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Fog is cloud in contact
with the ground. It occurs when moisture from the surface of the
Earth evaporates; as this evaporated moisture moves upward, it
cools and condenses into the familiar phenomenon of fog. Fog differs
from clouds in that fog touches the surface of the Earth, while
clouds do not. It 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.
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Steam fog is
the most localized form and is created by cold air passing over
much warmer water. Water vapor quickly enters the atmosphere by
evaporation and condensation occurs once the dewpoint has been
reached, thus creating a 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. Evening fog obscures London's Tower Bridge from passers
by.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 at the dewpoint
it condenses and fog forms.
Above Images Are From The NOAA
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