Southern England is one of the more sheltered parts of the UK, the windiest areas being in western and northern Britain, closer to the Atlantic. The strongest winds are associated with the passage of deep areas of low pressure close to or across the UK. The frequency and strength of these depressions is greatest in the winter half of the year, especially from December to February, and this is when mean speeds and gusts (short duration peak values) are strongest. The graph shows a typical variation of the monthly mean speeds and highest gusts.
The variation in monthly mean speeds (average of a continuous record) and highest gusts ('instantaneous' speed averaged over about 3 seconds) at Heathrow is shown below.
Another measure of wind exposure is the number of days when gale force is 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. Over most inland areas of the region the average is around 1-2 days per year but exposed places along the South Coast experience about 10 gales in an average year. Wind speed is sensitive to local topographic effects and land use. Places sheltered by hills or in extensive urban areas will have lower mean wind speeds and fewer days of gale, but can have strong gusts.
There have been several noteworthy gales affecting Southern England, accompanied by property damage and disruption to travel and power supplies. The most famous was the 'Great Storm' of 15-16 October 1987, considered to be the most severe to affect this region since that of November 1703. It brought a swathe of destruction across an area to the south and east of a line roughly from Southampton to London, with considerable damage to buildings, an estimated 15 million trees uprooted, disruption to power supplies and transport and 18 storm-related deaths. The highest speed recorded was a gust of 100 knots at Shoreham-by-Sea (West Sussex), where the hourly mean speed reached 72 knots. The scenes of widespread building and tree damage, transport and power disruption were repeated during the 'Burns Day Storm' of 25 January 1990 when gusts of 70-80 knots were recorded widely with 85 knots at Herstmonceux (East Sussex). Unlike the 1987 event, this storm struck during the day and consequently the death toll was higher (47 in the whole UK). The gale of 27 October 2002 again saw gusts of around 60-70 knots across the region.
The direction of the wind is defined as the direction from which the wind is blowing. As Atlantic depressions pass 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. Spring time tends to have a maximum frequency of winds from the north-east. This seasonal effect is due to high pressure building over Scandinavia at this time of year.
Coastal areas experience sea breezes from late spring through the summer, caused by the temperature differential between the sea and the warmer land. These sea breezes will often reach London, originating from either the North Sea or, occasionally, the English Channel.
The annual wind rose for Heathrow is typical of open, level locations across the region, with a prevailing south-westerly wind direction through the year.
A tornado is a violently-rotating column of air, caused by the rapid displacement of warm moist air by cold dense air often associated with the occurrence of active cold fronts. It will typically last for a few minutes, track across the land for 2 to 5 km and have a diameter of 20 to 100 metres. On average, 33 tornados are reported each year in the UK although the number can vary significantly from year to year. The UK has the highest frequency of reported tornadoes per unit area in the world, although they are nowhere near as intense as those reported in the USA.
An example of a tornado that affected southern England is the one that struck Kensal Rise in west London on 7 December 2006. Up to 150 homes were damaged and 6 people were injured in violent winds that lasted less than 1 minute. This was the first tornado to cause significant damage in London since one at Gunnersbury, west London, in December 1954. A further example is the tornado that struck Selsey (West Sussex) on 7 January 1998 causing an estimated £10M of damage to 1000 buildings. This part of the south coast seems somewhat prone to tornadoes, and Selsey also suffered damage in November 1986 and October 2000.
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