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In an area of low pressure
the air has a tendency to rise. This general upward motion
means that there is less pressure from the air pushing down
on the earth, in other words there is low pressure. As air
rises, it cools and if there is enough water vapour it may
condense to form clouds and rain. This is why a low pressure
is generally associated with wet weather.
Low pressures often
form on the boundary of warm and cold air. A small disturbance
may cause the pressure to fall along this boundary, causing
a circulation of air to develop. This is the beginning of
a low pressure, and the boundaries of warm and cold air become
weather fronts. The UK is currently seeing a succession of
these weather systems. This is common in autumn when there
is a larger temperature difference between the cold arctic
air and the warmer air over the sea.
Other common types
of lows are 'thermal lows'. One thermal low, a heat low, recurs
over Spain and Portugal. During a summer day, the Iberian
Peninsula gets much hotter than the surrounding sea. This
in turn heats the air nearer the ground, which rises. This
rising air lowers the pressure and a low forms. As the air
rises, if there is enough moisture present, intense thunderstorms
can develop.
Polar lows are another
type of thermal low, but as the name suggests, this low forms
in the polar regions. When air from the arctic moves south
across the sea, it is heated from below and the rising air
gives a region of low pressure. Polar lows are heated throughout
the day and night, so can become very intense. They are quite
common in the UK, and often bring heavy snow to the north-facing
coasts. They can cause problems for weather forecasters, as
they are often rather small, so difficult to locate and track.
Old hurricanes from
the Caribbean are also types of low pressure. Normally hurricanes
die away after drifting north into cooler waters, but sometimes
an old depression can intensify again on the way to the UK.
This will bring us very wet windy weather, as ex-hurricane
Iris did in September 1995. Fortunately this only happens
every few years, but as we are seeing at the moment, it is
not only ex-hurricanes that bring us heavy rain and strong
winds.
Other features in
the Weather Basics series
Air Pressure
Clouds
Day and Night
Fog and Mist
Fronts
Frost
High Pressure
Humidity
Jet Streams
Lightning
Mirages
Northern Lights
Rain or Showers?
Thunder
Watercycle
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