|
An area of high pressure
is usually associated with hot clear summer days, but it can
bring fog, frost and even cloud.
High pressure areas
are generally larger and move slower than low pressure. The
winds circulate around the centre in a clockwise, 'anticyclonic'
movement. However, the winds are generally weaker than those
around a low pressure, especially in the centre.
In a high pressure,
the air is generally slowly sinking, or 'subsiding'. As air
falls it warms, preventing clouds from forming. This is why
highs are generally clear. However, sometimes the ground may
be warm enough to cause some air to rise, and this can form
a layer of cloud.
High pressure does
not necessarily mean warm weather, a 'cold anticyclone' has
cold air near the ground. These cold anticyclones are common
in Siberia and Canada, where the cold air is cooling further
and subsiding. Another cold anticyclone is an area of high
pressure sandwiched between two areas of low pressure. This
is quite common during the British winters, giving us fog
and frost.
'Warm anticyclones'
include the Azores high. The Azores have a warm climate, which
normally indicates rising air, but at this latitude, the earth's
air has a tendency to sink and so a high pressure is formed.
This high pressure can extend all the way to the UK, giving
us a period of hot sunny weather.
Sometimes a high pressure
will 'get stuck', and can stay in the same position for over
a week. This is called a 'blocking high' and forces other
weather systems to go round it. These blocks often reoccur
in the same place, including just west of the UK and over
Scandinavia. This would give us some more unusual weather,
either dragging air from further north or further south than
usual.
A common phrase used
by weather forecasters is a 'ridge of high pressure'. This
often indicates settled weather. A ridge is an area of high
pressure that does not have a closed circulation, it either
extends from a high pressure or is sandwiched between a couple
of lows.
Although in the UK
we have been taught to love high pressure, in winter it can
bring frost and fog. In the summer it can cause problems with
smog, as the pollution gets 'caught' in the sinking air. However,
there is a chance that a high pressure will bring the sun
and in the UK we try to be optimistic!
Other features in
the Weather Basics series
Air Pressure
Clouds
Day and Night
Fog and Mist
Fronts
Frost
Humidity
Jet Streams
Lightning
Low
Pressure
Mirages
Northern Lights
Rain or Showers?
Thunder
Watercycle
|
