The sunniest parts of the United Kingdom are along the south coast of England. This is largely because the formation of convective (cumulus) cloud takes place over land and skies over the sea remain cloud-free. Many places along this south coast achieve annual average figures of around 1,750 hours of sunshine. The dullest parts of England are the mountainous areas, with annual average totals of less than 1,000 hours.
Mean daily sunshine figures reach a maximum in May or June, and are at their lowest in December. The key factor is, of course, the variation in the length of the day through the year, but wind and cloud play their part as well.
Facts and figures (bright sunshine)
- Maximum duration in a month: 383.9 hours at Eastbourne (East Sussex) in July 1911.
- Minimum duration in a month: 0.0 hours at Westminster (Greater London) in December 1890.
Rainfall in England varies widely. The Lake District is the wettest part, with average annual totals exceeding 2,000 mm (this is comparable with that in the western Highlands of Scotland). The Pennines and the moors of south-west England are almost as wet. However, all of East Anglia, much of the Midlands, eastern and north-eastern England, and parts of the south-east receive less than 700 mm a year.
Typically, it rains on about one day in three in England, perhaps somewhat more often in winter, though long, dry spells occur in most years.
Near the south coast there is an appreciable summer minimum and winter maximum of rainfall, with totals in July barely half those in January; western, northern and eastern coasts are more likely to see the driest month in spring and the wettest in late autumn. Inland, parts of the Midlands experience a summer rainfall maximum, which is a reflection of the higher frequency of thunderstorms in the more central and south-eastern parts of England. For example, at London and Birmingham, thunder occurs on an average of 15 days a year, but in the west and north-west the frequency declines to around eight days per year.
Facts and figures
- Maximum in a day (09-09 UTC): 279 mm at Martinstown (Dorset) on 18 July 1955.
There is a close relationship between surface isobars (lines joining points of equal air pressure) and wind speed and direction over open, level terrain. However, in mountain and moorland areas such as the Pennines, local topography also has a very significant effect with winds tending to be aligned along well-defined valleys. The most common direction from which the wind blows in England is the south-west, but in a climate which is extremely variable from day to day, winds from other directions are quite frequent and long spells of easterly or north-easterly winds are not unusual.
Over land, the roughness of the ground causes a decrease in the mean wind speed compared with that which occurs over the sea, with the size of the decrease depending on the nature of the terrain. In major towns and cities the overall mean speed is considerably reduced by the buildings, but local funnelling may occur and the wind can gust to about the same speed as in open country. It is this gustiness which causes much of the damage to buildings and trees on really windy days, though such days are comparatively rare.
A day of gale is defined as a day on which the mean wind speed at the standard measuring height of 10 m about ground attains a value of 34 knots (39 miles per hour, 17.2 metres per second) or more over any period of 10 minutes during the 24 hours. The strongest winds in the United Kingdom are associated with the passage of deep depressions across or close to the British Isles; these are most frequent during the winter, so that is when gales are most likely. These depressions are usually at their most intense over the open Atlantic Ocean; thus at low altitudes gales occur most frequently on the exposed western and northern coasts of both Britain and Ireland. For example, the Hebrides experience, on average, about 35 days of gale a year. In England, because of the protection afforded by Ireland, the most exposed coasts are those of Devon and Cornwall, and here there are about 15 days of gale a year. Inland, the number of days decreases to fewer than five days a year.
In general, wind speed increases with height, with the strongest winds being observed over the summits of hills and mountains. Wind data at high altitudes in England are very sparse, and it is not possible to quote data for the full 30-year period 1961-90; as an indication, however, Great Dun Fell in Cumbria (at 857 metres) averaged 114 days of gale a year during the period 1963 to 1976.
Facts and figures
- Highest gust recorded at a low-level site: 103 knots (118 m.p.h.) at Gwennap Head (Cornwall) on 15 December 1979.
Over England the mean annual temperature at low altitudes varies from about 8.5 °C to 11 °C, with the highest values occurring around or near to the coasts of Cornwall. The mean annual temperature decreases by approximately 0.5 °C for each 100 m increase in height so that, for example, Great Dun Fell in Cumbria (at 857 m) has an annual mean temperature of about 4 °C.
To a very large extent, winter temperature in the British Isles in influenced by the surface temperatures of the surrounding sea, which reach their lowest values in late February or early March. Around the coasts February is thus normally the coldest month, but inland there is little to choose between January and February as the coldest month.
The coldest nights are those on which there is little wind, skies are clear, and there is a covering of snow on the ground; the lowest temperatures occur away from the moderating influence of the sea, on the floors of inland valleys into which the cold air drains. It was under such conditions that the temperature fell to -26.1 °C, the lowest ever recorded in England, at Newport in Shropshire on 10 January 1982. Coastal areas do not experience such cold nights; as an example the lowest temperature ever recorded at Plymouth in Devon is -8.8 °C on 2 January 1979. At the opposite extreme, the highest winter temperatures are apt to occur in the lee of high ground. These high winter temperatures (up to 16 °C on rare occasions) occur when a moist south to south-westerly airflow warms up downwind after crossing mountains, an effect knows as the föhn after its more dramatic manifestations in the Alps.
July is normally the warmest month in England, and the highest temperatures of all have occurred in central districts furthest away from the cooling influence of the Atlantic. The highest temperature ever recorded in England is 38.5 °C at Brogdale, near Faversham in Kent on 10 August 2003, which is also the highest temperature ever recorded anywhere in the United Kingdom.
Facts and figures
- Air temperature (measured under standard conditions at 1.25 m above the ground).
- Highest recorded 38.5 °C at Brogdale, near Faversham in Kent on 10 August 2003.
- Lowest recorded -26.1 °C at Newport (Shropshire) on 10 January 1982.
Snow is comparatively rare near sea level in England, but much more frequent over hills. The average number of days each year when sleet or snow falls in England varies from about 10 or less in some south-western coastal areas to over 50 in the Pennines. Snow rarely lies on the ground at sea level before December or after March, and the average annual number of days with snow lying in England varies from five or less around the coasts to over 90 in parts of the Pennines. A day of snow lying is defined as one with snow covering at least half of the ground at 0900 UTC.
The number of days of snowfall and snow cover varies enormously from year to year. At many places in the last fifty years it has ranged from none at all in a number of winters to in excess of 30 days during the winters of 1946/47 and 1962/63. Even places near the coast experienced prolonged snow cover during these two winters. In heavy snowfalls there can be quite extensive drifting of the snow in strong winds, especially over the higher ground, resulting in severe dislocation of transport. Fortunately such occasions are comparatively rare.
Facts and figures
Many parts of England, especially those relatively remote from the industrial and populous areas of both Britain and mainland Europe, enjoy good visibility. This is particularly true of much of the coastline, the mountains and the moorlands.
Over high ground in England fog statistics are scarce, but because moist air often spreads across the country, hill fog can be both extensive and frequent and is a potential hazard to be borne in mind by walkers. Contrast the average 233 days of fog a year at 0900 UTC between 1963 and 1976 at Great Dun Fell in Cumbria (at 857 metres), with the 1961-90 figure of six days for Carlisle (at 26 metres) in the same county.