Fog
58991
225848579
2008-07-15T18:03:49Z
66.108.169.78
[[WP:UNDO|Undid]] revision 225848386 by [[Special:Contributions/66.108.169.78|66.108.169.78]] ([[User talk:66.108.169.78|talk]])
{{otheruses|Fog (disambiguation)}}
{{Refimprove|date=February 2008}}
[[Image:Fog from plane.JPG|thumb|right|350px|In this view, only downtown [[San Francisco]] and [[Sutro tower]] are free from fog]]
'''Fog''' is a [[cloud]] in contact with the ground. [[Stratus cloud]]s are usually the only clouds that touch the ground. Fog differs from other clouds only in that fog touches the surface of the Earth. The same cloud that is not fog on lower ground may be fog where it contacts higher ground such as hilltops or mountain ridges. Fog is distinct from [[mist]] only in its density. Fog is defined as cloud which reduces [[visibility]] to less than 1 [[kilometer|km]], whereas mist is that which reduces visibility to less than 2 km.
The foggiest place in the world is the [[Grand Banks]] off the island of [[Newfoundland (island)|Newfoundland]], [[Canada]]. Fog is frequent there as the Grand Banks is the meeting place of the cold [[Labrador Current]] from the north and the much warmer [[Gulf Stream]] from the south. The foggiest land areas in the world are [[Point Reyes]], [[California]], and [[Argentia, Newfoundland and Labrador|Argentia]], [[Newfoundland and Labrador]], both with over 200 foggy days a year. Even in generally warmer southern Europe, thick fog and localized fog is often found in lowlands and valleys, like the lower part of [[Po Valley]] and the [[Tiber]] Valley, especially in late autumn and winter.
==Characteristics==
[[Image:GGB in fog 2007 edit.jpg|[[Golden Gate Bridge]] in fog|thumb|left|350px]]
{{Weather nav|thumb|right}}
Fog forms when the difference between [[temperature]] and [[dewpoint]] is (5 [[°F]]) 3 [[°C]], or less.
Fog begins to form when [[water vapor]] (a colorless gas) [[condensation|condenses]] into tiny liquid water droplets in the air. Conversely, water vapor is formed by the [[evaporation]] of liquid water or by the [[sublimation (chemistry)|sublimation]] of ice. Since water vapor is colorless, it is actually the small liquid water droplets that are condensed from it that make water suspended in the atmosphere visible in the form of fog or any other type of cloud.
[[Image:Empirestatefoggy.jpg|[[Empire State Building]] in a foggy summer afternoon |thumb|left|350px]]
Fog normally occurs at a [[relative humidity]] near 100%. This can be achieved by either adding moisture to the air or dropping the ambient air temperature. Fog can form at lower humidities, and fog can sometimes not form with relative humidity at 100%. A reading of 100% relative humidity means that the air can hold no additional moisture and the air will then become [[supersaturated]] if additional moisture is added.
Fog formation does require all of the elements that normal cloud formation requires with the most important being [[condensation nuclei]]. When the air is saturated, additional moisture tends to condense rather than staying in the air as vapor. Condensation nuclei must be present in the form of dust, aeresols, pollutants, etc. for the water to condense upon. When there are exceptional amounts of condensation nuclei present, especially [[hydroscopic]] (water seeking such as salt, see below) then the water vapor may condense below 100% relative humidity.
Fog can form suddenly, and can dissipate just as rapidly, depending what side of the [[dewpoint]] the temperature is on. This phenomenon is known as flash fog.
Another type of formation also common is associated with sea fog. This is due to the peculiar effect of [[salt]]. 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. Researchers have found that under stress (intense sunlight, strong evaporation, etc.), kelp release particles of iodine which in turn become nuclei for condensation of water vapor.<ref>[http://www.eurekalert.org/pub_releases/2008-05/uom-ssc050608.php Stressed seaweed contributes to cloudy coastal skies, study suggests<!-- Bot generated title -->]</ref>
[[Image:Crepuscular rays with reflection in GGP.jpg|[[Crepuscular rays]] created by tree shadows over fog|250px|thumb]]
Fog occasionally produces [[Precipitation (meteorology)|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. This can occur when the fog layer is lifted and cooled sufficiently, or when it is forcibly compressed from above. Drizzle becomes freezing drizzle when the temperature at the surface drops below the freezing point.
The thickness of fog 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 airmass 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 fogbank it may contain will be "squashed" when the pressure is high, and conversely, may expand upwards when the pressure above it is lowering.
<!-- Deleted image removed: [[Image:Oasc 4728.JPG|thumb|350px|right|Fog over the hills in the [[Oaş Country]], [[Romania]]]] -->
=== Fog as a visibility hazard ===
[[Image:20080313 Foggy Street.jpg|thumb|right|350px|Light fog reducing visibility in suburban street. Cyclist is very hazy at about 200m (219 yards). The limit of visibility is about 400m (437 yards), which is before the end of the street.]]
Fog reduces [[visibility]] and thus contributes to accidents, particularly with modes of transportation. Ships, trains, cars and planes cannot see each other and collide. Notable examples of accidents due to fog include the [[July 28]], [[1945]] crash of a [[B-25 Mitchell]] into the [[Empire State Building]], and the [[July 25]], [[1956]] collision of the [[ocean liner]]s the {{SS|Andrea Doria}} and {{MS|Stockholm|1948|6}}. Although most sea vessels can penetrate fog using [[radar]], road vehicles have to travel slowly and use low-beam headlights. Localised fog is especially dangerous, as drivers can be caught by surprise. On busy highways, [[multiple-vehicle collision]]s have resulted.
In aviation, fog normally prevents aircraft from taking off or landing at airports other than those equipped with an [[Instrument Landing System]]. Fog can be especially deceptive for light aircraft because often they will overfly the airfield to assess the surface conditions before landing. When the pilot is overhead the field looking straight down, he will usually only be looking through about 100 or 200 meters of fog and if it is thin enough, the pilot will be able to see the runway with a minimal amount of interference. However, when the pilot lines up for the final approach phase, instead of looking vertically through the fog, he is now looking through it at an angle, which increases the amount of haze he has to see through, often completely obscuring the runway. This presents a hazard because the pilot may have developed a mindset whereby the visibility is (or soon will be) suitable to land when it is not.
The worst accident in aviation history occurred in the fog when 2 Boeing 747s collided in 1977 in [[Tenerife disaster|Tenerife]]. One 747 had clearance to taxi down a foggy runway and the other could not see any distance down the runway when the captain decided to take off without proper clearance.
At airports, some attempts have been made to develop methods (such as using heating or spraying salt particles) to aid fog dispersal. These methods enjoy some success at temperatures below freezing.
==Types==
{{Cite-section|date=June 2007}}
Fog can form in a number of ways, depending on how the cooling that caused the [[Water vapor#Condensation|condensation]] occurred:
[[Image:Radiation fog aubrey.JPG|thumb|350px|Early morning radiation fog on a cool morning in [[Texas]].
Temperature is 45[[°F]] or 7[[°C]].]]
[[Image:FogParticles.jpg|thumb|350px|Minute particles of water constitute this after dark radiation fog in [[Oregon]] with the ambient temperature -2 [[°C]].]]
[[Image:FogParticlesHighSpeed.jpg|thumb|350px|High speed photo of the above.]]
[[Image:Lakeontario steamfog wikipedia.JPG|thumb|right|350px|Minus 25 degree Celsius air temperature produces immense steam fog over Lake Ontario which is still 2 to 5 degrees Celsius above freezing.]]
'''Radiation fog''' is formed by the cooling of land after [[sunset]] by [[thermal radiation]] in calm conditions with clear [[sky]].
The cool ground produces [[condensation]] in the nearby air by [[heat conduction]]. In perfect calm the fog layer can be less than a [[meter]] deep but [[turbulence]] can promote a thicker layer. Radiation fogs occur at night, and usually does not last long after [[sunrise]]. Radiation fog is common in [[autumn]], and early [[winter]]. Examples of this phenomenon include the [[Tule fog]]. <ref>http://www.nwas.org/ej/pdf/2007-FTT1.pdf</ref>
'''Ground fog''' is fog that obscures less than 60% of the sky and does not extend to the base of any overhead clouds. However, the term is sometimes used to refer to radiation fog.
'''Advection fog''' occurs when moist air passes over a cool surface by [[advection]] ([[wind]]) and is cooled. It is common as a [[warm front]] passes over an area with significant [[snow]]pack. It's most common at [[sea]] when [[tropical]] air encounters cooler [[water]]s, including areas of cold water [[upwelling]], such as along the [[California]] coast. The advection of fog along the California coastline is propelled onto land by one of several processes. A cold front can push the marine layer coastward, 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]].
'''Steam fog''', also called '''evaporation fog''', is the most localized form and is created by cold air passing over much warmer water or moist land. It often causes freezing fog, or sometimes [[hoar frost]].
'''Precipitation fog''' (or ''frontal fog'') forms as [[precipitation (meteorology)|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.
'''Upslope fog''' forms when winds blow air up a [[slope]] (called [[orographic lift]]), [[adiabatic process|adiabatical 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]] [[valley]]s, often during winter. It is the result of a [[temperature inversion]] caused by heavier cold air settling into in a valley, with warmer air passing over the mountains above. It is essentially 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]].
'''Ice fog''' is any kind of fog where the droplets have [[freezing|frozen]] into extremely tiny [[crystal]]s of [[ice]] in midair. Generally this requires [[temperature]]s at or below −35 °C (−30 °F), making it common only in and near the [[Arctic]] and [[Antarctic]] regions. It is most often seen in urban areas where it is created by the freezing of water vapor present in automobile exhaust and combustion -products from heating and power generation. Urban ice fog can become extremely dense and will persist day and night until the temperature rises. Extremely small amounts of ice fog falling from the sky form a type of [[precipitation (meteorology)|precipitation]] called [[Diamond dust|ice crystals]], often reported in [[Barrow, Alaska]]. Ice fog often leads to the visual phenomenon of [[light pillar]]s.
'''Freezing fog''' occurs when liquid fog droplets freeze to surfaces, forming white [[rime ice]]. This is very common on [[mountain]] tops which are exposed to low clouds. It is equivalent to [[freezing rain]], and essentially the same as the ice that forms inside a [[freezer]] which is not of the "frostless" or "frost-free" type. In some areas such as in the State of Oregon, the term "freezing fog" refers 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".
'''Artificial fog''' is artificially generated fog that is usually created by vaporizing a water and [[glycol]]-based or [[glycerine]]-based fluid. The fluid is injected into a heated block, and evaporates quickly. The resulting pressure forces the vapor out of the exit. Upon coming into contact with cool outside air the vapor forms a fog—see [[fog machine]].
'''Garua fog''' is a type of fog which happens to occur near the western coast of Chile. The normal 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.
'''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 [[Evaporation|evaporating]].<ref name="Marshall>{{cite book |last=Marshall |first=Tim |authorlink=Timothy P. Marshall |coauthors=[[David Hoadley]] (sketches) |title=Storm Talk |publisher= |date=May 1995 |location=USA |pages=39 |isbn= }}</ref>
=== Fog shadows===
These fascinating shadows look odd since humans are not used to seeing shadows in three dimensions. The thin fog is just dense enough to be illuminated by the light that passes through the gaps in a structure or in a tree. As a result, the path of an object shadow through the "fog" appears darkened. In a sense, these shadow lanes are similar to [[crepuscular rays]], which are caused by cloud shadows, but here, they are caused by the shadows of solid objects.
<br clear="all"><!-- this was added because the galleries would disappear behind the right-aligned, larger image above -->
==Samples of fog shadows of different objects:==
<gallery>
Image:Fog_shadow_IMG_6557.jpg|Fog Shadow of a tree
Image:Fog_shadow_IMG_6503.jpg|Fog Shadow of [[Sutro Tower]]
Image:Fog_shadow_tv_tower.jpg|Fog Shadow of [[Sutro Tower]]
Image:Fog_shadow_of_GGB.jpg|Fog Shadow of the South Tower of the Golden Gate Bridge
Image:Fog shadow of a tree-crepuscular rays.JPG|Fog shadow of a tree
Image:IMG 4355a.jpg|Fog Shadow of the South Tower of the Golden Gate Bridge
Image:Fogshadow of windmill.jpg|Fog shadow of a [[windmill]]
</gallery>
==See also==
* [[Great Smog of 1952]]
* [[Pea soup#Pea soup fog|"Pea soup" fog]]. Fog found in British cities that have factories that burn soft coal.
* ''[[Thank You, Fog|Thank You, Fog: last poems]] by [[W. H. Auden]]''
* [[List of rail accidents]]
* [[Autoland]]
* [[Automotive lighting]]
* [[Foglamp]]
* [[Black ice]]
* [[Haboob]]
* [[Mist]]
* [[Pogonip (weather)]]
* [[Smog]]
* [[Tule fog]]
* [[Camanchacas]]
* [[Fog Investigation and Dispersal Operation (FIDO)|FIDO]]
* [[Fogging (photography)]]
* [[Anti-fog]]
* [[Runway visual range]]
* [[Transmissometer]]
* [[Visibility]]
* [[Whiteout (weather)]]
== References ==
{{reflist}}
* Maria K. Filonczuk, Daniel R. Cayan, Laurence G. Riddle, Variability of marine fog along the California coast, SIO-Reference, No 95-2, Climate Research Division, Scripps Institution of Oceanography, University of California San Diego, July 1995.
{{Commons|Fog}}
==External links==
*[http://www.theweatherprediction.com/habyhints/359/ What is the difference between ice fog and freezing fog?]
*[http://www.weather.gov/wwamap/wwatxtget.php?cwa=usa&wwa=dense%20fog%20advisory United States' current dense fog advisories] from [[NOAA]]
*[http://www.wrh.noaa.gov/satellite/ Current Western US fog satellite pictures from NOAA]
*[http://www.youtube.com/watch?v=r8FPWRj_TKo Lake Ontario Evaporative-Steam Fog Video]
*[http://www.ncdc.noaa.gov/oa/esb/?goal=weather&file=events/fog/ Social & Economic Costs of Fog] from "NOAA Socioeconomics" website initiative
{{Meteorological variables}}
{{Cloud types}}
[[Category:Aviation risks]]
[[Category:Weather]]
[[Category:Weather hazards]]
[[Category:Accidents involving fog]]
[[Category:Snow or ice weather phenomena]]
[[Category:Fog| ]]
[[Category:Psychrometrics]]
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