Fog

Fog is a collection of water droplets or ice crystals suspended in the air at or near the Earth's surface.^[1] While fog is a type of a cloud, 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).^[2]

Fog is distinguished from mist only by its density, as expressed in the resulting decrease in visibility: Fog reduces visibility to less than 1 km, whereas mist reduces visibility to no less than 1 km but less than 2 km. For aviation purposes in the UK, a visibility of less than 2 km but greater than 999 m is considered to be mist if the relative humidity is 95% or greater - below 95% haze is reported.

The foggiest place in the world is the Grand Banks off the island of Newfoundland, the meeting place of the cold Labrador Current from the north and the much warmer Gulf Stream from the south. Some of the foggiest land areas in the world include Argentina, Newfoundland, Labrador and Point Reyes, California, each with over 200 foggy days per year. Even in generally warmer southern Europe, thick fog and localized fog is often found in lowlands and valleys, such as the lower part of the Po Valley and the Arno and Tiber valleys, as well as on the Swiss plateau, especially in the Seeland area, in late autumn and winter. Other notably foggy areas include coastal Chile (in the south), coastal Namibia, and the Severnaya Zemlya islands.^[3]

Characteristics

Fog forms when the difference between temperature and dew point is generally less than 2.5 °C or 4 °F.^[4]

Fog begins to form when water vapor condenses into tiny liquid water droplets in the air. Conversely, water vapor is formed by the evaporation of liquid water or by the 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.

In this view, only downtown San Francisco and Sutro tower are free from fog

Fog normally occurs at a relative humidity near 100%.^[5] This can be achieved by either adding moisture to the air or dropping the ambient air temperature.^[5] 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; the air will become supersaturated if additional moisture is added.

Fog formation requires all of the elements that normal cloud formation requires, the most important being condensation nuclei, in the form of dust, aerosols, pollutants, etc., for the water to condense upon. When there are exceptional amounts of condensation nuclei present, especially hygroscopic particles such as salt, 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 dew point the temperature is on. This phenomenon is known as flash fog.^[6]

Low lying fog over the more cool ocean surface in Vancouver, Canada during the 2010 Winter Olympics that featured warmer temperatures mixed with high humidity.

Another common type of formation is associated with sea fog (also known as haar or fret). This is due to the peculiar effect of salt. 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. 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.^[7]

Fog occasionally 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.^[8] 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.

Fog as a visibility hazard

Dense Tule fog in Bakersfield, California. Visibility in this photo is less than 500 feet (150 metres).

Light fog reducing visibility on a suburban street. The 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.

Road signs are hard to see in the fog on Golden Gate Bridge.

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 28 July 1945 crash of a B-25 Mitchell into the Empire State Building, the 25 July 1956 collision of the ocean liners the SS Andrea Doria and MS Stockholm and more recently when the MV COSCO Busan struck the San Francisco – Oakland Bay Bridge. 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 collisions have resulted—such as Florida's Interstate 4 70-vehicle pileup that occurred on 9 January 2008.

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 looking straight down at the field, he will usually be looking through only about 100 or 200 meters of fog. If the fog 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 will be looking through it at an angle, thus increasing the amount of haze he has to see through, often completely obscuring the runway. This presents a hazard, because the pilot may think that the visibility is (or soon will be) suitable for landing when it is not.

The worst accident in aviation history occurred in the fog when two Boeing 747s collided in 1977 in the Tenerife airport disaster. One captain had clearance to taxi his airplane down a foggy runway; the other captain, who could not see down the runway, 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.^[9] These methods enjoy some success at temperatures below freezing.

Fog shadows

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 due to the same cause as crepuscular rays, which are the shadows of clouds, but in this case, they are the shadows of solid objects.

Fog shadow of the south tower of the Golden Gate Bridge

Fog shadow of Sutro Tower

Types

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

Fog over San Francisco Bay is a famous San Francisco fog

Minute particles of water constitute this after dark radiation fog in Oregon with the ambient temperature −2 °C (28 °F).

High speed photo of the above.

−25 °C (−13 °F) air temperature produces immense steam fog over Lake Ontario which is still 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 do not last long after sunrise. Radiation fog is common in autumn and early winter. Examples of this phenomenon include the Tule fog.^[10]

Ground fog is fog that obscures less than 60% of the sky and does not extend to the base of any overhead clouds.^[11] 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.^[12] It is common as a warm front passes over an area with significant snowpack. It is most common at sea when tropical air encounters cooler waters, 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.

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.^[13] It often causes freezing fog, or sometimes hoar frost.

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.

Upslope 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 the result of a temperature inversion caused by heavier cold air settling into 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, valley fog is often referred to as Tule fog.

Freezing fog occurs when liquid fog droplets freeze to surfaces, forming white soft or hard rime.^[13] 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".

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 (−30 °F), making it common only in and near the Arctic and Antarctic regions.^[14] 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 Barrow, Alaska. Ice fog often leads to the visual phenomenon of light pillars.

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 condenses and appears as fog.^[15]

Garua fog is a type of fog which happens to occur by the coast of Chile and Peru.^[16] 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 absorbs heat when melting and evaporating.^[17]

Ice fog or pogonip is rare, but can occur when temperatures are below -40 °C (-40 °F), when ice crystals freeze while suspended in the air, then stay.

Biological and human uses

Redwood forests in California receive approximately 30 to 40 percent of their moisture from coastal fog. Change in climate patterns could result in relative drought in these areas.^[18] Some coastal communities use fog nets to extract moisture from the atmosphere where groundwater pumping and rainwater collection are insufficient.

Specific locations

• Tule fog (California)
• Camanchacas (Chile)
• Pea soup fog (pre-Clean Air Act Britain)
• San Francisco fog

Fog permeates through the trees in Redwood National and State Parks	Golden Gate Bridge in fog	Empire State Building in a foggy summer afternoon	Crepuscular rays created by tree shadows over fog
The morning fog in the Rhine Valley between Lienz / Altstätten and Rüthi.	Ground fog in East Frisia (Moordorf)	Fog in the Botanical Garden of Curitiba, Southern Brazil	Fog at night
A foggy day (right) reduces visibility as compared to a sunny day (left)	The Space Shuttle Challenger moving through fog at the Kennedy Space Center, Florida.

See also

• Thank You, Fog: last poems by W. H. Auden

Technology

• Anti-fog
• Automotive lighting
• Decontamination foam
• FIDO
• Fog machine
• Fogging (photography)
• Foglamp
• Runway visual range
• Transmissometer

Weather

• Fog season
• Haboob (sandstorm)
• Mist
• Smog
• Whiteout (weather)

Notes

References

• Ahrens, C. (1991). Meteorology today: an introduction to weather, climate, and the environment. West Pub. Co. ISBN 978-0314809056.
• Allred, Lance (2009). Enchanted Rock: A Natural and Human History. University of Texas Press.
• Cowling, R. M., Richardson, D. M., Pierce, S. M. (2004). Vegetation of Southern Africa. Cambridge University Press.
• Filonczuk, Maria K., Cayan, Daniel R., Riddle, Laurence G. (1995). 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.
• Frost, H. (2004). Fog. Capstone Press. ISBN 978-0736820936.
• Gleissman, Stephe (2007). Agroecology: the ecology of sustainable food systems. CRC Press.
• Karukstis, K. K., Van Hecke, G. R. (2003). Chemistry connections: the basis of everyday phonemena. Academic Press.
• Marshall, T., Hoadley, D. (1995). Storm Talk. Tim Marshall.

External links

• Map of the United States showing days of heavy fog in each location
• United States' current dense fog advisories from NOAA
• Current Western US fog satellite pictures from NOAA
• Lake Ontario Evaporative-Steam Fog Video
• Social & Economic Costs of Fog from "NOAA Socioeconomics" website initiative