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Fog 1
Fog is formed by the cooling of the air to ‘dew point’ and the resulting condensation of water
vapour in the atmosphere at ground level. Anticyclonic conditions with few clouds can produce
radiation and advection fog.
Radiation fog results when a body of moist air, in contact with the ground surface is cooled to its
dew point. This commonly occurs at night under cloudless anticyclonic conditions with only a light
breeze blowing. Because the sky is clear, the ground surface cools rapidly by radiation and in turn
cools the air immediately above it. Once the dew point has been reached, condensation occurs.
The cooled layer of air is stirred by the light winds so that it cools the air above it to its dew point,
and so the fog grows deeper. Radiation fogs occur under temperature inversions, which prevent
the air from rising. They may persist for several days in winter if the sun is too weak to disperse
them. They are more common in autumn/early winter because the air is more moist as more
vegetation around than later in the winter.
Advection fog is formed when warm moist air moves over a cooler sea surface. This is commonly
formed around the coast of the Isle of Man in early summer under anticyclonic conditions. The sea
is at its coolest at this time. The lack of clouds in the anticyclone heats up the air above the land.
When this air moves gently over the sea it is cooled below its dew point and condensation occurs.
The lack of strong winds in an anticyclone can mean that fog persists most of the day around the
coasts.
No clouds  heat lost from ground at night  air in contact with cold ground cooled below dew
point-condensation occurs  very gentle winds fail to disperse fog but can mix cold air at surface
with air above so that the fog becomes deeper  the low angle of the sun in the winter means
that the heat from the sun is too weak to evaporate the water droplets the following morning so
the fog persists.
The anticyclonic conditions can create a temperature inversion when the air is cooler at the
surface than above. This means that the air can not rise and is trapped- so the fog persists,