The
answer is blowing in the wind |
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| Richard
taking it easy - his desk isn't normally that tidy! |
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February
was one of the warmest months for 30 years.
Points West weatherman Richard Angwin explains how the wind direction
can influence Wiltshire's weather. |
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I was
told many years ago by a senior forecaster in the Met Office that,
“If you can forecast the wind, the rest of the forecast will take
care of itself”.
This may seem like a sweeping statement but his forecasts were rarely
wrong.
And it is the wind, and more particularly, the wind direction which
holds the key to Wiltshire's weather.
This winter has been exceptionally mild. February was one of the mildest
in the last 30 years.
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| Winds
from the southwest originate over warm subtropical waters. |
The main
reason for this was the predominance of winds blowing from a southwesterly
direction. West to southwesterly winds originate over much warmer,
subtropical waters.
They do cool as they head towards us but a southwesterly is usually
a guarantee of mild weather.
Unfortunately those same southwesterlies are usually laden with moisture.
As they travel towards us over gradually cooling waters, the air cannot
hold as much moisture. That is why southwesterly winds are cloudy
and wet as well as mild.
When the wind blows from the south or southeast we tend to keep the
warmth but lose the moisture. In fact our warmest weather usually
occurs with winds from this direction.
This is hardly surprising - just think where these winds originated.
They blow over the Sahara Desert and then over the Mediterranean before
reaching us.
This wind direction is responsible for some of our biggest thunderstorm
outbreaks, as that warm African air picks up lots of moisture over
the Bay of Biscay before unleashing it over southern counties of the
UK.
Very occasionally, showers develop in these airstreams. But they underlying
air is warm and relatively dry.
Consequently only the largest raindrops can reach the ground before
evaporating. This has the unusual end product of very big individual
splashes of rain.
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| Dust
from the Sahara can result in a red coating on cars |
Rarely, those same shower clouds have picked up dust over the Sahara
and this can result in a covering of red dust once the rain has dried
up. Easterly winds are winds of extremes.
During the summer months winds blow across the dry, warm interior
of continental Europe. These winds can give us a good deal of fine,
warm weather.
In the winter however, those same winds have started life over the
permafrost of Siberia. They can give us very cold weather.
If they also manage to pick up moisture on their journey over the
North Sea then snow is the likely outcome. Winds blowing from the
northeast have also often started their journey from inside the Arctic
Circle.
But their journey over the cold North Sea is often longer. They therefore
pick up a lot of moisture and deposit it has sleet or snow.
These same northeasterly winds can spoil an otherwise fine day in
spring and autumn as low cloud spreads from the east coast, reaching
all the way to Wiltshire.
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| When
the north wind doth blow, we shall have snow.... |
“When the north wind doth blow, we shall have snow”, goes the old
saying. As a general rule it is a good one. But northerly winds often
bring us fine weather.
They have such a long land journey from the north of Scotland that
all their snow is dropped long before they reach us.
Northwesterly winds are usually associated with cool, showery weather.
They start life over cold seas between Newfoundland and Iceland but
as they travel over warmer waters big shower clouds usually develop.
So we have now come full circle around the points of the compass.
Each wind direction has its own characteristic type of weather and
each one affects the county in different ways.
Hopefully you can now see how.
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