South-west England is one of the more exposed areas of the UK, with wind speeds on average 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. The graph shows a typical variation of the monthly mean speed and highest gust.
The variation in monthly mean speeds (average of a continuous record) and highest gusts ('instantaneous' speed averaged over about 3 seconds) at St. Mawgan is shown below.
The graph shows that 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 both December and January have both had gusts to over 80 knots.
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 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 Scillies produced gusts of 91 knots at Lizard, 99 knots at St. Mary's (Scilly) and 103 knots at Gwennap Head. The gust at Gwennap Head is the highest recorded at a low-level site in England. The notorious storm on 15 October 1987 was not very severe from Cornwall to Avon, but the Channel Islands reported 85 knots that day. The Burn's 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.
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 Scillies the average is around 24 days per year, with a similar figure for exposed places in coastal Cornwall. The frequency is rather less to the north east especially inland. Plymouth (on the coast) has about 16 days per year, but Yeovilton in Somerset has 7 and Long Ashton near Bristol only 4. 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 illustrate the typical frequency of speeds and directions during the year and confirm the predominant SW direction. At Avonmouth, 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 are usually below 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.
