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High
pressure is good; sunny weather occurs when the pressure is high.
Skies are blue, winds are light and it never rains when the barometer
soars.
None
of the above is necessarily true, but as a general rule, our finest
weather happens under the influence of an area of high pressure
which is known, technically, as an anticyclone.
The air within an anticyclone is sinking
as it is drawn down from the higher part of the atmosphere,
the troposphere.  |
| Richard
Angwin |
The
air within an anticyclone is sinking as it is drawn down from the
higher part of the atmosphere, the troposphere.
As
this air descends it is warmed and it tends to dry out. This process
can extend through much of the atmosphere. The descent is generally
not conducive to cloud formation, but because the process does not
quite extend all the way down to the surface a layer of cloud can
be trapped very close to the surface.
Isobars
On
a weather chart an anticyclone shows itself as a series of circular
or elliptical shaped isobars.
A summer
anticyclone typically brings dry, mainly sunny weather with light
winds. Although it can be fairly cool by night, daytime temperatures
in the summer can be anywhere between 20 and 35 degrees Celsius.
The
actual temperature achieved depends very much on the wind direction.
If
the anticyclone lies over the North Sea, northeasterly winds can
produce rather cloudy weather, particularly along the east coast,
and temperatures may struggle to reach 20 degrees.
If
that same anticyclone is situated over continental Europe, southeasterly
winds can pull in very warm air from North Africa and our temperatures
can soar to well over 30 degrees.
Winter
weather
During
the winter months anticyclones are much less likely to influence
the weather over the West Country.
This
is because there are two semi-permanent areas of high pressure.
One is the Azores High which lies in the Atlantic ocean to the southwest
of the UK; the other is the Siberian High.
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Light winds, clear skies and long winter
nights can encourage air temperatures to fall very low.
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| Richard
Angwin |
Cold
air over the interior of the former USSR, intensifies during the
winter. This encourages the 'high' to build and it can occasionally
extend as far west as the UK, giving us an icy blast of cold easterly
winds. At the same time, the Azores high tends to migrates southwestwards
leaving the UK at the mercy of Atlantic frontal systems.
If,
during the winter months, the high pressure builds from the southwest,
our weather can be fairly mild as our temperatures are influenced
by the relatively warm waters around our coast. In such a situation
it can often be quite cloudy, leading to a type of weather referred
to as 'anticyclonic gloom'.
On
those occasions when the cloud is clear then the biggest problem
with anticyclones is fog and frost. Light winds, clear skies and
long winter nights can encourage air temperatures to fall very low.
If the air is also moist enough, fog patches also form across the
region, providing a double hazard for motorists.
Take
a reading
If
you tap your barometer during the development of high pressure you
will see that the indicator is pointing to 'dry' or 'very dry'.
Whilst this usually holds true, there are exceptions to the rule
and in both summer and winter, even a pressure in excess of 1030
millibars may not be enough to prevent heavy rain from falling from
an active weather front.
Anticyclones
are generally much slower moving than cyclones, or depressions as
they are more commonly known. This can result in long periods of
settled weather. In the summer this can be quite welcome if it results
in blue skies and high temperatures. But in the winter it can be
a different matter and days of grey and glum weather can become
rather depressing, as we are currently finding out.
by
Richard Angwin
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