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 Fog
and mist are both made of tiny water droplets suspended in
air. The difference between them is the density. Fog is denser
so contains more water droplets than mist. For pilots, if
you cannot see further than 1000 meters, then it is called
fog. For most people including motorists though, visibility
has to be less than 200 metres before it is classed as fog.
Fog forms in many different
ways, but generally the air near the ground can hold its moisture
for longer, so water droplets condense to form fog. The amount
of moisture that air can hold depends, amongst other things,
on temperature. The cooler the air is, the less moisture it
can hold, so fog can form. Fog usually clears soon after dawn
as the sun heats up the air again, allowing it to hold more
moisture.
At night, the ground
loses or 'radiates' heat. In turn the air near the ground
cools too and cannot hold as much moisture. Any excess moisture
condenses into 'radiation' fog, the UK's most common fog.
The longer the night and the clearer the skies, the more the
temperature can drop, so the denser the fog can become. However,
a wind can prevent fog from forming, because it 'mixes' the
cool air by the surface with the warmer air above, preventing
the air from getting cool enough for its water to condense.
Another way fog forms
is for air to travel over a colder area of land, or sea. This
will cool the air and its moisture will condense into fog.
This 'Advection' fog often forms by the coast, especially
in spring and summer when there is a greater difference between
the temperatures of the sea and land.
'Steam fog' is sometimes
seen rising from the ground after a shower. If the ground
is warm, the water from the shower may evaporate. If the air
above it is saturated and cannot hold any more water, the
excess moisture condenses and looks like steam. This 'steaming'
can also be seen at the Poles, when cold air sweeps over a
slightly warmer sea. The sea warms the air a little and some
water evaporates. The warmer damper air then rises, cools
again and the moisture condenses. The sea seems to be steaming
hot. Be warned though, it may look like a sauna, but it will
feel more like an ice bucket!
Other features in
the Weather Basics series
Air Pressure
Clouds
Day and Night
Fronts
Frost
High Pressure
Humidity
Jet Streams
Lightning
Low
Pressure
Mirages
Northern Lights
Rain or Showers?
Thunder
Watercycle
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