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Fog
Fog is cloud in contact with the ground. Fog reduces
visibility. Some vehicles have radar etc., cars have to drive slower and use
more lights. Especially dangerous is when fog is very localized, and the
driver is caught by surprise.
Fog - Golden Gate Bridge at Dawn
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All types of fog form when the relative humidity reaches 100%, and the air
temperature tries to drop below the dew point, pushing it lower by forcing the
water vapour to condense. 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 thermal (infrared) radiation in calm conditions with clear sky.
The cool ground then produces condensation in the nearby air by
conduction. In perfect calm the fog layer can be less than a metre deep
but turbulence can promote a thicker layer. Radiation fog is common in
autumn and usually does not last long past sunrise.
- Advection fog occurs when moist air passes over cool
ground by advection (wind) and is cooled. This form is most common at sea
when tropical air encounters cooler higher-latitude waters. It is also
extremely common as a warm front passes over an area with significant
snowpack.
- Steam fog is the most localized form and is created by
cold air passing over much warmer water. The air is quickly saturated by
evaporation and the condensation thus created is seen as wispy steam.
Steam fog is most common in polar regions, and around deeper and larger
lakes in late autumn and early winter. It is closely related to
lake-effect snow and lake-effect rain, and 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 vapour. The water vapour cools and increases
the moisture content of the air. As the air saturates below the cloud, fog
forms.
- Upslope fog forms when winds blow air up a slope
(called orographic lift), 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 valleys, often during
winter. It is the result of a temperature inversion caused by heavier cold
air settling into the valley, with warmer air passing over the mountains
above. It is essentially radiation fog confined by local topology, and can
last for several days in calm conditions.
- Ice fog is any kind of fog where the droplets have
frozen into extremely tiny crystals of ice in midair. Generally this
requires temperatures well below the freezing point, making it common only
in and near the Arctic and Antarctic regions. Extremely small amounts of
this falling from the sky form a type of precipitation called ice
crystals, often reported in Barrow, Alaska.
- Freezing fog is when liquid fog droplets freeze to
surfaces, forming white rime ice. This is very common on mountaintops
which are exposed to low clouds. It is equivalent to freezing rain, and
essentially the same as the ice which forms inside a freezer which is not
of the "frostless" or "frost-free" type. It
contains a high proportion of trapped air, making it appear white rather than
transparent, and giving it a density about one quarter of that of pure ice.
This article is licensed under the GNU Free Documentation License. It uses material from the
Wikipedia article "Fog"
and from
http://www.white-on.com
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