South-west England is one of the more exposed areas of
the UK, with average wind speeds only greater in western
Scotland. The strongest winds are associated with the passage
of deep depressions close to or across the British Isles.
The frequency and strength of depressions is greatest in
the winter half of the year and this is when mean speeds
and gusts are strongest. Figure 4 shows a typical
variation of the monthly mean speed and highest gust.
Figure 4. Monthly mean wind speed and
the highest monthly recorded gust for the period 1971-2000
at St Mawgan. The mean speed is the average of a continuous
record, but the gust is an 'instantaneous' speed averaged
over about three seconds.
In Figure 4 the months from November to March have
the highest mean speeds, with June to August having the
lightest winds. The peak gust follows a similar pattern
and in the past 30 years December and January have had gusts
to over 80 knots (note that 1 knot = 1.15 m.p.h., or 0.515
m/sec).
Other coastal areas are similar to St Mawgan, though mean
speeds generally decline towards to the north-east. Inland
areas have lower speeds which also decrease to the north-east.
At Yeovilton in lowland Somerset, for example, the mean
speeds are about two thirds of those at St Mawgan. Wind
speeds also tend to increase with increasing altitude. The
highest parts of Exmoor and Dartmoor are likely to have
speeds similar to those on the coast.
There have been several noteworthy strong-wind events in the
south-west. On 15 December 1979 severe gales in Cornwall and
the Isles of Scilly produced gusts of 91, 99 and 103 knots at
Lizard, St. Mary's (Isles of Scilly) and Gwennap Head respectively.
The Burns' Day storm (25 January 1990) caused considerable damage
to buildings and overturned vehicles as high winds were widespread.
Some stations had gusts which were the highest in the period
from 1971 to 2000 and included 84 and 85 knots at Plymouth and
St Mawgan respectively. Other reports were of 79 knots at Bristol
(on top of a building), 68 knots at Yeovilton and 74 knots at
Exeter Airport. The maximum hourly mean speed was 60 knots at
Plymouth and 45 knots at Yeovilton.
Figure 5. Wind rose for the period 1991-2000 at
Avonmouth, near Bristol (9 metres a.m.s.l.)
Figure 6. Wind rose for the period
1991-2000 at Plymouth (50 metres a.m.s.l.)
Another measure of wind exposure is the number of days when
gale force was reached. If the wind reaches a mean speed
of 34 knots or more over any ten consecutive minutes then
that day is classed as having a gale. Exposed headlands
and islands have the greatest frequency of days with gale.
In the Isles of Scilly the average is around 24 days per
year, with a similar figure for exposed places in coastal
Cornwall. It decreases to the north-east, especially inland.
Plymouth (on the coast) has about 16 days per year, but
Yeovilton in Somerset has seven, and Long Ashton in Avon
only four. Wind speed is sensitive to local topographic
effects and land use. Places sheltered by hills, or which
are in extensively wooded or urban areas will have lower
wind speeds and fewer days of gale.
The direction of the wind is defined as the direction from
which the wind is blowing. As Atlantic depressions pass
by the UK, the wind typically starts to blow from the south
or south-west, but later comes from the west or north-west
as the depression moves away. The range of directions between
south and north-west accounts for the majority of occasions
and the strongest winds nearly always blow from this range
of directions. The wind roses in Figures 5 and 6
illustrate the typical frequency of speeds and directions
during the year and confirm the predominant SW direction.
At Avonmouth (Figure 5), the relatively high frequency
of NE winds (and lack of SE and NW winds) reflects its situation
on the Bristol Channel, aligned NE-SW. East or north-east
winds can also be strong if depressions pass along the English
Channel. Spring time also tends to have a maximum of winds
from the north-east, due to the build up of high pressure
over Scandinavia at this time of year. Periods of very light
or calm winds with no preferred direction occur usually
less than 6% of the time in the islands and coastal districts.
This increases to about 15% of the total time in inland
areas to the north-east and can be compared with a typical
inland station in central England that would have light
winds for around 25% of the time.
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