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1Definition

2Formation

3Types

4Freezing conditions

5Topographical influences

6Sea and coastal fog

7Visibility effects

8Shadows

9Sound propagation and acoustic effects

10Record extremes

11As a water source

12Artificial fog

13Historical references

14See also

Toggle See also subsection

14.1Technology

14.2Weather

15References

16Further reading

17External links

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Fog

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العربيةAragonésArmãneashtiAsturianuअवधीAzərbaycancaتۆرکجهবাংলাBanjarBân-lâm-gúБеларускаяБеларуская (тарашкевіца)भोजपुरीБългарскиBosanskiCatalàЧӑвашлаČeštinaChiShonaCymraegDagbanliDanskالدارجةDeutschडोटेलीEestiΕλληνικάEmiliàn e rumagnòlEspañolEsperantoEuskaraفارسیFrançaisFryskFurlanGaeilgeGaelgGalegoГӀалгӀайગુજરાતીगोंयची कोंकणी / Gõychi Konknni客家語/Hak-kâ-ngî한국어Հայերենहिन्दीHrvatskiIdoBahasa IndonesiaInterlinguaÍslenskaItalianoעבריתJawaಕನ್ನಡქართულიҚазақшаKiswahiliKreyòl ayisyenКыргызчаLatinaLatviešuLietuviųLombardMagyarМакедонскиMalagasyमराठीმარგალურიBahasa Melayu閩東語 / Mìng-dĕ̤ng-ngṳ̄МокшеньМонголNa Vosa VakavitiNederlandsNedersaksiesनेपाली日本語NapulitanoNorsk bokmålNorsk nynorskOccitanଓଡ଼ିଆOʻzbekcha / ўзбекчаਪੰਜਾਬੀپنجابیPolskiPortuguêsRomânăRuna SimiРусскийСаха тылаSakizayaᱥᱟᱱᱛᱟᱲᱤShqipSicilianuSimple EnglishSlovenčinaSlovenščinaŚlůnskiکوردیСрпски / srpskiSrpskohrvatski / српскохрватскиSundaSuomiSvenskaTagalogதமிழ்TaqbaylitТатарча / tatarçaไทยTürkçeУкраїнськаاردوVahcuenghVènetoVepsän kel’Tiếng ViệtWalonWinarayWolof吴语粵語Žemaitėška中文

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From Wikipedia, the free encyclopedia

Atmospheric phenomenon

For other uses, see Fog (disambiguation) and Foggy (disambiguation).

Light fog in Bangladesh

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Fog is a visible aerosol consisting of tiny water droplets or ice crystals suspended in the air at or near the Earth's surface.[1][2] Fog can be considered a type of low-lying cloud usually resembling stratus, and is heavily influenced by nearby bodies of water, topography, and wind conditions. 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[edit]

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 the 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]

For aviation purposes in the United Kingdom, a visibility of less than 5 km (3.1 mi) but greater than 999 m (3,278 ft) is considered to be mist if the relative humidity is 95% or greater; below 95%, haze is reported.[5][full citation needed]

Formation[edit]

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 of fog 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[edit]

Fog can form in a number of ways, depending on how the cooling that caused the condensation occurred.

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 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 (see San Francisco fog). 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.

Evaporation fog or steam fog forms over bodies of water overlain by much colder air; this situation can also lead to the formation of steam devils, which look like their dust counterparts.[22] 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.

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. 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.

Ice fog forms in very low temperatures and can be the result of other mechanisms mentioned here, as well as the exhalation of moist warm air by herds of animals. 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.

Freezing fog, which deposits rime, is composed of droplets of supercooled water that freeze to surfaces on contact.[23]

Precipitation fog (or frontal fog) forms as precipitation falls into drier air below the cloud, the liquid droplets evaporate into water vapor. The water vapor cools and at the dewpoint it condenses and fog forms.

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.[24]

Upslope fog forms when moist air is going up the slope of a mountain or hill (orographic lifting) which condenses into fog on account of adiabatic cooling, and to a lesser extent the drop in pressure with altitude.

Freezing conditions[edit]

Freezing fog occurs when liquid fog droplets freeze to surfaces, forming white soft or hard rime.[25] 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.[26] 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".[26][27]

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 described a pogonip which surrounded the main characters, killing one of them.

The phenomenon is also extremely 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 due to 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 LaPine, 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.[28] 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. Extremely small amounts of ice fog falling from the sky form a type of precipitation called ice crystals, often reported in Utqiaġvik, Alaska. Ice fog often leads to the visual phenomenon of light pillars.

Pogonip fog in Virginia City, Nevada, from an early 20th-century postcard

A tree in a field during extreme cold with frozen fog

Ice fog on Pyhäjärvi, Tampere during sunset.

Topographical influences[edit]

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.

Sea and coastal fog[edit]

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.[29]

Sea smoke, also called steam fog or evaporation fog, is the most localized form and is created by cold air passing over warmer water or moist land.[25] It often causes freezing fog, or sometimes hoar frost.

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 deposition of liquid water on hard surfaces. Camanchaca is a similar, dense fog.

Fog rolls into Seattle from the sea

Sea fog or "fret" encroaching on Brighton Pier

Sea fog in the Arctic Ocean near the island of Jan Mayen

Visibility effects[edit]

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[edit]

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 acoustic effects[edit]

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.

Molecular effect: 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]

Temperature effect: 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[edit]

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[edit]

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] Some animals, including insects, depend on wet fog as a principal source of water, particularly in otherwise desert climes, as along many African coastal areas. Some coastal communities use fog nets to extract moisture from the atmosphere where groundwater pumping and rainwater collection are insufficient. Fog can be of different type according to climatic conditions.

Artificial fog[edit]

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, and 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.[35] Such fog machines are primarily used for entertainment applications.

Historical references[edit]

See also: Pea soup fog, Fog of war, and Theatrical smoke and fog

The presence of fog has often played a key role in historical events, such as strategic battles. One example is the Battle of Long Island (27 August 1776), when American general George Washington and his command were able to evade imminent capture by the British Army, using fog to conceal their escape. Another example is D-Day (6 June 1944) during World War II, when the Allies landed on the beaches of Normandy, France during fog conditions. Both positive and negative results were reported from both sides during that battle, due to impaired visibility.[36]

See also[edit]

Technology[edit]

Anti-fog

Automotive lighting

Decontamination foam

Fog Investigation and Dispersal Operation (FIDO)

Fog collection

Foghorn

Fog machine

Fogging (photography)

Fog lamp

Head-up display

Runway visual range

Transmissometer

Weather[edit]

Cloud

Fog season

Haboob (sandstorm)

Mist

San Francisco fog

Smoke

Smog

Whiteout (weather)

Vog

Stratus

References[edit]

^ Gultepe, Ismail, ed. (2 January 2008). "Fog Visibility and Forecasting". Fog and Boundary Layer Clouds. Springer. p. 1126. ISBN 978-3-7643-8418-0. Archived from the original on 3 September 2016. The international definition of fog consists of a suspended collection of water droplets or ice crystal near the Earth's surface... Reprint from Pure and Applied Geophysics. 164 (6–7). 2007.{{cite journal}}: CS1 maint: untitled periodical (link)

^ "What's the Difference Between Fog and Clouds?". NOAA. 2022.

^ 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. (Thomas, P. (2005). Standard practice for the design and operation of supercooled fog dispersal projects. American Society of Civil Engineers. p. 3. ISBN 0-7844-0795-9. Archived from the original on 3 September 2016. Retrieved 27 March 2016.) 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.

^ "Federal Meteorological Handbook Number 1: Chapter 8 – Present Weather" (PDF). Office of the Federal Coordinator for Meteorology. 1 September 2005. pp. 8–1, 8–2. Archived from the original (PDF) on 21 May 2011. Retrieved 9 October 2010.

^ Annex 3 (17th ed.), July 2010

^ "Fog – AMS Glossary". Archived from the original on 27 March 2013. Retrieved 16 March 2013.

^ "Fog" (PDF). National Weather Service. 2022.

^ Robert Penrose Pearce (2002). Meteorology at the Millennium. Academic Press. p. 66. ISBN 978-0-12-548035-2. Retrieved 2 January 2009.

^ "Virga and Dry Thunderstorms". National Weather Service Office, Spokane, Washington. Archived from the original on 22 May 2009.

^ Bart van den Hurk; Eleanor Blyth (2008). "Global maps of Local Land-Atmosphere coupling" (PDF). KNMI. Archived from the original (PDF) on 25 February 2009. Retrieved 2 January 2009.

^ Krishna Ramanujan; Brad Bohlander (2002). "Landcover changes may rival greenhouse gases as cause of climate change". National Aeronautics and Space Administration Goddard Space Flight Center. Archived from the original on 3 June 2008. Retrieved 2 January 2009.

^ National Weather Service JetStream (2008). "Air Masses". Archived from the original on 24 December 2008. Retrieved 2 January 2009.

^ Michael Pidwirny (2008). "CHAPTER 8: Introduction to the Hydrosphere (e). Cloud Formation Processes". Physical Geography. Archived from the original on 20 December 2008. Retrieved 1 January 2009.

^ "Front". Glossary of Meteorology. American Meteorological Society. 25 April 2012. Archived from the original on 10 October 2018.

^ Roth, David M. (14 December 2006). "Unified Surface Analysis Manual" (PDF). Hydrometeorological Prediction Center. Archived (PDF) from the original on 29 September 2006. Retrieved 9 October 2010.

^ FMI (2007). "Fog And Stratus – Meteorological Physical Background". Zentralanstalt für Meteorologie und Geodynamik. Archived from the original on 6 July 2011. Retrieved 7 February 2009.

^ a b Gleissman, Stephe (2007). Agroecology: the ecology of sustainable food systems. CRC Press. p. 73. ISBN 0849328454.

^ Allred, Lance (2009). Enchanted Rock: A Natural and Human History. University of Texas Press. p. 99. ISBN 0292719639.

^ Cox, Robert E. Applying Fog Forecasting Techniques using AWIPS and the Internet Archived 29 October 2007 at the Wayback Machine. National Weather Service, 2007. nwas.org

^ Climate education update: News and information about climate change for teachers and students Archived 27 May 2010 at the Wayback Machine. Atmospheric Radiation Measurement. Climate Research Facility. U.S. Department of Energy. education.arm.gov

^ Frost, Helen (2004). Fog. Capstone Press. p. 22. ISBN 978-0-7368-2093-6.

^ "Fog Facts". Fast Facts for Kids. 2022.

^ "NWS Glossary". National Weather Service. 2022.

^ Marshall, T., Hoadley, D. (1995). Storm Talk. Tim Marshall.[full citation needed]

^ a b Understanding Weather – Fog Archived 31 January 2009 at the Wayback Machine. BBC Weather. bbc.co.uk

^ a b "pogonip". Merriam-Webster.com Dictionary.

^ "Pogonip – Definition from the Dictionary.com". Archived from the original on 22 February 2014. Retrieved 2 January 2013.

^ Haby, Jeff. What is the difference between ice fog and freezing fog? Archived 8 January 2006 at the Wayback Machine theweatherprediction.com

^ Stressed seaweed contributes to cloudy coastal skies, study suggests Archived 11 May 2008 at the Wayback Machine, eurekalert.org

^ "Arctic Sea Smoke". encyclopedia.com. Archived from the original on 6 May 2016.

^ Cowling, R. M., Richardson, D. M., Pierce, S. M. (2004). Vegetation of Southern Africa. Cambridge University Press. p. 192. ISBN 0521548012.

^ "Does fog have a dampening effect on sounds?". thenakedscientists.com. 14 June 2009. Archived from the original on 16 January 2015.

^ "How fog can play tricks on your ears?". katu.com. Archived from the original on 12 April 2015.

^ Joyce, Christopher (23 February 2010). "Fog Fluctuations Could Threaten Giant Redwoods". NPR. Archived from the original on 27 January 2016.

^ Karukstis, K. K., Van Hecke, G. R. (2003). Chemistry connections: the basis of everyday phonemena. Academic Press. p. 23. ISBN 0124001513.

^ Tardif, Robert M. (2007). Characterizing fog and the physical mechanisms leading to its formation during precipitation in a coastal area of the northeastern United States. Bibcode:2007PhDT........70T.

Under "[ ^ "Federal Meteorological Handbook Number 1: Chapter 8 – Present Weather" (PDF). Office of the Federal Coordinator for Meteorology. 1 September 2005. pp. 8–1, 8–2. Retrieved 9 October 2010. ] " ….

Actually use the following link- http://www.ofcm.gov/publications/fmh/FMH1/FMH1.pdf and proceed to Chapter 8, etc.

Further reading[edit]

Ahrens, C. (1991). Meteorology today: an introduction to weather, climate, and the environment. West Pub. Co. ISBN 978-0-314-80905-6.

Corton, Christine L. London Fog: The Biography (2015)

Riddle, Laurence G.; Cayan, Daniel R.; Filonczuk, Maria K. (1 July 1995). "Variability of Marine Fog Along the California Coast". {{cite journal}}: Cite journal requires |journal= (help)

Lu, Chunsong; Liu, Yangang; Niu, Shengjie; Zhao, Lijuan; Yu, Huaying; Cheng, Muning (30 January 2014). "Examination of microphysical relationships and corresponding microphysical processes in warm fogs". Acta Meteorologica Sinica. 27 (6): 832–848. doi:10.1007/s13351-013-0610-0. S2CID 2471958.

Lu, Chunsong; Niu, Shengjie; Tang, Lili; Lv, Jingjing; Zhao, Lijuan; Zhu, Bin (July 2010). "Chemical composition of fog water in Nanjing area of China and its related fog microphysics". Atmospheric Research. 97 (1–2): 47–69. Bibcode:2010AtmRe..97...47L. doi:10.1016/j.atmosres.2010.03.007.

External links[edit]

Wikimedia Commons has media related to Fog.

Social & Economic Costs of Fog from "NOAA Socioeconomics" website initiative

United States' current dense fog advisories from NOAA

Current Western US fog satellite pictures from NOAA

vteMeteorological data and variablesGeneral

Adiabatic processes

Advection

Buoyancy

Lapse rate

Lightning

Surface solar radiation

Surface weather analysis

Visibility

Vorticity

Wind

Wind shear

Condensation

Cloud

Cloud condensation nuclei (CCN)

Fog

Convective condensation level (CCL)

Lifted condensation level (LCL)

Precipitable water

Precipitation

Water vapor

Convection

Convective available potential energy (CAPE)

Convective inhibition (CIN)

Convective instability

Convective momentum transport

Conditional symmetric instability

Convective temperature (Tc)

Equilibrium level (EL)

Free convective layer (FCL)

Helicity

K Index

Level of free convection (LFC)

Lifted index (LI)

Maximum parcel level (MPL)

Bulk Richardson number (BRN)

Temperature

Dew point (Td)

Dew point depression

Dry-bulb temperature

Equivalent temperature (Te)

Forest fire weather index

Haines Index

Heat index

Humidex

Humidity

Relative humidity (RH)

Mixing ratio

Potential temperature (θ)

Equivalent potential temperature (θe)

Sea surface temperature (SST)

Temperature anomaly

Thermodynamic temperature

Vapor pressure

Virtual temperature

Wet-bulb temperature

Wet-bulb globe temperature

Wet-bulb potential temperature

Wind chill

Pressure

Atmospheric pressure

Baroclinity

Barotropicity

Pressure gradient

Pressure-gradient force (PGF)

Velocity

Maximum potential intensity

vteCloud genera and selected species, supplementary features, and other airborne hydrometeors - WMO Latin terminology except where indicatedMesosphericExtreme-level80–85 kmNoctilucent (NLC)Polar mesospheric clouds

Noctilucent type I veils

Noctilucent type II bands

Noctilucent type III billows

Noctilucent type IV whirls

StratosphericVery high-level15–30 kmNacreous polar stratospheric clouds (PSC)

Cirriform nacreous

Lenticular nacreous

Nitric acid and waterpolar stratospheric clouds (PSC)

No differentiated sub-types; tends to resemble cirrostratus

TroposphericHigh-level3–18 kmCirrus (Ci)Species

Cirrus castellanus (Ci cas)

Cirrus fibratus (Ci fib)

Cirrus floccus (Ci flo)

Cirrus spissatus (Ci spa)

Cirrus uncinus (Ci unc)

Ci-only varieties

Cirrus intortus (Ci in)

Cirrus vertebratus (Ci ve)

Cirrocumulus (Cc)Species

Cirrocumulus castellanus (Cc cas)

Cirrocumulus floccus (Cc flo)

Cirrocumulus lenticularis (Cc len)

Cirrocumulus stratiformis (Cc str)

Cirrostratus (Cs)Species

Cirrostratus fibratus (Cs fib)

Cirrostratus nebulosus (Cs neb)

High-level-onlymutatus cloud

Mutatus non-height specific (see below)

Medium-level2–8 kmAltocumulus (Ac)Species

Altocumulus castellanus (Ac cas)

Altocumulus floccus (Ac flo)

Altocumulus lenticularis (Ac len)

Altocumulus stratiformis (Ac str)

Altocumulus volutus (Ac vol)

Altostratus (As)

Altostratus undulatus

Nimbostratus (Ns) Multi-levelVarieties

Nimbostratus virga (Ns vir)

Low-level0–2 kmCumulonimbus (Cb) Towering verticalSpecies

Cumulonimbus calvus (Cb cal)

Cumulonimbus capillatus (Cb cap)

Cb-only supplementary features

Cumulonimbus cauda ((cau) Tail cloud)

Cumulonimbus incus (inc)

Cumulonimbus murus ((mur) Wall cloud)

Cb-only accessories and other

Cumulonimbus flumen ((Cb flu) Beaver tail)

Overshooting top

Hot tower

Cumulus (Cu)Variable vertical extentSpecies

Fractus

Cumulus humilis (Cu hum)

Cumulus mediocris (Cu med)

(Cumulus congestus (Cu con) (Cumulus castellanus (unofficial alternative name for Cu con)) (ICAO term for Cu con and "Cu cas" is Towering cumulus [Tcu]))

Other

Horseshoe

Stratus (St)Species

Stratus fractus (St fra)

Stratus nebulosus (St neb)

St-only genitus cloud and other

Stratus silvagenitus (St sil)

Fog (Fg) Surface level

Stratocumulus (Sc)Species

Stratocumulus castellanus (Sc cas)

Stratocumulus floccus (Sc flo)

Stratocumulus lenticularis (Sc len)

Stratocumulus stratiformis (Sc str)

Stratocumulus Undulatus

Stratocumulus volutus (Sc vol)

Low-level-onlysupplementary features

Arcus ((arc) Shelf)

Tuba ((tub) Funnel cloud)

Low-level-onlyaccessory cloud and other

Pileus (pil)

Velum (vel)

Pannus (pan)

Other- Actinoform cloud (Stratocumulus)

Non-heightspecificVarieties

Duplicatus (du)

Lacunosus (la)

Opacus (op)

Perlucidus (pe)

Radiatus (ra)

Translucidus (tr)

Undulatus (un)

Supplementary features

Asperitas (asp)

Cavum (cav)

Fluctus (flu)

Mamma (mam)

Praecipitatio (pra)

Virga (vir)

Mother clouds and human-made clouds

(Mother cloud)+genitus (e.g. cumulogenitus (cugen)

(Mother cloud)+mutatus (e.g. cumulomutatus (cumut)

Homogenitus (hogen)

Homomutatus (homut)

Authority control databases: National

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Germany

Israel

United States

Japan

Czech Republic

Retrieved from "https://en.wikipedia.org/w/index.php?title=Fog&oldid=1211580880"

Categories: FogWeather hazards to aircraftSnow or ice weather phenomenaRoad hazardsClouds, fog and precipitationHazards of outdoor recreationHidden categories: CS1 maint: untitled periodicalWebarchive template wayback linksAll articles with incomplete citationsArticles with incomplete citations from July 2019Articles with short descriptionShort description matches WikidataUse dmy dates from June 2021Articles with incomplete citations from February 2010All articles with unsourced statementsArticles with unsourced statements from November 2016Articles with unsourced statements from September 2014Articles with unsourced statements from February 2010Articles with unsourced statements from December 2021CS1 errors: missing periodicalCommons link is on WikidataArticles with BNF identifiersArticles with BNFdata identifiersArticles with GND identifiersArticles with J9U identifiersArticles with LCCN identifiersArticles with NDL identifiersArticles with NKC identifiers

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Fog | Definition, Formation, Types, & Facts | Britannica

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fog enveloping the Golden Gate Bridge, San Francisco

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fog, cloud of small water droplets that is near ground level and sufficiently dense to reduce horizontal visibility to less than 1,000 metres (3,281 feet). The word fog also may refer to clouds of smoke particles, ice particles, or mixtures of these components. Under similar conditions, but with visibility greater than 1,000 metres, the phenomenon is termed a mist or haze, depending on whether the obscurity is caused by water drops or solid particles.How fog formsOverview of how fog forms.(more)See all videos for this articleFog is formed by the condensation of water vapour on condensation nuclei that are always present in natural air. This results as soon as the relative humidity of the air exceeds saturation by a fraction of 1 percent. In highly polluted air the nuclei may grow sufficiently to cause fog at humidities of 95 percent or less. Growth of the drops may be helped by the absorption of certain soluble gases, notably sulfur dioxide to form dilute sulfuric acid. The relative humidity of the air can be increased by three processes: cooling of the air by adiabatic expansion; mixing two humid airstreams having different temperatures; and direct cooling of the air by radiation.

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The first process, adiabatic expansion, is responsible for the formation of clouds and plays a part in the formation of upslope fogs that are formed by the forced ascent of humid air up the sides of hills and mountains.The mixing process is manifest when air that has been in contact with a wet ground or water surface having a different temperature from that of the air above is mixed with this air.The most stable fogs occur when the surface is colder than the air above; that is, in the presence of a temperature inversion. Fogs also can occur when cold air moves over a warm, wet surface and becomes saturated by the evaporation of moisture from the underlying surface. Convection currents, however, tend to carry the fog upward as it forms, and it appears to rise as steam or smoke from the wet surface. This is the explanation of steam fogs that are produced when cold Arctic air moves over lakes, streams, inlets of the sea, or newly formed openings in the pack ice; hence, the term Arctic sea smoke.fogFormation of stratus cloud and fog beneath a frontal surface.(more)Advection fog is formed by the slow passage of relatively warm, moist, stable air over a colder wet surface. It is common at sea whenever cold and warm ocean currents are in close proximity and may affect adjacent coasts. A good example is provided by the frequent dense fogs formed off the Grand Banks of Newfoundland in summer, when winds from the warm Gulf Stream blow over the cold Labrador Current. It also may occur over land, especially in winter when warm air blows over frozen or snow-covered ground. Advection fogs occur most readily with winds of about 5 metres per second (10 miles per hour), sufficiently light to maintain a temperature contrast between air and surface and not strong enough to produce turbulent mixing through a considerable depth of the atmosphere. Typical advection fogs extend up to heights of a few hundred metres and sometimes also occur together with radiation fogs.

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Radiation fog forms over land on calm, clear nights when loss of heat by radiation cools the ground and chills the air in the lowest few metres to below the dew-point temperature. Once dense fog has formed, the top of the fog replaces the ground as the effective surface cooled by radiation, and the fog increases progressively in depth as long as there is sufficiently moist air above it. The development of a strong temperature inversion tends to stabilize the fog and suppress air motions, but slow, turbulent stirring motions usually are present and probably are important in maintaining the fog. They do so by replacing the air in the lowest layers—which is losing moisture by deposition on the ground—with moister air from above. Typical inland radiation fogs reach to heights of 100 to 200 metres.fogFormation of inversion fog.(more)Inversion fogs are formed as a result of a downward extension of a layer of stratus cloud, situated under the base of a low-level temperature inversion. They are particularly prevalent off western coasts in tropical regions during the summer, when the prevailing winds blow toward the Equator and cause the upwelling of cold water along the coast. Air that passes over the cold water becomes chilled, its relative humidity rises, and it becomes trapped under the inversion. Subsequent nocturnal cooling may then cause a stratus layer to form and build down to the ground to form an inversion fog.Frontal fog forms 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.When the air temperature falls below 0 °C (32 °F) the fog droplets become supercooled. At temperatures between 0 and −10 °C (32 and 14 °F), only a small proportion of the droplets freeze, and the fog is composed mainly or entirely of liquid water. However, at lower temperatures, more and more droplets freeze, so that below about −35 °C (−31 °F)—and certainly below −40 °C (−40 °F)—the fog is composed entirely of ice crystals. The visibility in an ice fog is often considerably worse than that in a water fog containing the same concentration of condensed water.Although it is convenient to classify fogs according to the physical processes that produce saturation of the air, it is difficult to apply such a clear-cut classification in practice. Typically, more than one of the processes acts at the same time, and their relative importance varies from case to case and with time. Probably no two fogs are controlled by exactly the same combination of factors, a fact that makes forecasting the formation and dispersal of fog difficult.Learn about how the fog in the Atacama Desert of Chile irrigates Chañaral provinceIn the Atacama Desert of Chile, fresh water is extracted from fog and used to irrigate crops of Aloe vera in Chañaral province.(more)See all videos for this articleIn most areas subject to fog, the frequency and persistence of the fogs show a marked seasonal dependence. Conditions favourable to the formation of radiation fog—namely, clear skies and light winds—often occur in the central regions of anticyclones (see anticyclone) and ridges of high pressure, so that the advantages of dry, settled weather are often nullified by the occurrence of fog, especially in autumn and winter. Advection fog may occur in any season of the year and at any time of day or night and is not restricted to conditions of light winds and clear skies. Over land it is especially liable to occur in winter, when mild, damp air flows over a frozen or snow-covered surface. Over the coastal waters of the British Isles, it occurs chiefly in late spring and early summer when the sea is still cold.

Dense fog presents one of the greatest hazards to aviation and to nearly all forms of surface transportation. Modern aircraft generally are not allowed to take off or land if the visibility along the runway is less than 600 metres. In many countries, especially those in temperate latitudes, fog causes widespread dislocation and delay in transportation systems on several days each year. The Editors of Encyclopaedia BritannicaThis article was most recently revised and updated by Adam Augustyn.

fog

ducationSign InMenuDonateENCYCLOPEDIC ENTRYENCYCLOPEDIC ENTRYfogfogFog is a cloud that touches the ground.Grades5 - 12+SubjectsEarth Science, Geography, Physical Geography‌‌‌‌‌‌‌‌‌‌‌‌‌‌Loading ...Powered byArticleVocabularyFog is a cloud that touches the ground. Fog can be thin or thick, meaning people have difficulty seeing through it. In some conditions, fog can be so thick that it makes it hard to drive safely because it obscures the road and other cars. Even monuments like London Bridge, in London, England, or the Golden Gate Bridge, in San Francisco, California, are almost impossible to see in thick fog.Fog shows up when water vapor, or water in its gaseous form, condenses. During condensation, molecules of water vapor combine to make tiny liquid water droplets that hang in the air. You can see fog because of these tiny water droplets. Water vapor, a gas, is invisible.Fog happens when it’s very, very humid. There has to be a lot of water vapor in the air for fog to form.In order for fog to form, dust or some kind of air pollution needs to be in the air. Water vapor condenses around these microscopic solid particles. Sea fog, which shows up near bodies of salty water, is formed as water vapor condenses around bits of salt.Depending on the humidity and temperature, fog can form very suddenly and then disappear just as quickly. This is called flash fog.Fog is not the same thing as mist. Fog is denser than mist. This means fog is more massive and thicker than mist. There are more water molecules in the same amount of space in a fog. Fog cuts visibility down to one kilometer, meaning it will prevent you from seeing further away than one kilometer from where you’re standing. Mist can reduce visibility to between one and two kilometers.Types of FogThere are several different types of fog, including radiation fog, advection fog, valley fog, and freezing fog.Radiation fog forms in the evening when heat absorbed by the Earth’s surface during the day is radiated into the air. As heat is transferred from the ground to the air, water droplets form. Sometimes people use the term “ground fog” to refer to radiation fog. Ground fog does not reach as high as any of the clouds overhead. It usually forms at night. Fog that is said to “burn off” in the morning sun is radiation fog.Advection fog forms when warm, moist air passes over a cool surface. This process is called advection, a scientific name describing the movement of fluid. In the atmosphere, the fluid is wind. When the moist, warm air makes contact with the cooler surface air, water vapor condenses to create fog. Advection fog shows up mostly in places where warm, tropical air meets cooler ocean water. The Pacific coast of the United States, from Washington to California, is often covered in advection fog. The cold California Current, which runs along the western coast of North America, is much cooler than the warm air along the coast.Valley fog forms in mountain valleys, usually during winter. Valley fog develops when mountains prevent the dense air from escaping. The fog is trapped in the bowl of the valley. In 1930, vapor condensed around particles of air pollution in the Meuse Valley, Belgium. More than 60 people died as a result of this deadly valley fog.Freezing fog happens when the liquid fog droplets freeze to solid surfaces. Mountaintops that are covered by clouds are often covered in freezing fog. As the freezing fog lifts, the ground, the trees, and even objects like spider webs, are blanketed by a layer of frost. The white landscapes of freezing fog are common in places with cold, moist climates, such as Scandinavia or Antarctica.Fog CatchersMany ancient cultures collected water from fog by placing large pots under trees and shrubs. As the water from fog collected on these objects, the pots collected the water. This method of water collection was effective, but not as effective as collecting rainwater or other liquid water.Today, engineers are working on more sophisticated ways to collect water from fog. The most effective way has been the development of “fog catchers.” Fog catchers are very large screens constructed in arid areas. As fog glides in, water droplets form around the thin screens and drip to the collection pools below. In one day, a single screen can collect more than a hundred gallons of water.The village of Bellavista, Peru, relies on fog catchers. Bellavista is an area that has little access to liquid water—no rivers, lakes, or glaciers are nearby. Wells dry up quickly. Water for irrigation and human consumption is threatened. Every year, however, huge fogs blow in from the Pacific Ocean. In 2006, the community invested in a series of fog catchers outside of town. Now, the residents of Bellavista have enough water to irrigate trees and gardens, as well as provide for their own drinking and hygiene needs.Engineers warn that fog catchers will only work in small areas. Still, engineers and politicians are working on ways to make more powerful fog catchers that will perhaps reduce the need for people to rely so much on groundwater.Fast FactGrand BanksThe foggiest place in the world is Grand Banks, a spot in the Atlantic Ocean off the island of Newfoundland, Canada. The cold Labrador Current from the north and the warm Gulf Stream current from the east create prime conditions for thick fog to form almost every day.Fast FactPea SouperA "pea souper" is a type of fog that forms when water condenses around microscopic particles of coal. This fog is often a brownish-yellow color, leading to the name. Pea soupers are common in areas that burn coal for energy.The London Fog of 1952, which killed 12,000 people around the urban center of London, England, was a pea souper. The Great Fog led to legislation that regulated the coal industry and air pollution in the United Kingdom.Articles & ProfilesNational Geographic News: Fog Catchers Bring Water to Parched VillagesNational Weather Service: Types of FogAudio & VideoNational Geographic Channel: Killer FogCreditsMedia CreditsThe audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.WritersKim RutledgeMelissa McDanielSantani TengHilary HallTara RamroopErin SproutJeff HuntDiane BoudreauHilary CostaIllustratorsMary Crooks, National Geographic SocietyTim GuntherEditorsJeannie Evers, Emdash Editing, Emdash EditingKara WestEducator ReviewerNancy WynneProducerNational Geographic SocietyotherLast UpdatedOctober 19, 2023User PermissionsFor information on user permissions, please read our Terms of Service. If you have questions about how to cite anything on our website in your project or classroom presentation, please contact your teacher. They will best know the preferred format. When you reach out to them, you will need the page title, URL, and the date you accessed the resource.MediaIf a media asset is downloadable, a download button appears in the corner of the media viewer. If no button appears, you cannot download or save the media.TextText on this page is printable and can be used according to our Terms of Service.InteractivesAny interactives on this page can only be played while you are visiting our website. You cannot download interactives.Related ResourcesNational Geographic Headquarters 1145 17th Street NW Washington, DC 20036ABOUTNational Geographic SocietyNatGeo.comNews and ImpactContact UsExploreOur ExplorersOur ProgramsEducationNat Geo LiveStorytellers CollectiveTraveling ExhibitionsJoin UsWays to GiveApply for a GrantCareersdonateget updatesConnectNational Geographic Society is a 501 (c)(3) organization. © 1996 - 2024 National Geographic Society. All rights reserved.Privacy Notice|Sustainability Policy|Terms of Service|Code of Eth

What's the Difference Between Fog and Clouds? | NOAA SciJinks – All About Weather

What's the Difference Between Fog and Clouds? | NOAA SciJinks – All About Weather

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What's the Difference Between Fog and Clouds?

The Short Answer:

Clouds and fog both form when water vapor condenses or freezes to form tiny droplets or crystals in the air, but clouds can form at many different altitudes while fog only forms near the ground.

Both fog and clouds are formed when water vapor condenses or freezes to form tiny droplets or crystals in the air. So why are they two different things?

Fog forms only at low altitudes.

Clouds can form at many different altitudes. They can be as high as 12 miles above sea level or as low as the ground. Fog is a kind of cloud that touches the ground. Fog forms when the air near the ground cools enough to turn its water vapor into liquid water or ice.

A cool December fog. Image Credit: Jonathan Zander

There are many different types of fog, too. Ice fog forms when the air near the ground is cold enough to turn the water in fog into ice crystals. Ice fog forms only at extremely cold temperatures. Ice fog is common in parts of Alaska and Canada.

Another kind of fog is freezing fog. Ice crystals form in the air when it’s cold enough and particles like dust or smoke in the air provide a “seed” for the ice crystal to form around. Sometimes it is cold enough, but the air does not have any particles. In this case, water in the air becomes “supercooled.” This supercooled water is a liquid, but it is colder than the freezing point (32ºF). When it comes into contact with cold surfaces such as roads and sidewalks, it instantly forms a dangerous icy layer.

One of the most troubling kinds of fog is called “super fog.” Super fog forms when smoke from wildfires and water vapor come together to form an extremely dense fog. The smoke provides particles for the water vapor to condense around. This combination of smoke and water vapor is a dangerous one. A super fog is so dense that you would not be able to see your own hand in front of your face. Superfogs create very hazardous driving conditions.

Fog and smoke from a brush fire combined to make a super fog, resulting in this massive car accident in Florida in 2008. Photo used with permission of Orlando Sentinel, Copyright 2008.

Fog can be a big problem for humans, especially when we have to drive or fly through it.

How can we prepare for fog?

Thousands of driving accidents happen each year because of fog. Fog also creates trouble for air travelers. Foggy conditions create dangerous flying conditions and can delay or cancel flights.

Pilots and drivers can get some help from space, though. Two types of satellites from the National Oceanic and Atmospheric Administration (NOAA) monitor fog from high in the sky. The first is called a geostationary satellite. These satellites orbit Earth in the same exact time that it takes for Earth to make a full rotation. Orbiting Earth in such a way allows the satellite to hover over one location, providing a bird's eye view.

The second kind is a polar satellite. The orbits of these satellites cross over each of the poles. Earth rotates under these satellites as they make the trip from pole to pole. Because the Earth rotates while the satellite makes its orbit, it is able to see nearly every part of Earth’s surface.

NOAA is developing a new generation of geostationary and polar satellites. These satellites will be able to take very high-resolution photos of clouds and fogs. This information can tell pilots or drivers where to expect fog, and can help save lives.

Satellite data can be used to predict the likelihood of fog forming. Current geostationary and polar satellites, however, are capable only of producing a low-resolution picture, like the image on the left. The next generation of geostationary and polar satellites, the GOES-R series and JPSS, will be able to produce a much more detailed and accurate image, like the image on the right.

But what about the difference between fog and smog? Are they the same thing?

The word smog comes from a combination of the words smoke and fog. It refers to human-caused air pollution that creates a hazy cloud near the ground similar to fog. Instead of being formed from water vapor, though, modern smog is formed from pollution. The most common source of smog is car emissions. It can contain many different pollutants. In the past, burning large amounts of coal also formed smog. Smog is not only a problem for visibility, but it can create serious health problems as well.

The Egyptian city of Cairo covered in a dense smog. Credit: Sturm58 via Wikimedia Commons.

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Radiation Fog

This type of fog forms at night under clear skies with calm winds when heat absorbed by the earth’s surface during the day is radiated into space. As the earth’s surface continues to cool, provided a deep enough layer of moist air is present near the ground, the humidity will reach 100% and fog will form. Radiation fog varies in depth from 3 feet to about 1,000 feet and usually remains stationary. This type of fog can reduce visibility to near zero at times and make driving very hazardous.

Valley fog is a type of radiation fog. When air along ridgetops and the upper slopes of mountains begins to cool after sunset, the air becomes dense and heavy and begins to drain down into the valley floors below. As the air in the valley floor continues to cool due to radiational cooling, the air becomes saturated and fog forms. Valley fog can be very dense at times. This type of fog tends to dissipate very quickly once the sun comes up and starts to evaporate the fog layer.

 

Valley fog in Harrison County, Indiana.  Scott Taylor

 

Advection Fog

Advection fog often looks like radiation fog and is also the result of condensation. However, the condensation in this case is caused not by a reduction in surface temperature, but rather by the horizontal movement of warm moist air over a cold surface, such as warm moist air flowing over snow. Advection fog can sometimes be distinguished from radiation fog by its horizontal motion along the ground.

 

Fog invading downtown Louisville from the Ohio River.  WHAS

 

 

Freezing Fog

Freezing fog occurs when water droplets remain in the liquid state until they come into contact with a surface upon which they can freeze. As a result, any object the freezing fog comes into contact with will become coated with ice. 

 

Freezing fog in Jefferson Memorial Forest.  Tony Bright

 

Evaporation or Mixing Fog

This type of fog forms when sufficient water vapor is added to the air by evaporation and the moist air mixes with cooler, relatively drier air. The two common types are steam fog and frontal fog. Steam fog forms when cold air moves over warm water. When the cool air mixes with the warm moist air over the water, the moist air cools until its humidity reaches 100% and fog forms. This type of fog takes on the appearance of wisps of smoke rising off the surface of the water.

The other type of evaporation fog is known as frontal fog. This type of fog forms when warm raindrops evaporate into a cooler drier layer of air near the ground. Once enough rain has evaporated into the layer of cool surface, the humidity of this air reaches 100% and fog forms. 

 

Hail Fog

Hail fog is an unusual type of fog that forms shortly after a heavy hailstorm.  The cold balls of ice fall into warm, very moist air near the surface.  As the hail accumulates on the ground, it cools the air just above the ground to the dew point, resulting in fog.  The fog forms when winds are light, and it usually quite patchy and shallow.

 

Hail fog along Interstate 65 near Horse Cave, Kentucky.  Brian Fugiel

 

Remember, whenever you drive into dense fog ALWAYS slow down. This will allow you to increase the distance between your car and any cars in front of you that you may not be able to see due to the thickness of the fog. It is also important to switch your headlights to low beams.

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fog

1 of 2

noun

ˈfȯg 

fäg

Synonyms of fog

1

a

: vapor condensed to fine particles of water suspended in the lower atmosphere that differs from cloud only in being near the ground

b

: a fine spray or a foam for firefighting

2

: a murky condition of the atmosphere or a substance causing it

3

a

: a state of confusion or bewilderment

spent the morning in a fog

b

: something that confuses or obscures

hid behind a fog of rhetoric

4

: cloudiness or partial opacity in a developed photographic image caused by chemical action or stray radiation

fogless

ˈfȯg-ləs 

ˈfäg-

adjective

fog

2 of 2

verb

fogged; fogging

transitive verb

1

: to cover, envelop, or suffuse with or as if with fog

fog the barns with pesticide

2

: to make obscure or confusing

accusations which fogged the real issues

3

: to make confused

4

: to produce fog on (something, such as a photographic film) during development

intransitive verb

1

: to become covered or thick with fog

2

a

: to become blurred by a covering of fog or mist

b

: to become indistinct through exposure to light or radiation

Synonyms

Noun

daze

dazedness

haze

muddle

spin

swoon

Verb

becloud

befog

blur

cloud

confuse

muddy

obfuscate

See all Synonyms & Antonyms in Thesaurus 

Examples of fog in a Sentence

Noun

Heavy fog made it difficult to see the road.

a climate marked by heavy fogs

The fog reduced visibility to a quarter of a mile.

This problem has me in a fog.

Verb

The steam from the pot was fogging the window near the stove.

The bathroom was all fogged up after my shower.

politicians who try to fog the issue instead of taking a stand

See More

Recent Examples on the WebNoun

The intention is to immerse our audience in the confusion, the cacophony, the fog of war.

—Los Angeles Times Staff, Los Angeles Times, 8 Mar. 2024

When another biopsy shipment was diverted from Heathrow Airport to another airport in Manchester, England, because of fog, the sample seemed doomed, Dr. Hill said.

—Delthia Ricks, New York Times, 7 Mar. 2024

Lows are mainly in the upper 40s to low 50s, and some patchy fog may form late.

—Ian Livingston, Washington Post, 4 Mar. 2024

During the musical's penultimate song, as Caden Marshall poured his heart out on stage, an overly productive fog machine set off the Broadway Theatre Center's fire alarm, sending everyone outside for some minutes until the all-clear was given.

—Jim Higgins, Journal Sentinel, 2 Mar. 2024

Your sword of certainty can cut through the fog of doubt.

—Steph Koyfman, Condé Nast Traveler, 27 Feb. 2024

There will be some patchy fog on Saturday from 7 to 8 a.m., followed by mostly sunny skies, the agency said in its seven-day forecast.

—Hanh Truong, Sacramento Bee, 23 Feb. 2024

With notes of dark chocolate and dates, the Flatiron blend is perfect for those crisp mornings when chilly fog still clings to the treetops.

—Jaina Grey, WIRED, 17 Feb. 2024

In the early morning hours of May 1, 1940, as a thick fog blanketing Lake Superior transformed into a ferocious storm, the crew of the S.S. Arlington started to worry.

—Ellen Wexler, Smithsonian Magazine, 16 Feb. 2024

Verb

But Kerry Abbott noticed condensation fogging the windows — a sign someone might be inside.

—Kate Talerico, The Mercury News, 25 Jan. 2024

Advertisement Newsom, whose responsibilities go beyond that of lieutenant governor — which are, essentially, fogging a mirror and sitting on various boards and commissions — took a more reasoned and responsible approach.

—Mark Z. Barabak, Los Angeles Times, 27 Dec. 2023

The turmoil from the attack fogged Meg’s ability to speed past the drama, as social media often poked fun at her and, in some cases, took Lanez’s side in the public drama.

—Carl Lamarre, Billboard, 7 Feb. 2024

Our camera instantly fogged up and the concrete walls and arches beaded sweat.

—Matt Gutman, ABC News, 21 Jan. 2024

Beyond that, this helmet is also quite lightweight, warm, and ventilated to keep little ones’ heads from sweating and goggles from fogging up.

—Hillary Maglin, Travel + Leisure, 16 Dec. 2023

But the drug fogged her thinking, keeping her from driving.

—Elizabeth Cooney, STAT, 15 Dec. 2023

Inside Dorian restaurant the windows are fogging up with quiet conversations, candles blink, and a gleaming, rarefied Holland Park evening is coming to life.

—Danielle Pergament, Allure, 14 Dec. 2023

For an enjoyable day on the slopes, choose a pair of goggles that will protect your eyes and won’t fog up.

—Douglas Schnitzspahn, Travel + Leisure, 13 Dec. 2023

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These examples are programmatically compiled from various online sources to illustrate current usage of the word 'fog.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History

Etymology

Noun

probably back-formation from foggy

First Known Use

Noun

1544, in the meaning defined at sense 1a Verb

circa 1592, in the meaning defined at transitive sense 1

Time Traveler

The first known use of fog was

in 1544

See more words from the same year

Phrases Containing fog

fog light

ice fog

fog lamp

brain fog

Dictionary Entries Near fog

fofarraw

fog

fogas

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Merriam-Webster

“Fog.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/fog. Accessed 12 Mar. 2024.

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Kids Definition

fog

1 of 2

noun

ˈfȯg 

ˈfäg

1

a

: fine particles of water floating in the atmosphere near the ground

b

: a fine spray or a foam for firefighting

2

: a gloomy condition of the atmosphere or a substance causing it

3

: a state of mental confusion

spent the morning in a fog

fog

2 of 2

verb

fogged; fogging

1

: to cover or become covered with or as if with fog

2

: to make confused

Medical Definition

fog

transitive verb

ˈfäg, ˈfȯg 

fogged; fogging

: to blur (a visual field) with lenses that prevent a sharp focus in order to relax accommodation before testing vision

More from Merriam-Webster on fog

Nglish: Translation of fog for Spanish Speakers

Britannica English: Translation of fog for Arabic Speakers

Britannica.com: Encyclopedia article about fog

Last Updated:

12 Mar 2024

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What is Fog? - Earth Networks

at is Fog? - Earth Networks

ProductsDecision Support & CollaborationColumn 1Sferic ProtectSferic MapsSferic MobileSferic ConnectSferic SirenMeteorological ServicesData & Analytical Model DeliveryColumn 1Sferic APISferic Data StreamSevere Weather AnalyticsLightning DataProductsWeather Sensors & NetworksColumn 1Greenhouse GasLightningTotal Lightning NetworkWeather Stations & SensorsProductsAccessoriesColumn 1Real-Time HD Weather DisplayVideo CamerasProductsResourcesPricing GuideSevere Weather GuidesCompanyCareersISO 9001:2015Contact UsLog InSupport(301) 250-4000 What is Fog?This page includes everything you ever wanted to know about fog and why it happens. What is Fog?Welcome to Fog 101!We’ve all seen fog before, but do you know how it forms? What’s the difference between fog and mist, anyway?This Weather 101 Guide on wind will answer all your questions. Our meteorologists helped create this guide so you can clear up the difference between fact and myth.Use the buttons below to jump to a specific question or read the entire guide and become an expert. Don’t forget to share this page with anyone who might be interested! The ultimate guide to #fog (20 min. read) Click To Tweet What Is Fog & How Is Fog Formed? Types of Fog Fog DangersDon’t Have Time to Read the Entire Guide Now?Download a PDF version of the guide you can access later. Keep Scrolling to Start Reading!What is Fog? Fog is a visible aerosol comprising tiny water droplets or ice crystals suspended in the air at or near the Earth’s surface. Nearby bodies of water, topography, and weather conditions are three factors that influence fog. You can think of it as a low-lying cloud. Fog most resembles stratus clouds, or low-lying, horizontally layered clouds. It is difficult to see through because of the varying concentrations of the water droplets. How is Fog Formed?You can watch Earth Networks Meteorologist, Fred Allen, explain how it forms in the video below or you can scroll down to read about it. Fog forms when the difference between air temperature and dew point is less that 4.5 degrees Fahrenheit or 2.5 degrees Celsius. When water vapor condenses, it turns into tiny droplets of varying concentration in the air. Dew Point: The temperature below which water droplets start to condense and form dew or frost. Dew point is a surface weather observation. It usually forms at a relative humidity of about 100%, which occurs when there is increased moisture in the air or when the temperature is decreasing.Fog can be a little tricky. Sometimes it forms at lower humidities. Other times, it fails to form with a relative humidity of 100%. More on FormationAnother way to look at fog formation is as when air at or near the earth’s surface becomes saturated. Air in this area becomes saturated by any of these three processes: Cooling Addition of moisture Mixing with another air parcelSince you’re probably not a meteorologist, we think this explanation is sufficient for you to understand the variable nature of fog and the different ways it can form.Forecasting As we’ve mentioned before, this can be a little difficult to forecast. Dew and frost also form when relative humidity levels approach 100%. So how do forecasters know which condition you’ll experience?To get fog, you need a thicker layer of saturated air at the surface. You also need a light breeze to help mix the atmosphere, but not too strong or drier air higher in the atmosphere will mix out the higher moisture near the surface.Another factor to think about is that when it is calm, it’s harder for it to form.What Causes Fog in the Morning? This is one of the most frequently asked questions on the subject. Now that you know a little bit about what fog is and how it forms, do you think you could answer this question?Answer: Fog forms in the morning because it is the coolest time of the day when the temperature drops to the dew point temperatures and the relative humidity approaches 100%. There are instances where dew points rise to the air temperature, but common morning fog is created as the atmosphere cools. Back to Top ↑Types of FogSince it forms in a number of ways, there are also various types of fog. This section includes definitions for several types of fog, including: evaporation, freezing, and radiation fog – just to name a few!Fog vs. MistBefore we get into different types, we’re going to address a common debate: Fog vs. mist!This may come as a shock, but there really is no difference between fog and mist. These terms are interchangeable, with the only difference pertaining to observation visibility for aviation. This is how the National Weather Service’s glossary defines the two terms:Mist: A visible aggregate of minute water particles suspended in the atmosphere that reduces visibility to less than 7 statute miles, but greater than or equal to 5/8 statute miles. It does not reduce visibility as much as fog and is often confused with drizzle.Fog: Fog is water droplets suspended in the air at the Earth’s surface. Fog is often hazardous when the visibility is reduced to ¼ mile or less.Now that we’ve got that out of the way, we can go through the different types!AdvectionThis occurs when moist air passes over a cool surface by the wind and is cooled. Advection fog typically happens when a warm front passes over an area with significant snow pack.It is even more common at sea when moist air encounters cooler waters, including areas of upswelling. We typically see this along the California Coast (specifically in San Francisco).Although strong winds can prevent fog, this can form with windy conditions. Markedly warmer and humid air blowing over a snowpack can continue to generate advection fog at elevated velocities up to 50 mph or more. This fog moves in a turbulent, comparatively shallow layers.Evaporation/SteamAlso known as lake fog, evaporation or steam fog forms over bodies of water. It typically forms during the fall season when water temperatures don’t cool right away but air temperature does.As a mass of dry, cold air moves over a warmer lake, the lake conducts warm, moist air into the air mass above. The transport between the lake and air events out and creates fog. This fluffy-looking fog is deep enough to block some sunlight.FreezingFreezing fog occurs when temperatures are below freezing and is composed of droplets of supercooled water that freeze to surfaces on contact.FrontalFrontal fog forms when raindrops, falling from a relatively warm air above a frontal surface, evaporate into cooler air close to the Earth’s surface. This causes it to become saturated. This can result from a very low frontal stratus cloud subsiding to the surface level in the absence of any lifting agent after the front passes. It forms in the same way a stratus cloud near a front does.GroundThis type is close to the ground. It obscures less than 60% of the sky and does not extend to the base of any overhead clouds. HailHail fog sometimes occurs after hail accumulates 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 is actually a ground fog that tends to be localized, however it can be dense and abrupt. IceDifferent from freezing, ice fog is only seen in the polar and arctic regions when temperatures reach 14 degrees Fahrenheit. At this point, air is too cold to contain supercooled water droplets so it forms tiny ice crystals.It can be beautiful, especially when associated with the diamond dust form of precipitation, in which tiny 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.PrecipitationPrecipitation fog forms as precipitation falls into cold, drier air below the cloud and evaporates into water vapor. The water vapor cools and at the dew point it condenses. When it condenses, it creates fog. This is common with warm fronts but can occur with cold fronts as well only if it’s not moving too fast. RadiationRadiation fog happens after sunset when the land cools by infrared thermal radiation in calm conditions with a clear sky. The cooling ground then cools adjacent air by conduction. This causes 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. Turbulence can promote a thicker layer.Radiation fog doesn’t typically last long after sunrise, but can linger all day during the winter months – especially in areas bounded by high ground. It is more common in the fall and early winter and when it rains the night before. Rain helps moisten up the soil and create higher dew points. This makes it easier for the air to become saturated and form fog.Upslope The last type of fog on our list is upslope fog. The name of this fog describes it pretty well. Upslope fog forms when moist air is going up the slope of a mountain or hill. This movement condenses fog through adiabatic cooling and the drop in pressure with altitude.Adabatic cooling causes sinking air to warm and rising air to cool. As moist winds blow toward a mountain, it up glides and this causes the air to rise and cool to meet up with the dew point temperature. This fog can be seen on the top of mountains. Back to Top ↑Fog DangersWhen most people think of severe weather, they think of thunderstorms, hurricanes, or tornadoes. However, fog can be dangerous too.Although not as impactful as the conditions mentioned above, it presents its own set of risks and dangers to humans.The major danger fog presents visibility. While visibility can be in an issue in industries like mining and construction, fog is most hazardous to drivers, marines, and aviation.MotoristsFog causes many car accidents each year. From 2016-2017, roughly 5.89 weather-related vehicle crashes occurred each year in the U.S., according to the Federal Highway Administration. These include wrecks during falling precipitation, slippery pavement, and fog.How dangerous is fog to motorists? When breaking out fog-related accidents, the annual averages are as follows: 25,451 crashes 464 deaths 8,902 injuresThat means fog-related accidents claim more lives each year than tornadoes (60 people on average). This is because fog distorts your perception of speed and distance.How can you drive safer?Don’t speed! You may even have to drive slower than the speed limit so you have more time to judge your surroundings. The most important thing you can do while driving in foggy weather is to take your time and stay cautious.Another good tip is to never use your high beams. High beams actually make it harder to see as the water vapor will scatter more light back at you. Use fog lights if you have them but never use high beams! There are also some vehicle telematics that protect drivers from bad weather, like fog,MarinersFog is dangerous to those on the water because it can form quickly and catch boaters off guard. Because of the time it can take to stop or turn a marine vessel, we usually consider fog as “dense” for mariners if it reduces visibility to less than 1 mile.Visibility can be reduced to a few feet, which can disorient even the most experienced boaters and create dangerous situations. The international standard for describing reduced visibility in marine forecasts are as followed: Very Poor: Less than 0.5 nautical miles Poor: 0.5 to less than 2 nautical miles Moderate: 2 to 5 nautical miles Good: Greater than 5 nautical milesBoaters can stay safe weather by: Slowing down Turning on all running lights Listening for sounds of nearby boats, buoys, and land Using radar to help locate dangers Staying put until it’s clear AviationFlying in fog can be quite a challenge, even for the most experienced pilots. The dangers are most evident during landing and takeoff procedures and flying at lower altitudes.According to the U.S. National Weather Service, around 440 people are killed due to weather-related aviation accidents including the conditions of low visibilities and ceilings each year. Our recent airport industry survey found that 43% of all delays are due to adverse weather conditions, like fog.If you are planning a flight and it’s foggy or may become foggy, follow the following safety guidelines: Get the latest forecasts, advisories, and meteorological advice to help make your flight safe Consider changing plans to avoid flying in fog Follow the Federal Aviation Administration’s mandated guidelines and flight rules for the specific flight category based on visibility and ceiling heightKnow the layout of the airport you are departing from and arriving to, including the length and orientation of the runwayBack to Top ↑Learn MoreFog is both a common and dangerous weather condition comes in various forms and has a serious impact on travelers by land, air, and sea. Feel like an expert? If you still have questions please let us know on Twitter and our meteorologists will get back to you as soon as possible. My #fog question is: Click To TweetIf you feel confident in your fog knowledge, it’s time to increase your understanding of another aspect of meteorology. Check out other topics like lightning detection, floods, and wind on our Weather 101 Resources Guide.Back to Top ↑Learn what we can do for your business contact us today! 12410 Milestone Center Dr., Suite 300

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6.8: Fog

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6.8: Fog

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Page ID9935

Roland StullUniversity of British Columbia

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6.8.1. Types6.8.2. Idealized Fog Models6.8.2.1. Advection FogHIGHER MATH • Advection Fog6.8.2.2. Radiation Fog6.8.2.3. Dissipation of Well-Mixed (Radiation and Advection) Fogs

6.8.1. Types

Fog is a cloud that touches the ground. The main types of fog are:

upslope

radiation

advection

precipitation or frontal

steam

They differ in how the air becomes saturated.

Upslope fog is formed by adiabatic cooling in rising air that is forced up sloping terrain by the wind. Namely, it is formed the same way as clouds. As already discussed in the Water Vapor chapter, air parcels must rise or be lifted to their lifting condensation level (LCL) to form a cloud or upslope fog.

Radiation fog and advection fog are formed by cooling of the air via conduction from the cold ground. Radiation fog forms during clear, nearlycalm nights when the ground cools by IR radiation to space. Advection fog forms when initially-unsaturated air advects over a colder surface.

Precipitation fog or frontal fog is formed by adding moisture, via the evaporation from warm rain drops falling down through the initially-unsaturated cooler air below cloud base.

Steam fog occurs when cold air moves over warm humid surfaces such as unfrozen lakes during early winter. The lake warms the air near it by conduction, and adds water by evaporation. However, this thin layer of moist warm air near the surface is unsaturated. As turbulence causes it to mix with the colder air higher above the surface, the mixture becomes saturated, which we see as steam fog.

6.8.2. Idealized Fog Models

By simplifying the physics, we can create mathematical fog models that reveal some of the fundamental behaviors of different types of fog.

6.8.2.1. Advection Fog

For formation and growth of advection fog, suppose a fogless mixed layer of thickness zi advects with speed M over a cold surface such as snow covered ground or a cold lake. If the surface potential temperature is θsfc, then the air potential temperature θ cools with downwind distance according to

\(\ \begin{align} \theta=\theta_{s f c}+\left(\theta_{o}-\theta_{s f c}\right) \cdot \exp \left(-\frac{C_{H}}{z_{i}} \cdot x\right)\tag{6.8}\end{align}\)

where θo is the initial air potential temperature, CH is the heat transfer coefficient (see the Heat Budgets chapter), and x is travel distance over the cold surface. This assumes an idealized situation where there is sufficient turbulence caused by a brisk wind speed to keep the boundary layer well mixed.

Advection fog forms when the temperature drops to the dew-point temperature Td. At the surface (more precisely, at z = 10 m), θ ≈ T . Thus, setting θ ≈ T = Td at saturation and solving the equation above for x gives the distance over the lake at which fog first forms:

\(\ \begin{align} x=\frac{z_{i}}{C_{H}} \cdot \ln \left(\frac{T_{o}-T_{s f c}}{T_{d}-T_{s f c}}\right)\tag{6.9}\end{align}\)

Surprisingly, neither the temperature evolution nor the distance to fog formation depends on wind speed.

For example, advection fog can exist along the California coast where warm humid air from the west blows over the cooler “Alaska current” coming from further north in the Pacific Ocean.

Advection fog, once formed, experiences radiative cooling from fog top. Such cooling makes the fog more dense and longer lasting as it can evolve into a well-mixed radiation fog, described in the next subsection.

Dissipation of advection fog is usually controlled by the synoptic and mesoscale weather patterns. If the surface becomes warmer (e.g., all the snow melts, or there is significant solar heating), or if the wind changes direction, then the conditions that originally created the advection fog might disappear. At that point, dissipation depends on the same factors that dissipate radiation fog. Alternately, frontal passage or change of wind direction might blow out the advection fog, and replace the formerly-foggy air with cold dry air that might not be further cooled by the underlying surface.

Sample Application

Fog formation: A layer of air adjacent to the surface (where P = 100 kPa) is initially at temperature 20°C and relative humidity 68%. (a) To what temperature must this layer be cooled to form radiation or advection fog? (b) To what altitude must this layer be lifted to form upslope fog? (c) How much water must be evaporated into each kilogram of dry air from falling rain drops to form frontal fog? (d) How much evaporation (mm of lake water depth) from the lake is necessary to form steam fog throughout a 100 m thick layer? [Hint: Use eqs. from the Water Vapor chapter.]

Find the Answer

Given: P = 100 kPa, T = 20°C, RH = 68%, ∆z = 100 m

Find: a) Td=?°C, b) zLCL=?m, c) rs=?g kg–1 d) d=?mm

Using Table 4-1: es = 2.371 kPa.

Using eq. (4.14): e = (RH/100%)·es = (68%/100%)·( 2.371 kPa) = 1.612 kPa

(a) Knowing e and using Table 4-1: Td = 14°C.

(b) Using eq. (4.16):

zLCL = a· [T – Td] = (0.125 m K–1)· [(20+273)K – (14+273)K] = 0.75 km

(c) Using eq. (4.4), the initial state is:

r = ε·e/(P–e) = 0.622 · (1.612 kPa) / (100 kPa – 1.612 kPa) = 0.0102 g g–1 = 10.2 g kg–1

The final mixing ratio at saturation (eq. 4.5) is:

rs = ε·es /(P–es) = 0.622·(2.371 kPa) / (100 kPa – 2.371 kPa) = 0.0151 g g–1 = 15.1 g kg–1.

The amount of additional water needed is

∆r = rs – r = 15.1 – 10.2 = 4.9 gwater kgair–1

(d) Using eqs. (4.11) & (4.13) to find absolute humidity

ρv = ε · e· ρd / P = (0.622)·(1.612 kPa)·(1.275 kg·m–3)/(100 kPa) = 0.01278 kg·m–3

ρvs = ε · es· ρd / P = (0.622)·(2.371 kPa)·(1.275 kg·m–3)/(100 kPa) = 0.01880 kg·m–3

The difference must be added to the air to reach saturation: ∆ρ = ρvs – ρv = ∆ρ = (0.01880–0.01278) kg·m–3 = 0.00602 kg·m–3

But over A =1 m2 of surface area, air volume is

Volair =A · ∆z = (1 m2)·(100 m) = 100 m3.

The mass of water needed in this volume is

m = ∆ρ · Volair = 0.602 kg of water.

But liquid water density ρliq = 1000 kg·m–3: Thus,

Volliq = m / ρliq = 0.000602 m3

The depth of liquid water under the 1 m2 area is

d = Volliq / A = 0.000602 m = 0.602 mm

Check: Units OK. Physics OK.

Exposition: For many real fogs, cooling of the air and addition of water via evaporation from the surface happen simultaneously. Thus, a fog might form in this example at temperatures warmer than 14°C.

HIGHER MATH • Advection Fog

Derivation of eq. (6.8):

Start with the Eulerian heat balance, neglecting all contributions except for turbulent flux divergence:

\(\frac{\partial \theta}{\partial t}=-\frac{\partial F_\textParseError: EOF expected (click for details)Callstack:

at (Bookshelves/Meteorology_and_Climate_Science/Practical_Meteorology_(Stull)/06:_Clouds/6.08:_Fog), /content/body/div[2]/div[1]/div[2]/div/p[3]/span, line 1, column 3

(\theta)}{\partial z}\)

where θ is potential temperature, and F is heat flux. For a mixed layer of fog, F is linear with z, thus:

\(\frac{\partial \theta}{\partial t}=-\frac{F_\text{z turb zi}(\theta)-F_\text{z turb sfc}(\theta)}{z_{i}-0}\)

If entrainment at the top of the fog layer is small, then Fz turb zi(θ) = 0, leaving:

\(\frac{\partial \theta}{\partial t}=\frac{F_\text {z turb sfc}(\theta)}{z_{i}}=\frac{F_{H}}{z_{i}}\)

Estimate the flux using bulk transfer eq. (3.21). Thus:

\(\frac{\partial \theta}{\partial t}=\frac{C_{H} \cdot M \cdot\left(\theta_{s f c}-\theta\right)}{z_{i}}\)

If the wind speed is roughly constant with height, then let the Eulerian volume move with speed M:

\(\frac{\partial \theta}{\partial t}=\frac{\partial \theta}{\partial x} \frac{\partial x}{\partial t}=\frac{\partial \theta}{\partial x} \cdot M\)

Plugging this into the LHS of the previous eq. gives:

\(\frac{\partial \theta}{\partial x}=\frac{C_{H} \cdot\left(\theta_{s f c}-\theta\right)}{z_{i}}\)

To help integrate this, define a substitute variable s = θ – θsfc , for which ∂s = ∂θ . Thus:

\(\frac{\partial S}{\partial x}=-\frac{C_{H} \cdot s}{z_{i}}\)

Separate the variables: \(\frac{d s}{s}=-\frac{C_{H}}{z_{i}} d x\)

Which can be integrated (using the prime to denote a dummy variable of integration):

\(\int_{s^{\prime}=s_{0}}^{s} \frac{d s^{\prime}}{s^{\prime}}=-\frac{C_{H}}{z_{i}} \int_{x^{\prime}=0}^{x} d x^{\prime}\)

Yielding:

\(\ln (s)-\ln \left(s_{o}\right)=-\frac{C_{H}}{z_{i}} \cdot(x-0)\)

or

\(\ln \left(\frac{s}{s_{o}}\right)=-\frac{C_{H}}{z_{i}} \cdot x\)

Taking the antilog of each side (i.e., exp):

\(\frac{s}{s_{o}}=\exp \left(-\frac{C_{H}}{z_{i}} \cdot x\right)\)

Upon rearranging, and substituting for s:

\(\theta-\theta_{s f c}=\left(\theta-\theta_{s f c}\right)_{o} \cdot \exp \left(-\frac{C_{H}}{z_{i}} \cdot x\right)\)

But θsfc is assumed constant, thus:

\(\ \begin{align} \theta=\theta_{s f c}+\left(\theta_{o}-\theta_{s f c}\right) \cdot \exp \left(-\frac{C_{H}}{z_{i}} \cdot x\right)\tag{6.8}\end{align}\)

Sample Application

Air of initial temperature 5°C and depth 200 m flows over a frozen lake of surface temperature –3°C. If the initial dew point of the air is –1°C, how far from shore will advection fog first form?

Find the Answer

Given: To = 5°C, Td = –1°C, Tsfc = –3 °C, zi =200 m

Find: x = ? km.

Assume smooth ice: CH = 0.002 .

Use eq. (6.9): x = (200 m/0.002)· ln[(5–(–3))/(–1–(–3))] = 138.6 km

Sketch, where eq. (6.8) was solved for T vs. x:

Check: Units OK. Physics OK.

Exposition: If the lake is smaller than 138.6 km in diameter, then no fog forms. Also, if the dew-point temperature needed for fog is colder than the surface temperature, then no fog forms.

6.8.2.2. Radiation Fog

For formation and growth of radiation fog, assume a stable boundary layer forms and grows, as given in the Atmospheric Boundary Layer chapter. For simplicity, assume that the ground is flat, so there is no drainage of cold air downhill (a poor assumption). If the surface temperature Ts drops to the dew-point temperature Td then fog can form (Fig. 6.13). The fog depth is the height where the nocturnal temperature profile crosses the initial dew-point temperature Tdo.

Figure 6.13 Stable-boundary-layer evolution at night leading to radiation fog onset, where Ts is near-surface air temperature, Tdo is original dew-point temperature, and TRL is the original temperature. Simplified, because dew formation on the ground can decrease Td before fog forms, and cold-air downslope drainage flow can remove or deposit cold air to alter fog thickness.

The time to between when nocturnal cooling starts and the onset of fog is

\(\ \begin{align} t_{o}=\frac{a^{2} \cdot M^{3 / 2} \cdot\left(T_{R L}-T_{d}\right)^{2}}{\left(-F_{H}\right)^{2}}\tag{6.10}\end{align}\)

where a = 0.15 m1/4·s1/4 , TRL is the residual-layer temperature (extrapolated adiabatically to the surface), M is wind speed in the residual layer, and FH is the average surface kinematic heat flux. Faster winds and drier air delay the onset of fog. For most cases, fog never happens because night ends first.

Once fog forms, evolution of its depth is approximately

\(\ \begin{align} z=a \cdot M^{3 / 4} \cdot t^{1 / 2} \cdot \ln \left[\left(t / t_{o}\right)^{1 / 2}\right]\tag{6.11}\end{align}\)

where to is the onset time from the previous equation. This equation is valid for t > to.

Liquid water content increases as a saturated air parcel cools. Also, visibility decreases as liquid water increases. Thus, the densest (lowest visibility) part of the fog will generally be in the coldest air, which is initially at the ground.

Sample Application

Given a residual layer temperature of 20°C, dew point of 10°C, and wind speed 1 m s–1. If the surface kinematic heat flux is constant during the night at –0.02 K·m s–1, then what is the onset time and height evolution of radiation fog?

Find the Answer

Given: TRL = 20°C, Td = 10°C, M = 1 m s–1, FH = –0.02 K·m s–1

Find: to = ? h, and z vs. t.

Use eq. (6.10):

\(\begin{aligned} t_{O} &=\frac{\left(0.15 \mathrm{m}^{1 / 4} \cdot \mathrm{s}^{1 / 4}\right)^{2} \cdot(1 \mathrm{m} / \mathrm{s})^{3 / 2} \cdot\left(20-10^{\circ} \mathrm{C}\right)^{2}}{\left(0.02^{\circ} \mathrm{C} \cdot \mathrm{m} / \mathrm{s}\right)^{2}} =1.563 \mathrm{h} \end{aligned}\)

The height evolution for this wind speed, as well as for other wind speeds, is calculated using eq. (6.11):

Check: Units OK. Physics OK.

Exposition: Windier nights cause later onset of fog, but stimulates rapid growth of the fog depth.

Initially, fog density decreases smoothly with height because temperature increases smoothly with height (Fig. 6.14a). As the fog layer becomes optically thicker and more dense , it reaches a point where the surface is so obscured that it can no longer cool by direct IR radiation to space.

Figure 6.14 (a) Stratified fog that is more dense and colder at ground. (b) Well-mixed fog that is more dense and colder at the top due to IR radiative cooling.

Instead, the height of maximum radiative cooling moves upward into the fog away from the surface. Cooling of air within the nocturnal fog causes air to sink as cold thermals. Convective circulations then turbulently mix the fog.

Very quickly, the fog changes into a well-mixed fog with wet-bulb-potential temperature θW and total-water content rT that are uniform with height. During this rapid transition, total heat and total water averaged over the whole fog layer are conserved. As the night continues, θW decreases and rL increases with time due to continued radiative cooling.

In this fog the actual temperature decreases with height at the moist adiabatic lapse rate (Fig. 6.14b), and liquid water content increases with height. Continued IR cooling at fog top can strengthen and maintain this fog. This fog is densest near the top of the fog layer.

6.8.2.3. Dissipation of Well-Mixed (Radiation and Advection) Fogs

During daytime, solar heating and IR cooling are both active. Fogs can become less dense, can thin, can lift, and can totally dissipate due to warming by the sun.

Stratified fogs (Fig. 6.14a) are optically thin enough that sunlight can reach the surface and warm it. The fog albedo can be in the range of A = 0.3 to 0.5 for thin fogs. This allows the warm ground to rapidly warm the fog layer, causing evaporation of the liquid drops and dissipation of the fog.

For optically-thick well-mixed fogs (Fig. 6.14b), albedoes can be A =0.6 to 0.9. What little sunlight is not reflected off the fog is absorbed in the fog itself. However, IR radiative cooling continues, and can compensate the solar heating. One way to estimate whether fog will totally dissipate at time t in the future is to calculate the sum QAk of accumulated cooling and heating (see the Atmospheric Boundary Layer chapter) during the time period (t – to) since the fog first formed:

\(\begin{align} Q_{A k}=F_{H . n i g h t} \cdot\left(t-t_{o}\right)+(1-A) \cdot \frac{F_{H . \max } \cdot D}{\pi} \cdot\left[1-\cos \left(\frac{\pi \cdot\left(t-t_{S R}\right)}{D}\right)\right] \tag{6.12}\end{align}\)

where FH.night is average nighttime surface kinematic heat flux (negative at night), and FH.max is the amplitude of the sine wave that approximates surface kinematic sensible heat flux due to solar heating (i.e., the positive value of insolation at local noon). D is daylight duration hours, t is hours after sunset, to is hours after sunset when fog first forms, and tSR is hours between sunset and the next sunrise.

The first term on the right should be included only when t > to, which includes not only nighttime when the fog originally formed, but the following daytime also. This approach assumes that the rate FH.night of IR cooling during the night is a good approximation to the continued IR cooling during daytime.

The second term should be included only when t > tSR, where tSR is sunrise time. (See the Atmospheric Boundary Layer Chapter for definitions of other variables). If there was zero fog initially, but fog forms during the night as QAk becomes negative, then that fog will dissipate after sunrise at a time when QAk first becomes positive (neglecting other factors such as advection). This is illustrated in the figure in the Sample Application box at right.

While IR cooling happens from the very top of the fog, solar heating occurs over a greater depth in the top of the fog layer. Such heating underneath cooling statically destabilizes the fog, creating convection currents of cold thermals that sink from the fog top. These upside-down cold thermals continue to mix the fog even though there might be net heating when averaged over the whole fog. Thus, any radiatively heated or cooled air is redistributed and mixed vertically throughout the fog by convection.

Such heating can cause the bottom part of the fog to evaporate, which appears to an observer as lifting of the fog (Fig. 6.15a). Sometimes the warming during the day is insufficient to evaporate all the fog. When night again occurs and radiative cooling is not balanced by solar heating, the bottom of the elevated fog lowers back down to the ground (Fig. 6.15b).

In closing this section on fogs, please be aware that the equations above for formation, growth, and dissipation of fogs were based on very idealized situations, such as flat ground and horizontally uniform heating and cooling. In the real atmosphere, even gentle slopes can cause katabatic drainage of cold air into the valleys and depressions (see the Regional Winds chapter), which are then the favored locations for fog formation. The equations above were meant only to illustrate some of the physical processes, and should not to be used for operational fog forecasting.

Sample Application

Initially, total water is constant with height at 10 g kg–1, and wet-bulb potential temperature increases with height at rate 2°C/100m in a stratified fog. Later, a well-mixed fog forms with depth 100 m. Plot the total water and θW profiles before and after mixing.

Find the Answer

Check: Units OK. Physics OK.

Exposition: Dashed lines show initial profiles, solid are final profiles. For θW, the two shaded areas must be equal for heat conservation. This results in a temperature jump of 1°C across the top of the fog layer for this example. Recall from the Water Vapor and Atmospheric Stability chapters that actual temperature within the saturated (fog) layer decreases with height at the moist adiabatic lapse rate (lines of constant θW).

Figure 6.15 (a) Heating during the day can modify the fog of Fig. 6.14b, causing the base to lift and the remaining elevated fog to be less dense (b) If solar heating is insufficient to totally dissipate the fog, then nocturnal radiative cooling can re-strengthen it.

Sample Application(§)

When will fog dissipate if it has an albedo of A = (a) 0.4; (b) 0.6; (c) 0.8 ? Assume daylight duration of D = 12 h. Assume fog forms at to = 3 h after sunset.

Given: FH night = –0.02 K·m s–1, and FH max day = 0.2 K·m s–1. Also plot the cumulative heating.

Find the Answer

Given: (see above) Let t = time after sunset.

Use eq. (6.12), & solve on a spreadsheet:

(a) t = 16.33 h = 4.33 h after sunrise = 10:20 AM

(b) t = 17.9 h = 5.9 h after sunrise = 11:54 AM

(c) Fog never dissipates, because the albedo is so large that too much sunlight is reflected off of the fog, leaving insufficient solar heating to warm the ground and dissipate the fog.

Check: Units OK. Physics OK.

Exposition: Albedo makes a big difference for fog dissipation. There is evidence that albedo depends on the type and number concentration of fog nuclei.

This page titled 6.8: Fog is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Roland Stull via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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How is fog formed? - BBC Science Focus Magazine

How is fog formed? - BBC Science Focus Magazine

Subscribe to BBC Science Focus MagazinePrevious IssuesPodcastQ&ANewsFuture techNatureSpaceHuman bodyEveryday sciencePlanet EarthNewslettersHow is fog formed?Fog occurs when air saturated with water vapour is cooled suddenly, and there are several different ways this can happen.Try 3 issues for £5 when you subscribe to BBC Science Focus Magazine!Holly SpannerPublished: November 18, 2022 at 11:00 amFog is made up of molecules of water vapour, suspended in the air as tiny droplets of water but lingering close to the surface. Essentially, fog is just cloud that touches Earth’s surface and it forms the same way that clouds do. High humidity is a major contributing factor to the formation of fog, and depending on the percentage (as well as temperature), fog can appear and disappear very suddenly.

Water in the vapour state is transparent and invisible. The warmer the air, the more kinetic energy it has, and so the more water molecules it can keep jostling around as vapour.

If warm air containing lots of water vapour cools down suddenly, the water molecules slow down too much and are unable to stay in vapour form. Instead, they clump together into tiny droplets of liquid water. The droplets are still small enough to hang suspended in the air currents, but now they appear opaque because light reflects off the air/water interface.

Radiation fog

Radiation fog © Dan Bright

Radiation fog forms over land on calm, clear nights when heat absorbed by the Earth’s surface during the day is radiated into the air. As the heat escapes upwards, air close to the surface is cooled until it reaches saturation.

Cold air holds less water vapour than warm air, and the water vapour condenses into fog. Radiation fog will usually ‘burn off’ as the ground begins to warm again, but during winter months it can persist all day.

Radiation fog is also known as shallow fog or ground fog when it occurs in a narrow enough layer, situated below average eye-level on land (around 2m), or below around 10m at sea.

Valley fog

Valley Fog © Dan Bright

Valley fogusually forms in the lowest parts of a valley as cold, dense air settles and condenses, forming fog. It’s confined by local topography, such as hills or mountains, and can persist for several days.

Advection fog

Advection fog © Dan Bright

Advection fogforms when horizontal winds push warm, moist, air over a cool surface, where it condenses into fog. It’s common at sea, where warm, tropical air moves over cooler water. Advection fog can cover wide areas, and the Golden Gate Bridge in San Francisco Bay is often shrouded in advection fog.

Sea fog, a type of advection fog, can occur when warm, wet, air rolls off the land and onto the colder sea, or when a warm weather front hits a cold ocean current. In the UK, the north-east coast is very prone to sea fog because of the cold waters of the North Sea.

Upslope fog

Upslope fog © Dan Bright

Upslope fog is a type of hill fog and forms when the wind blows moist air up a slope, hill, or mountain, which cools as it rises. As it cools, the moisture condenses, and fog is formed as it continues to drift up the slope.

Evaporation fog

Evaporation fog © Dan Bright

Evaporation fog is similar to advection fog, and forms as cold air passes over moist land or warm water. When the warmer water evaporates into the low bands of air, it warms the air and causes it to rise. As this warm, moist air rises, it mixes with the colder air until its humidity reaches 100 per cent, and fog is formed. You'll often see evaporation fog over lakes, ponds and even outdoor swimming pools.

Why does altitude affect air temperature?

Imagine the atmosphere made up of parcels of air: the higher they are, the less strongly they’re compressed by the weight of the overlying atmosphere, and so the bigger their volume can be. Such expansion requires energy, and as the total energy in the parcel of air is fixed, it comes at the expense of thermal energy – thus lowering the temperature.

Does sound travel further on foggy days?

"Sound travels through the air as pressure waves rhythmically moving air molecules back and forth. Fog contains water droplets that scatter more of the sound energy, thus damping the sound and reducing the distance at which you can hear it," explains physicist Robert Matthews.

So, case closed? Not quite – because both the basic theory and experiments don’t take account of all the conditions under which fog forms.

"On warmer days where the humidity is especially high, the water molecules in the air are more agitated and can only form the tiniest droplets, which have a negligible effect on the sound waves."

"This damp air also has a higher density than dry air, which means that the sound waves can travel more effectively and be heard over a greater distance," Matthews adds.

Does sound travel further on foggy days? © Getty Images

About our expert, Professor Robert Matthews

After studying physics at Oxford, Robert became a science writer. He’s visiting professor in science at Aston University.

Read more about fog:

What is a fogbow?

Does sound travel further on foggy days?

How thick is the thickest fog?

How do thunderstorms form?

Asked by: Rich French, London

To submit your questions email us at questions@sciencefocus.com (don't forget to include your name and location)Share this articleHolly SpannerStaff Writer, BBC Science Focus

Holly is the Staff Writer for BBC Science Focus and looks after the popular Q&A section. She holds an MSc (distinction) in Geoscience (Space and Climate Physics) from UCL, and specialises in astronomy and earth sciences. Before joining Our Media, she worked as a geoenvironmental consultant and engineer, informing on ground risk and remediation in the north of England, while hunting for exoplanets in her spare time.

Alongside this, she was regional editor of a popular theatre website for nine years, and has a solid background in curating and developing content for the digital space. Holly is also an artist and illustrator, and regular contributor to the craft website, Gathered. She has previously worked with the RSPB, English Heritage, Disney, Pilot and Brother, as well as on BBC Good Food magazine, Home Style magazine, and Papercraft inspirations magazine. She enjoys photography and has a sustained interest in antiques.FacebookTwitterInstagramPinterestYoutubeRSSTerms & ConditionsPrivacy policyCookies policyCode of conductLicensingMagazine subscriptionsContact UsManage preferencesThis website is owned and published by Our Media Ltd. www.ourmedia.co.uk© Our Media 2023