Editing Fog

{{short description|Atmospheric phenomenon}}
{{Other uses|Fog (disambiguation)|Foggy (disambiguation)}}
{{Use dmy dates|date=June 2021}}

[[File:A foggy winter morning.jpg|thumb|upright=1.35|Light fog in Bangladesh]]
{{Weather}}

'''Fog''' is a visible [[aerosol]] consisting of tiny [[water]] [[drop (liquid)|droplets]] or [[ice crystals]] suspended in the air at or near the [[Earth]]'s surface.<ref name=DefinitionFog>{{cite book |quote=The international definition of fog consists of a suspended collection of water droplets or ice crystal near the Earth's surface... |title=Fog and Boundary Layer Clouds |chapter=Fog Visibility and Forecasting |editor-last=Gultepe |editor-first=Ismail |isbn=978-3-7643-8418-0 |page=1126 |chapter-url=https://books.google.com/books?id=QwzHZ-wV-BAC&pg=PA1126 |archive-url=https://web.archive.org/web/20160903104636/https://books.google.com/books?id=QwzHZ-wV-BAC&pg=PA1126 |archive-date=3 September 2016 |date=2 January 2008|publisher=Springer }} Reprint from {{cite journal |journal=Pure and Applied Geophysics |title=none |volume=164 |year=2007 |issue=6–7}}</ref><ref>{{cite web |url=https://scijinks.gov/fog/ |title=What's the Difference Between Fog and Clouds? |publisher=NOAA |date=2022}}</ref> Fog can be considered a type of low-lying [[cloud]] usually resembling [[stratus cloud|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. [[Marine layer|Sea fog]], which shows up near bodies of [[saline water]], is formed as water vapor condenses on bits of salt. Fog is similar to, but less transparent than, [[mist]].

== Definition ==
The term ''fog'' is typically distinguished from the more generic term ''cloud'' in that fog is low-lying, and the moisture in the fog is often generated locally (such as from a nearby body of water, like a lake or ocean, or from nearby moist ground or [[marsh]]es).<ref name=DistinguishCloud>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. ({{cite book |title=Standard practice for the design and operation of supercooled fog dispersal projects |last=Thomas |first=P. |year=2005 |page=3 |publisher=American Society of Civil Engineers |isbn=0-7844-0795-9 |url=https://books.google.com/books?id=AIvBjD_HXpEC&pg=PR2 |access-date=27 March 2016 |archive-date=3 September 2016 |archive-url=https://web.archive.org/web/20160903082350/https://books.google.com/books?id=AIvBjD_HXpEC&pg=PR2 |url-status=dead }}) 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.</ref> By definition, fog reduces [[visibility]] to less than {{cvt|1|km|mi}}, whereas [[mist]] causes lesser impairment of visibility.<ref>{{cite web |url=http://www.ofcm.gov/fmh-1/pdf/H-CH8.pdf |title=Federal Meteorological Handbook Number 1: Chapter 8 – Present Weather |date=1 September 2005 |pages=8–1, 8–2 |access-date=9 October 2010 |publisher=Office of the Federal Coordinator for Meteorology |url-status=dead |archive-url=https://web.archive.org/web/20110521015053/http://www.ofcm.gov/fmh-1/pdf/H-CH8.pdf |archive-date=21 May 2011}}</ref> For aviation purposes in the United Kingdom, a visibility of less than {{cvt|5|km|mi}} but greater than {{cvt|999|m|ft}} is considered to be mist if the [[relative humidity]] is 95% or greater; below 95%, [[haze]] is reported.<ref>{{citation |title=Annex 3 |edition=17th |date=July 2010}}</ref>{{Full citation needed|date=February 2010}}

== Formation ==
{{see also|Cloud physics}}
[[File:FogParticles.jpg|thumb|right|Minute droplets of water constitute this after-dark radiation fog, with an ambient temperature of {{cvt|-2|C|F|0}}. Their [[motion blur|motion trails]] are captured as streaks.]]
[[File:FogParticlesHighSpeed.jpg|thumb|right|A close-up view of water droplets forming fog. Those [[defocus aberration|outside]] the camera lens's [[depth of field]] appear as [[backscatter (photography)|orbs]].]]

Fog forms when the difference between air temperature and [[dew point]] is less than {{cvt|2.5|C-change|lk=on}}.<ref>{{cite web|url=http://glossary.ametsoc.org/wiki/Fog|title=Fog – AMS Glossary|access-date=16 March 2013|url-status=live|archive-url=https://web.archive.org/web/20130327012718/http://glossary.ametsoc.org/wiki/Fog|archive-date=27 March 2013}}</ref><ref>{{cite web |url=https://www.weather.gov/media/zhu/ZHU_Training_Page/fog_stuff/fog_guide/fog.pdf |title=Fog |publisher=National Weather Service |date=2022}}</ref> Fog begins to form when [[water vapor]] [[condensation|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;<ref name="convection">{{cite book|author=Robert Penrose Pearce|year=2002|url=https://books.google.com/books?id=QECy_UBdyrcC&pg=PA66 |title=Meteorology at the Millennium|publisher=Academic Press|page=66|isbn=978-0-12-548035-2 |access-date=2 January 2009}}</ref> precipitation or [[virga]] falling from above;<ref>{{cite web|website=[[National Weather Service]] Office, Spokane, Washington|url=http://www.wrh.noaa.gov/otx/outreach/ttalk/virga.php |title=Virga and Dry Thunderstorms|url-status=live|archive-url=https://web.archive.org/web/20090522112015/http://www.wrh.noaa.gov/otx/outreach/ttalk/virga.php|archive-date=22 May 2009}}</ref> daytime heating evaporating water from the surface of oceans, water bodies, or wet land;<ref>{{cite web|author1=Bart van den Hurk |author2=Eleanor Blyth |year=2008 |url=http://www.knmi.nl/~hurkvd/Loco_workshop/Workshop_report.pdf |title=Global maps of Local Land-Atmosphere coupling |publisher=KNMI |access-date=2 January 2009 |url-status=dead |archive-url=https://web.archive.org/web/20090225074154/http://www.knmi.nl/~hurkvd/Loco_workshop/Workshop_report.pdf |archive-date=25 February 2009 }}</ref> [[transpiration]] from plants;<ref>{{cite web|author1=Krishna Ramanujan |author2=Brad Bohlander |year=2002 |url=http://www.gsfc.nasa.gov/topstory/20020926landcover.html|title=Landcover changes may rival greenhouse gases as cause of climate change|publisher=[[National Aeronautics and Space Administration]] [[Goddard Space Flight Center]] |access-date=2 January 2009 |url-status=dead |archive-url = https://web.archive.org/web/20080603022239/http://www.gsfc.nasa.gov/topstory/20020926landcover.html |archive-date = 3 June 2008}}</ref> cool or dry air moving over warmer water;<ref>{{cite web|author=[[National Weather Service]] JetStream|year=2008|url=http://www.srh.weather.gov/srh/jetstream/synoptic/airmass.htm|title=Air Masses|access-date=2 January 2009|url-status=dead|archive-url=https://web.archive.org/web/20081224062959/http://www.srh.weather.gov/srh/jetstream/synoptic/airmass.htm|archive-date=24 December 2008}}</ref> and lifting air over mountains.<ref name="MT">{{cite web|author=Michael Pidwirny|year=2008|url= http://www.physicalgeography.net/fundamentals/8e.html |title=CHAPTER 8: Introduction to the Hydrosphere (e). Cloud Formation Processes|publisher=Physical Geography|access-date=1 January 2009|url-status=live|archive-url= https://web.archive.org/web/20081220230524/http://www.physicalgeography.net/fundamentals/8e.html |archive-date=20 December 2008}}</ref> Water vapor normally begins to condense on [[Cloud condensation nuclei|condensation nuclei]] such as dust, ice, and salt in order to form clouds.<ref>{{cite web|url=http://glossary.ametsoc.org/wiki/Front |website=Glossary of Meteorology|date=25 April 2012|publisher=[[American Meteorological Society]]|title= Front |url-status=live|archive-url=https://web.archive.org/web/20181010010455/http://glossary.ametsoc.org/wiki/Front |archive-date= 10 October 2018}}</ref><ref name="DR">{{cite web|author=Roth, David M.|title=Unified Surface Analysis Manual|date=14 December 2006|access-date=9 October 2010 |publisher= [[Hydrometeorological Prediction Center]] |url=http://www.wpc.ncep.noaa.gov/sfc/UASfcManualVersion1.pdf |url-status=live|archive-url=https://web.archive.org/web/20060929004553/http://www.hpc.ncep.noaa.gov/sfc/UASfcManualVersion1.pdf |archive-date=29 September 2006}}</ref> Fog, like its elevated cousin [[Stratus cloud|stratus]], is a stable cloud deck which tends to form when a cool, stable air mass is trapped underneath a warm air mass.<ref>{{cite web|author=FMI|year=2007|url= http://www.zamg.ac.at/docu/Manual/SatManu/main.htm?/docu/Manual/SatManu/CMs/FgStr/backgr.htm |title=Fog And Stratus – Meteorological Physical Background|publisher=Zentralanstalt für Meteorologie und Geodynamik|access-date=7 February 2009|url-status=live|archive-url=https://web.archive.org/web/20110706085616/http://www.zamg.ac.at/docu/Manual/SatManu/main.htm?%2Fdocu%2FManual%2FSatManu%2FCMs%2FFgStr%2Fbackgr.htm|archive-date=6 July 2011}}</ref>

Fog normally occurs at a [[relative humidity]] near 100%.<ref name="gliess">Gleissman, Stephe (2007). ''Agroecology: the ecology of [[sustainable food systems]].'' [[CRC Press]]. p. 73. {{ISBN|0849328454}}.</ref> This occurs from either added moisture in the air, or falling ambient air temperature.<ref name="gliess" /> 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 [[supersaturation|supersaturated]] if additional moisture is added.

Fog commonly produces [[precipitation]] in the form of [[drizzle]] or very light snow. Drizzle occurs when the humidity attains 100% and the minute cloud droplets begin to coalesce into larger droplets.<ref>Allred, Lance (2009). ''Enchanted Rock: A Natural and Human History.'' [[University of Texas Press]]. p. 99. {{ISBN|0292719639}}.</ref> 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 ==
Fog can form multiple ways, depending on how the cooling occurred that caused the condensation.

'''Radiation fog''' is formed by the cooling of land after sunset by [[infrared]] [[thermal radiation]] in calm conditions with a clear sky. The cooling ground then cools adjacent air by [[Thermal conduction|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]].<ref>Cox, Robert E. [http://www.nwas.org/ej/pdf/2007-FTT1.pdf Applying Fog Forecasting Techniques using AWIPS and the Internet] {{webarchive|url=https://web.archive.org/web/20071029004355/http://www.nwas.org/ej/pdf/2007-FTT1.pdf |date=29 October 2007 }}. [[National Weather Service]], 2007. nwas.org</ref>

'''Ground fog''' is fog that obscures less than 60% of the sky and does not extend to the base of any overhead clouds.<ref>[http://education.arm.gov/outreach/publications/08augnewsltr.pdf Climate education update: News and information about climate change for teachers and students] {{webarchive|url=https://web.archive.org/web/20100527095541/http://education.arm.gov/outreach/publications/08augnewsltr.pdf |date=27 May 2010 }}. Atmospheric Radiation Measurement. Climate Research Facility. [[U.S. Department of Energy]]. education.arm.gov</ref> However, the term is usually a synonym for shallow radiation fog; in some cases the depth of the fog is on the order of tens of centimetres over certain kinds of terrain with the absence of wind.

[[File:San francisco in fog with rays.jpg|thumb|upright|[[San Francisco fog|Advection fog layer in San Francisco]] with the [[Golden Gate Bridge]] and skyline in the background]]
[[File:Sydney Harbour Fog - panoramio.jpg|thumb|upright|Advection fog over [[Sydney Harbour]] and the [[Sydney Opera House]], Australia]] 
'''Advection fog''' occurs when moist air passes over a cool surface by [[advection]] (wind) and is cooled.<ref>{{cite book |last = Frost|first = Helen|date =2004|title =Fog|publisher= Capstone Press|page = 22|isbn = 978-0-7368-2093-6|url = https://books.google.com/books?id=YJ-ZDmX4MkQC&pg=PA22}}</ref> 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 [[San Francisco fog|along the California coast]]. A strong enough temperature difference over water or bare ground can also cause advection fog.

Although strong winds often mix the air and can disperse, fragment, or prevent many kinds of fog, markedly warmer and humid air blowing over a snowpack can continue to generate advection fog at elevated velocities up to {{cvt|50|mph|km/h|0|order=flip}} or more – this fog will be in a turbulent, rapidly moving, and comparatively shallow layer, observed as a few centimetres/inches in depth over flat farm fields, flat urban terrain and the like, and/or form more complex forms where the terrain is different such as rotating areas in the lee of hills or large buildings and so on.

Fog formed by advection along the California coastline is propelled onto land by one of several processes. A cold front can push the marine layer coast-ward, an occurrence most typical in the spring or late fall. During the summer months, a low-pressure trough produced by intense heating inland creates a strong pressure gradient, drawing in the dense marine layer. Also, during the summer, strong high pressure aloft over the desert southwest, usually in connection with the summer [[monsoon]], produces a south to southeasterly flow which can drive the offshore marine layer up the coastline; a phenomenon known as a "southerly surge", typically following a coastal heat spell. However, if the monsoonal flow is sufficiently turbulent, it might instead break up the marine layer and any fog it may contain. Moderate turbulence will typically transform a fog bank, lifting it and breaking it up into shallow convective clouds called [[stratocumulus]].

'''Frontal fog''' forms in much the same way as stratus cloud near a front when raindrops, falling from relatively warm air above a frontal surface, evaporate into cooler air close to the Earth's surface and cause it to become saturated. The water vapor cools and at the dewpoint it condenses and fog forms. This type of fog can be the result of a very low frontal stratus cloud subsiding to surface level in the absence of any lifting agent after the front passes.

'''Hail fog''' sometimes occurs in the vicinity of significant [[hail]] accumulations due to decreased temperature and increased moisture leading to saturation in a very shallow layer near the surface. It most often occurs when there is a warm, humid layer atop the hail and when wind is light. This ground fog tends to be localized but can be extremely dense and abrupt. It may form shortly after the hail falls; when the hail has had time to cool the air and as it [[Phase transition|absorbs heat]] when melting and evaporating.<ref name="Marshall">Marshall, T., Hoadley, D. (1995). ''Storm Talk''. Tim Marshall.{{full citation needed|date=July 2019}}</ref>

=== Freezing conditions ===
'''{{visible anchor|Freezing fog}}''' occurs when liquid fog droplets freeze to surfaces, forming white soft or hard [[rime ice]].<ref>{{cite web |url=https://w1.weather.gov/glossary/index.php?word=FOG |title=NWS Glossary |publisher=National Weather Service |date=2022}}</ref><ref name="bbcw" /> 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 [[Supercooling|super-cooled]], filling the air with small ice crystals similar to very light snow. It seems to make the fog "tangible", as if one could "grab a handful".
[[File:Aerial View of freezing fog in the Okanagan Highlands.webm|thumb|[[Aerial photography#Aerial video|Aerial video]] of freezing fog in the [[Okanagan Highlands]] ]]

In the [[western United States]], freezing fog may be referred to as '''pogonip'''.<ref name="merriam-webster">{{Cite Merriam-Webster|pogonip}}</ref> 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".<ref name="merriam-webster" /><ref name="dictionary.com">{{cite web|url=http://dictionary.reference.com/browse/pogonip?s=t|title=Pogonip – Definition from the Dictionary.com|access-date=2013-01-02|url-status=live|archive-url=https://web.archive.org/web/20140222133643/http://dictionary.reference.com/browse/pogonip?s=t|archive-date=22 February 2014}}</ref>
In ''[[The Old Farmer's Almanac]],'' in the calendar for December, the phrase "Beware the Pogonip" regularly appears. In his anthology ''Smoke Bellew'', [[Jack London]] describes a pogonip which surrounded the main characters, killing one of them.

The phenomenon is common in the inland areas of the Pacific Northwest, with temperatures in the {{convert|10|to|30|°F|°C|abbr=on}} range. The [[Columbia Plateau]] experiences this phenomenon most years during [[temperature inversion]]s, sometimes lasting for as long as three weeks. The fog typically begins forming around the area of the [[Columbia River]] and expands, sometimes covering the land to distances as far away as [[La Pine, Oregon]], almost {{convert|150|mi|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 {{convert|-35|°C|°F|abbr=on}}, making it common only in and near the [[Arctic]] and [[Antarctic]] regions.<ref>Haby, Jeff. [http://www.theweatherprediction.com/habyhints/359/ What is the difference between ice fog and freezing fog?] {{webarchive|url=https://web.archive.org/web/20060108091322/http://www.theweatherprediction.com/habyhints/359/ |date=8 January 2006 }} theweatherprediction.com</ref> It is most often seen in urban areas where it is created by the freezing of water vapor present in automobile exhaust and combustion products from heating and power generation. Urban ice fog can become extremely dense and will persist day and night until the temperature rises. It can be associated with the [[diamond dust]] form of precipitation, in which very small crystals of ice form and slowly fall. This often occurs during blue sky conditions, which can cause many types of halos and other results of refraction of sunlight by the airborne crystals. Ice fog often leads to the visual phenomenon of [[light pillar]]s.

<gallery mode="packed">
File:PostcardVirginiaCityNVPogonipCirca1907.jpg|Pogonip fog in [[Virginia City, Nevada]], from an early 20th-century postcard
File:Tree in field during extreme cold with frozen fog.png|A tree in a field during extreme cold with frozen fog
File:Näsijärvi, Tampere, Finland. January 2019.jpg|Ice fog on [[Pyhäjärvi (Tampere region)|Pyhäjärvi]], [[Tampere]] during sunset.
</gallery>

=== Topographical influences ===
{{multiple image
| footer    = 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)|Rio de Janeiro state]], [[Brazil]]
| width     = 220
| image1    = Nascer do sol em meio à neblina, na Pedra do Sino.jpg
| alt1      =
| image2    = Nascer do Sol no Dedo de Deus.jpg
| alt2      =
}}

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

'''Valley fog''' forms in mountain [[valley]]s, 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 (California)|Central Valley]], valley fog is often referred to as [[tule fog]].
[[File:High Desert Fog.jpg|thumb|[[Yucca Valley, California]] [[tule fog]]]]

=== Sea and coastal areas ===
'''Sea fog''' (also known as ''[[Haar (fog)|haar]]'' or ''fret'') is heavily influenced by the presence of [[sea spray]] and microscopic airborne salt crystals. Clouds of all types require minute [[Hygroscopy|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.<ref>[http://www.eurekalert.org/pub_releases/2008-05/uom-ssc050608.php Stressed seaweed contributes to cloudy coastal skies, study suggests] {{webarchive|url=https://web.archive.org/web/20080511185319/http://www.eurekalert.org/pub_releases/2008-05/uom-ssc050608.php |date=11 May 2008 }}, eurekalert.org</ref>

'''[[Sea smoke]]''', also called '''steam fog''' or '''evaporation fog''', is created by cold air passing over warmer water or moist land.<ref name=bbcw>[http://www.bbc.co.uk/weather/features/understanding/fog.shtml Understanding Weather – Fog] {{webarchive|url=https://web.archive.org/web/20090131104702/http://www.bbc.co.uk/weather/features/understanding/fog.shtml |date=31 January 2009 }}. [[BBC Weather]]. bbc.co.uk</ref> It may cause freezing fog or sometimes [[hoar frost]]. This situation can also lead to the formation of [[steam devils]], which look like their [[dust devil|dust counterparts]].<ref>{{cite web |date=2022 |title=Fog Facts |url=https://www.fastfactsforkids.com/weather-facts/fog-facts-for-kids |publisher=Fast Facts for Kids}}</ref> Lake-effect fog is of this type, sometimes in combination with other causes like radiation fog. It tends to differ from most advective fog formed over land in that it is (like [[lake-effect snow]]) a convective phenomenon, resulting in fog that can be very dense and deep and looks fluffy from above. '''Arctic sea smoke''' is similar to sea smoke but occurs when the air is very cold. Instead of condensing into water droplets, columns of freezing, rising, and condensing water vapor is formed. The water vapor produces the ''sea smoke fog'' and is usually misty and smoke-like.<ref>{{cite encyclopedia|title=Arctic Sea Smoke|url=http://www.encyclopedia.com/topic/Arctic_sea_smoke.aspx|encyclopedia=encyclopedia.com|url-status=live|archive-url=https://web.archive.org/web/20160506012219/http://www.encyclopedia.com/topic/Arctic_sea_smoke.aspx|archive-date=6 May 2016}}</ref>

'''[[Garúa]] fog''' near the coast of [[Chile]] and [[Peru]]<ref>Cowling, R. M., Richardson, D. M., Pierce, S. M. (2004). ''Vegetation of Southern Africa''. [[Cambridge University Press]]. p. 192. {{ISBN|0521548012}}.</ref> occurs when typical fog produced by the sea travels inland but suddenly meets an area of hot air. This causes the water particles of fog to shrink by evaporation, producing a "transparent mist". Garua fog is nearly invisible, yet it still forces drivers to use windshield wipers because of condensation onto cooler hard surfaces. '''[[Camanchaca]]''' is a similar dense fog.

<gallery mode="packed">
File:Seattle Columbia Pano2.jpg|Fog rolls into [[Seattle]] from the sea
File:Sea fog encroaching on Brighton pier.jpg|Sea fog or "fret" encroaching on [[Brighton Pier]]
File:MS Europa vor der Insel Jan Mayen im Nebel - 2011.png|Sea fog in the [[Arctic Ocean]] near the island of [[Jan Mayen]]
</gallery>

== Visibility effects ==
[[File:Свјетлопис аутопута у Србији, магле и Сунца.jpg|thumb|right|Fog in Serbia|upright]]
[[File:20080313 Foggy Street.jpg|thumb|left|Light fog reduces visibility on a suburban street, rendering the cyclist very hazy at about {{convert|200|m|yd|abbr=on}}. The limit of visibility is about {{convert|400|m|yd|abbr=on}}, 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 collision]]s.

The aviation travel industry is affected by the severity of fog conditions. Even though modern [[Autoland|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 ==
[[File:Fog shadow tv tower.jpg|thumb|upright|[[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 ==
{{see also|Acoustic location|Acoustic shadow|Foghorn}}
Sound typically travels fastest and farthest through solids, then liquids, then gases such as the atmosphere. Sound is affected during fog conditions due to the small distances between water droplets, and air temperature differences.

Though fog is essentially liquid water, the many droplets are separated by small air gaps. High-pitched sounds have a high frequency, which in turn means they have a short wavelength. To transmit a high frequency wave, air must move back and forth very quickly. Short-wavelength high-pitched sound waves are reflected and refracted by many separated water droplets, partially cancelling and dissipating their energy (a process called "[[Damping ratio|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 [[foghorn]]s use a low-pitched tone.<ref>{{cite web|title=Does fog have a dampening effect on sounds?|url=http://www.thenakedscientists.com/HTML/questions/question/2327/|work=thenakedscientists.com|date=14 June 2009|url-status=live|archive-url=https://web.archive.org/web/20150116132528/http://www.thenakedscientists.com/HTML/questions/question/2327/|archive-date=16 January 2015}}</ref>

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.<ref>{{cite web|title=How fog can play tricks on your ears?|url=http://www.katu.com/blogs/weather/34619514.html|work=katu.com|url-status=live|archive-url=https://web.archive.org/web/20150412212345/http://www.katu.com/blogs/weather/34619514.html|archive-date=12 April 2015}}</ref>

== Record extremes ==
Particularly foggy places include{{citation needed|date=November 2016}} [[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|date=September 2014}} 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 Valley|Tiber]] valleys in Italy; [[Ebro]] Valley in northeastern Spain; as well as on the [[Swiss plateau]], especially in the [[Seeland (Switzerland)|Seeland]] area, in late autumn and winter.{{Citation needed|date=February 2010}} Other notably foggy areas include coastal [[Chile]] (in the south); coastal [[Namibia]]; [[Nord, Greenland]]; and the [[Severnaya Zemlya]] islands.{{Citation needed|date=December 2021}}

== As a water source ==
[[Sequoia sempervirens|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.<ref>{{cite web|url=https://www.npr.org/templates/story/story.php?storyId=123771983&ft=1&f=1001|author=Joyce, Christopher|date=23 February 2010|title=Fog Fluctuations Could Threaten Giant Redwoods|website=[[NPR]]|url-status=live|archive-url=https://web.archive.org/web/20160127091016/http://www.npr.org/templates/story/story.php?storyId=123771983&ft=1&f=1001|archive-date=27 January 2016}}</ref> 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 net]]s 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 ==

[[File:Verisure zerovision.jpg|thumb|An artificial opacifying fog triggered remotely to scare off burglars.]]

'''Artificial fog''' is man-made fog that is usually created by vaporizing a water- and [[glycol]]- or [[glycerine]]-based fluid. The fluid is injected into a heated metal block which evaporates quickly. The resulting pressure forces the vapor out of a vent. Upon coming into contact with cool outside air, the vapor condenses in microscopic droplets and appears as fog.<ref>Karukstis, K. K., Van Hecke, G. R. (2003). ''Chemistry connections: the basis of everyday phonemena.'' Academic Press. p. 23. {{ISBN|0124001513}}.</ref> Such [[fog machine]]s are primarily used for [[theatrical smoke and fog|entertainment applications]].

== Historical references ==
{{see also|Pea soup fog|Fog of war|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 1776 [[Battle of Long Island]] 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 [[Normandy landings|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.<ref>{{cite book |last1=Tardif |first1=Robert M. |title=Characterizing fog and the physical mechanisms leading to its formation during precipitation in a coastal area of the northeastern United States |date=2007 |bibcode=2007PhDT........70T |url=https://www.proquest.com/openview/91ad9e73f6805860be94dadcc9d40c44/1 }}</ref>

== See also ==
=== Technology ===
* [[Anti-fog]]
* [[Automotive lighting]]
* [[Decontamination foam]]
* [[Fog Investigation and Dispersal Operation (FIDO)]]
* [[Fog collection]]
* [[Fogging (photography)]]
* [[Fog lamp]]
* [[Head-up display]]
* [[Runway visual range]]
* [[Transmissometer]]

=== Weather ===
* [[Fog season]]
* [[Smoke]]
* [[Smog]]
* [[Whiteout (weather)]]
* [[Vog]]

== References ==
{{Reflist|30em}}
Under "[ ^ "Federal Meteorological Handbook Number 1: Chapter 8 – Present Weather" (PDF). Office of the Federal Coordinator for Meteorology. 1 September 2005. pp.&nbsp;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 ==
* 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)
* {{cite journal |last1=Riddle |first1=Laurence G. |last2=Cayan |first2=Daniel R. |last3=Filonczuk |first3=Maria K. |title=Variability of Marine Fog Along the California Coast |date=1 July 1995 |url=https://escholarship.org/uc/item/2kc7x97f }}
* {{cite journal |last1=Lu |first1=Chunsong |last2=Liu |first2=Yangang |last3=Niu |first3=Shengjie |last4=Zhao |first4=Lijuan |last5=Yu |first5=Huaying |last6=Cheng |first6=Muning |title=Examination of microphysical relationships and corresponding microphysical processes in warm fogs |journal=Acta Meteorologica Sinica |date=30 January 2014 |volume=27 |issue=6 |pages=832–848 |doi=10.1007/s13351-013-0610-0 |s2cid=2471958 }}
* {{cite journal |last1=Lu |first1=Chunsong |last2=Niu |first2=Shengjie |last3=Tang |first3=Lili |last4=Lv |first4=Jingjing |last5=Zhao |first5=Lijuan |last6=Zhu |first6=Bin |title=Chemical composition of fog water in Nanjing area of China and its related fog microphysics |journal=Atmospheric Research |date=July 2010 |volume=97 |issue=1–2 |pages=47–69 |doi=10.1016/j.atmosres.2010.03.007 |bibcode=2010AtmRe..97...47L }}

== External links ==
{{Commons}}
{{Wikiquote|Fog}}
* [http://www.ncdc.noaa.gov/oa/esb/?goal=weather&file=events/fog/ Social & Economic Costs of Fog] from "NOAA Socioeconomics" website initiative
* [http://www.weather.gov/wwamap/wwatxtget.php?cwa=usa&wwa=dense%20fog%20advisory United States' current dense fog advisories] from [[National Oceanic and Atmospheric Administration|NOAA]]
* [http://www.wrh.noaa.gov/satellite/ Current Western US fog satellite pictures from NOAA]

{{Aerosol terminology|state=expanded}}
{{Meteorological variables}}
{{Cloud types}}

{{Authority control}}

[[Category:Fog| ]]
[[Category:Weather hazards to aircraft]]
[[Category:Snow or ice weather phenomena]]
[[Category:Road hazards]]
[[Category:Clouds, fog and precipitation]]
[[Category:Hazards of outdoor recreation]]