like (AIS) and provide positional data and target detection to mitigate these dangers, though they cannot fully eliminate risks from undetected small vessels or in dense fog. For rail and urban transport, fog obscures signals and track markers, forcing speed reductions and causing widespread delays. To counter these effects, employs (RVR) systems, which measure the distance a pilot can see markings or lights along the centerline, enabling low-visibility operations down to 50 meters in Category III instrument landing systems during fog. On roads, vehicles are equipped with fog lamps that emit low, wide beams to improve contrast without , while protocols mandate reduced speeds—typically to 30-50 km/h in dense fog—and increased following distances to enhance reaction times. Maritime and rail sectors similarly rely on automated alerts and procedural slowdowns, integrated with and AIS, to maintain safety margins in impaired conditions.

Optical Phenomena

Fog's optical phenomena arise primarily from the interaction of with its suspended water droplets, which typically range from 1 to 15 micrometers in diameter. dominates this process, as the droplet sizes are comparable to visible wavelengths, leading to efficient forward of and resulting in fog's characteristic white or gray appearance. This scattering attenuates shorter wavelengths like blue light more than longer ones, contributing to the overall desaturated hue observed in dense fog. One striking visual effect is the fogbow, a pale, nearly colorless arc similar to a but formed by and interference in very small fog droplets, often less than 100 micrometers. Unlike rainbows, which rely on larger raindrops for and dispersion into colors, fogbows appear white because the tiny droplets diffract all wavelengths similarly, producing a broad, ghostly bow opposite the sun. , or "god rays," also emerge in foggy conditions, where streams through gaps in clouds or fog layers, casting elongated shadows that appear to converge due to perspective; the fog scatters light along these beams, making them visible as bright shafts against darker backgrounds. The represents another dramatic illusion, occurring when an observer's shadow is projected onto a fog or cloud bank by sunlight from behind, often magnified enormously and encircled by a glory—a series of concentric colored rings caused by around the observer's . Around artificial lights in fog, creates luminous halos or , where light bends around droplets, forming iridescent rings that reduce contrast and enhance the perceived glow of urban areas by back-scattering light toward the ground. Historically, London's "pea-soupers"—dense fogs mixed with coal smoke and from the 19th and early 20th centuries—exhibited a distinctive yellow-brown tint due to absorption and by and particles, exacerbating loss beyond pure meteorological fog. To profile fog droplet sizes and optical properties, lidar systems measure backscattered light, as the intensity and depolarization of returned signals depend on droplet distribution via Mie theory; for instance, dual-wavelength s can invert these profiles to estimate effective radii and concentrations, aiding in fog characterization without direct sampling.

Acoustic Effects

Fog primarily affects through mechanisms, including by droplets and absorption to viscous and interactions with the droplets. is more pronounced for high-frequency sounds above 1 kHz, as the becomes comparable to droplet sizes (typically 5-20 micrometers), resulting in greater deflection and loss of directional coherence, which contributes to the characteristic muffling of sounds in foggy conditions. Absorption by the adds a modest excess loss, on the order of 0.1-1 dB per kilometer for audible frequencies, depending on fog density and droplet composition. Contrary to a persistent myth that fog significantly bends or refracts sound waves, thereby misleading the perceived direction of sources, acoustic propagation in fog experiences negligible refraction overall. The speed of sound remains approximately 340 m/s, largely unimpeded by the fog medium, as density variations are minor compared to temperature or wind gradients in clear air. Apparent directional distortions often stem from localized pockets of uneven fog density or auditory illusions amplified by reduced visibility, rather than systematic wave bending. Low-frequency sounds, below 1 kHz, suffer less and absorption in fog layers, enabling enhanced transmission over distances where higher frequencies would be damped. This principle underpins the design of foghorns, which typically operate in the 200-500 Hz range to maximize audibility for warnings, as longer wavelengths interact minimally with droplets. Experiments in the quantified these effects, showing that dense can reduce audible sound signals by 20-30% compared to clear air over typical propagation paths, with scaling roughly with the square of due to dominance. Acoustic techniques, leveraging travel-time variations and profiles, have been explored to estimate thickness and structure noninvasively.

Extremes and Records

Duration and Density Records

The Grand Banks off Newfoundland, Canada, holds the record for the longest annual duration of sea-level fog, with visibility less than 900 meters persisting for an average of more than 120 days per year due to the interaction of the cold and warm waters. This advection-driven fog often envelops the region for weeks at a time, contributing to its reputation as one of the foggiest marine areas globally. In terms of density, extreme fog events can reduce to near-zero levels through high droplet concentrations. For example, visibilities as low as 3 meters have been recorded in dense events. A notable instance occurred in , , in August 2022, where skiers reported some of the densest encountered, with dropping below 10 meters amid unusually thick following heavy snowfall. Such conditions highlight localized extremes in orographic formation. The longest continuous spell of dense fog on record affected northern in January 2024, lasting over 30 days and extending across a 2,500-kilometer swathe from , , to , . This event, driven by persistent low-level temperature inversions and calm winds, marked the longest such episode since satellite observations began in 2014, surpassing the previous record of about three weeks from the 2019-20 winter. Point Reyes, California, experiences the highest frequency of foggy days in the United States, averaging approximately 200 days per year primarily from coastal fog rolling in from the . This high incidence, concentrated from through , results from the of cold marine waters interacting with warmer coastal air. For ice fog, Fairbanks, Alaska, records extreme occurrences during prolonged subzero temperatures, with historical data indicating up to 36 days of ice fog in a single winter (e.g., 1969), though medians have declined from 16.5 days per winter in the mid-20th century to 6 days in recent decades below -40°C. These events form when freezes directly into tiny crystals in the highly polluted, inversion-trapped air of the interior valley, often lasting hours to days per outbreak.

Notable Historical Events

One of the most deadly fog-related events in history occurred in from December 5 to 9, 1952, when a combination of cold weather, windless conditions, and industrial pollution created a dense smog-fog that enveloped the city, leading to an estimated 4,000 excess deaths from respiratory issues and other illnesses. This catastrophe, which reduced visibility to mere yards and halted transportation, prompted widespread public outcry and investigations into air quality, ultimately resulting in the passage of the Clean Air Act 1956, which restricted coal burning in urban areas and established smoke control zones. In 1871, persistent thick fog along the coast near contributed to a series of maritime disasters. These events, spanning weeks of unrelenting fog, highlighted the perils of Pacific navigation and spurred the U.S. government to accelerate lighthouse and fog signal construction, such as the activation of the Pigeon Point Lighthouse fog horn in September 1871 to aid vessels approaching the . During the 1930s era on the , severe combined with high winds generated massive dust storms that created fog-like conditions, reducing visibility to near zero and blanketing farms in choking clouds that destroyed crops and livestock. These "black blizzards" and dust-laden fog exacerbated and economic hardship, contributing to the of approximately 2.5 million people from the Plains states by 1940 as families sought better opportunities elsewhere. During , persistent adverse weather in the , including dense fog and low visibility, delayed Allied planning for , the D-Day invasion of , postponing the original June 5, 1944, launch date by 24 hours to June 6. Forecasters' predictions of improving conditions amid the fog and storms allowed to proceed, averting potential disaster from even worse weather later in the month.

Human Uses

Water Harvesting

Fog harvesting utilizes large vertical mesh nets to capture water droplets from natural fog, offering a passive and sustainable freshwater source in arid and semi-arid regions where traditional water supplies are limited. The predominant technique involves Raschel polypropylene mesh with a 35% shade coefficient, designed to intercept fog droplets larger than 10 μm—the typical size range for effective collection—through aerodynamic impaction as wind propels the fog toward the net. Yields from these systems generally range from 1 to 10 liters per square meter of mesh per day, varying with local fog density, wind speed, and duration of fog events. Notable implementations include fog collection arrays in Chile's , the world's driest non-polar region, where large fog collectors at sites like the Center provide water for ecological restoration and community use. In , the Dar Si Hmad project's extensive network of nets in the Mountains supplies potable water to approximately 500 people in rural villages, reducing reliance on distant and contaminated sources. Along Peru's coastal deserts near , community-scale fog net installations collectively harvest around 5,000 liters per day, supporting household needs and small-scale irrigation in areas plagued by chronic . The collection process depends on to drive into the , where droplets impact the fibers, coalesce into larger rivulets, and drain via into underlying gutters connected to storage tanks for distribution. Standard mesh systems achieve collection efficiencies of 20-50%, determined by factors like , droplet , and airflow dynamics. This energy-free approach incurs low operational costs—primarily initial setup and maintenance—enabling its application in supporting and projects, such as irrigating drought-resistant crops in remote outposts. Advancements in have focused on nano-coated meshes, incorporating superhydrophobic or patterned surfaces that enhance droplet mobility and reduce , boosting collection yields by up to 40% compared to untreated nets and potentially doubling output in high- environments through optimized coalescence. Despite these benefits, fog harvesting faces challenges including seasonal variability, where yields fluctuate with fog and intensity, often limited to specific climatic windows, and structural vulnerability to high winds that can tear or deform meshes.

Artificial Generation

Artificial fog is generated through various engineered methods primarily for entertainment, training, and industrial applications, utilizing controlled vaporization or sublimation to produce visible aerosols mimicking natural fog. Common devices include fog machines that heat a glycol- or water-based fluid to create vapor, which then condenses into suspended droplets upon cooling, forming dense or hazy effects suitable for atmospheric enhancement. These machines typically employ heated coils or ultrasonic transducers; in the former, a pump delivers the fluid to a heating element where it vaporizes at temperatures around 300–400°C before rapid cooling in ambient air produces droplets, while ultrasonic variants use high-frequency vibrations (1.7–2.4 MHz) from a piezoelectric ceramic to agitate the fluid surface, ejecting fine mist without heat. For haze effects, these produce smaller droplets in the 1–5 μm range, allowing prolonged suspension and light diffusion without rapid settling. Another prevalent technique involves sublimation, where solid (CO₂) pellets or blocks are placed in warm , causing rapid phase change to gas that cools surrounding air and condenses atmospheric moisture into low-lying fog hugging the ground for 1–5 minutes per batch. This method is favored in theaters for its ground-level, ethereal spread without machinery noise, often using specialized "pea-souper" devices to contain and direct the effect. For more dynamic bursts, CO₂ jets in concerts release compressed CO₂ through nozzles, expanding it into a cold, dense vapor up to 10 meters high, creating instantaneous, dramatic fog pillars synchronized with music or lighting. In industrial contexts, artificial fog serves practical simulations and control purposes. Firefighting employs glycol-based fog as a non-toxic analog to replicate challenges and behavior in controlled environments, allowing safe practice of search-and-rescue techniques. Pest utilizes thermal or cold foggers to disperse insecticides in ultra-low volume droplets (10–50 μm), enabling uniform coverage over crops or urban areas for and vector management without excessive residue. applications leverage portable fog generators for obscurants, producing dense screens to simulate conditions or conceal movements during tactical exercises. Health considerations for artificial fog exposure emphasize short-term safety with caveats for prolonged use. Glycol-based fogs are generally deemed safe for brief occupational exposures under well-ventilated conditions, as approved by regulatory bodies, but can irritate respiratory tracts, eyes, and skin due to fine particulates, potentially exacerbating or causing "theater cough" in performers. A 2021 study examined fog's interaction with respiratory aerosols during the , finding that glycol fog reduced airborne particle suspension time by promoting coalescence and settling, suggesting potential utility in ventilation assessments without increasing transmission risk. In the film industry, artificial fog production scales with production demands, with rental costs for professional machines and fluid in Hollywood typically ranging from $100 to $450 per day, enabling cost-effective creation of moody atmospheres in scenes like those in major blockbusters. Emerging innovations include solar-powered misting systems that integrate photovoltaic panels with ultrasonic or high-pressure nozzles to generate fog for outdoor events or cooling, reducing reliance on grid electricity and enabling remote deployment in off-grid locations.

Cultural Aspects

In History and Literature

The English word "fog," denoting a thick near the ground, derives from fok, meaning "spray" or "drifting snow," which entered via Danish fog referring to spray, shower, or snowdrift. This etymological root evokes images of atmospheric veils, aligning with Norse mythological concepts of mist-shrouded realms like , the "mist home" associated with primordial fog and obscurity in ancient Scandinavian lore. In literature, fog frequently symbolizes societal oppression and personal uncertainty. ' Bleak House (1853) portrays London's pervasive fog as a suffocating entity that mirrors the era's legal and social chaos, creeping into every aspect of urban life to obscure truth and exacerbate isolation. , drawing from his maritime experiences, employed sea fogs in works like (1899) to represent moral ambiguity and the disorienting veil of , where mist blurs boundaries between reality and illusion. The Victorian "pea-soupers"—dense, yellow-tinged smogs from coal smoke—further inspired Arthur Conan Doyle's canon, such as in (1892), where fog-laden streets foster intrigue, concealment, and deductive revelation amid the gloom. Historically, fog emerged as a formidable adversary in 19th-century navigation, with shipping logs documenting it as a primary cause of collisions and strandings due to reduced visibility, often termed a silent peril on busy sea lanes. Twentieth-century American literature extended fog's metaphorical reach into existential themes. Jack Kerouac's (1957) uses vivid depictions of "hunger-making raw fog" along coastal routes to underscore the protagonists' disorientation and search for meaning, blurring the lines between physical travel and inner turmoil in a post-war landscape of fleeting connections.

In Art and Mythology

In the , fog serves as a potent motif for conveying atmospheric depth, , and emotional ambiguity. British Romantic painter J.M.W. Turner's 1842 oil Snow Storm – Steam-Boat off a Harbour's Mouth depicts a battling turbulent seas amid swirling and , emphasizing the chaotic interplay of light and form in foggy conditions. Critic lauded the work as "one of the very grandest statements of sea-motion, and light" ever rendered on canvas. Similarly, American expatriate captured the hazy veils of Victorian London's Thames fogs in his nocturnes, such as Nocturne: Blue and Silver – Battersea Reach (1870–1875), where diffused light softens industrial forms into ethereal compositions. Whistler embraced these effects, declaring in 1879 that London's fogs were "lovely" and positioning himself as their foremost interpreter. Fog's symbolic role in underscores themes of uncertainty and the sublime, representing the blurred boundary between human perception and nature's vastness. German artist Caspar David Friedrich's Wanderer above the Sea of Fog (1818) portrays a solitary figure gazing over a mist-enshrouded alpine expanse, evoking introspection amid ambiguity and the awe-inspiring unknown. This motif persisted into modern , where harnessed fog's transformative qualities in . In works like Morning Mist, Merced River and Bridalveil Fall, Yosemite Valley, (c. 1945), rising mists create layered depth and a sense of serene isolation, amplifying the landscape's moody grandeur. In mythology, fog often functions as a liminal veil bridging mortal and otherworldly realms. Celtic lore features the féth fíada, a magical mist wielded by the Tuatha Dé Danann to render themselves invisible and access the fairy sidhe, transforming familiar landscapes into gateways for the supernatural. Japanese folklore similarly associates fog with spectral visitations; yūrei, vengeful ghosts, manifest on misty nights, their pale forms emerging from the haze to haunt the living, as exemplified by funayūrei sea spirits that lure sailors during foggy storms. Across cultural traditions, fog symbolizes mystical concealment and spiritual presence. In East Asian ink wash painting, artists like those of the Song dynasty used diluted ink and blank spaces to depict mist-shrouded mountains, fostering a sense of infinite depth and ethereal harmony with nature. 19th-century European chromolithographs romanticized alpine fogs as mystical veils, reproducing luminous scenes of the Alps where mists evoked wonder and the transcendent, popularizing these motifs in illustrated travelogues and prints.

Detection and Prediction

Observation Methods

Visibility meters, particularly transmissometers, are widely used for real-time measurement of fog by quantifying the of over a fixed baseline, such as 100 m, to determine the meteorological optical range (). These instruments emit a of from a transmitter to a receiver and calculate based on the of received to transmitted intensity, following Koschmieder's , where MOR is inversely proportional to the extinction coefficient derived from the . Transmissometers are essential at airports and highways for safety-critical applications, providing continuous data with high , though they require regular to account for baseline soiling or alignment issues. Ceilometers and lidars enable vertical profiling of fog layers by emitting beams and analyzing returns from particles and droplets, achieving vertical resolutions as fine as 1 m to identify base, top, and thickness. These eye-safe systems, operating at wavelengths like 910 nm, detect the strong signal from dense droplets, distinguishing from higher clouds through ratios or gradient analysis in the profile. Deployed in automated stations, they offer unattended operation for monitoring evolution, with applications in for height determination during low-visibility events. Remote sensing techniques complement ground-based observations; satellite-borne imagers, such as those on polar-orbiting platforms, infer presence by measuring brightness temperatures sensitive to liquid paths in the atmosphere, capable of detecting low paths on the order of 10–20 g/m² or greater associated with layers. Ground-based radars detect echoes from droplets via their at S- or C-band frequencies, though sensitivity is limited to denser cases due to weak returns from small particles (radii ~10 μm). These methods provide broad spatial coverage for mapping, particularly over oceans where data are sparse. Disdrometers serve as in situ particle spectrometers for fog microphysics, optically sizing and counting droplets from 2 to 50 μm by analyzing shadows or patterns in a sampled air volume, yielding size distributions and . Instruments like the Fog Monitor aspirate ambient air through a and use a beam to measure individual droplet transit times and sizes, enabling parameterization of from droplet spectra via empirical relations like V = 3.91 / β, where β is the extinction coefficient. Such measurements are crucial for validating fog models and understanding droplet evolution in real-time field campaigns. Human observers remain vital for qualitative fog assessment, employing standardized codes under WMO protocols to report present , such as code 40 for fog at the time or 42 for fog that has become thinner, alongside manual estimates using landmarks or charts. These reports, transmitted hourly from staffed stations, include details on fog patches (code 41) or sky through fog (codes 45-49), ensuring global consistency in synoptic exchange. Augmentation with webcams allows visual verification and dissemination of real-time imagery, bridging gaps in automated networks for comprehensive fog monitoring.

Forecasting Techniques

Forecasting techniques for fog rely on a combination of (NWP) models, statistical approaches, ensemble methods, and localized nowcasting systems to predict onset, duration, and dissipation. These methods integrate atmospheric data to address the challenges posed by fog's small-scale formation processes, often influenced by synoptic conditions such as stable air masses and high . Accurate predictions are crucial for , transportation, and , with ongoing advancements improving reliability through refined parameterizations and . Numerical weather prediction models, such as the Weather Research and Forecasting (WRF) model, are widely used for fog forecasting by simulating boundary-layer processes and microphysical interactions. The Thompson aerosol-aware microphysics scheme within WRF explicitly resolves droplet evolution, including activation and droplet growth, which enhances the representation of fog formation in and scenarios. Sensitivity analyses of this scheme in WRF demonstrate improved simulations of fog cases, particularly in capturing low-level moisture convergence and visibility reductions during nocturnal events. For instance, evaluations show that the Thompson scheme outperforms simpler bulk schemes in predicting fog microphysics over complex terrain, though challenges remain in resolving sub-grid scale . Statistical methods, including algorithms trained on historical meteorological data, have emerged as powerful tools for prediction, especially in the . models, for example, analyze surface observations like , , and to forecast occurrence, achieving high accuracy in detecting radiation events characterized by under clear skies. One such model, developed using 18 years of data from Brazilian airports, reports a proportion correct of 90% and a probability of detection of 96% for episodes between 03 and 11 UTC, enabling predictions of onset and dissipation times with reduced false alarms compared to traditional thresholds. These approaches excel in post-processing NWP outputs or standalone applications, leveraging patterns from past events to handle data sparsity in fog-prone regions. Ensemble forecasting combines outputs from multiple global models, such as the European Centre for Medium-Range Weather Forecasts (ECMWF) and (GFS), to generate probabilistic maps of fog likelihood and . By perturbing initial conditions and physics parameterizations, ensembles quantify uncertainty in fog development, with post-processing techniques like parametric models calibrating forecasts to improve reliability. For example, ECMWF ensemble predictions provide probability fields for near-surface below 1 km, outperforming deterministic runs by reducing bias in low- events. Studies on ECMWF ensembles show that statistical post-processing, such as using ordered , enhances , particularly for fog influenced by coastal synoptic patterns. Local nowcasting utilizes —dense networks of automated weather stations equipped with and sensors—to deliver short-term alerts within one hour. These systems monitor rapid changes in relative approaching 100% and near-surface inversions, triggering warnings based on thresholds derived from assimilation. observations, such as those from the Oklahoma , enable high-resolution mapping of -prone areas by integrating sensor readings every 5-15 minutes, supporting aviation decisions in regions with frequent nocturnal . This approach complements broader models by focusing on microscale variations, with relative maps identifying likely zones during calm, clear nights. Climate trends are increasingly incorporated into long-term fog forecasting, revealing decreases in coastal fog frequency due to global warming. Projections for indicate a 12-20% reduction in coastal fog by mid-century (around 2070) under moderate emission scenarios, driven by warmer sea surface temperatures and altered stratification that suppress fog formation. These trends, based on regional models, impact water harvesting and ecosystems while necessitating adaptive forecasting adjustments.

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