Climate of the United Kingdom

The current climate of United Kingdom is classified as temperate, with warm summers, cool winters and plentiful precipitation throughout the year. The principle factors of influence on the climate include the UK's northerly latitude (which ranges from 50° to 60° N), its close proximity to the Atlantic Ocean and, especially, the warming of the waters around the British Isles by the Gulf Stream. The weather can be notoriously changeable from one day to the next but temperature variations throughout the year are small.

The climate of the United Kingdom is significantly influenced by the maritime tropical, maritime polar, continental polar and continental tropical air masses.

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A deep depression near Iceland on 27 October 2000

The UK is at the boundary of convergence between the warm tropical air to the South and the cold polar air to the north. In this area, the large temperature variation creates instability and this a major factor that influences the notoriously changeable and often unsettled weather the UK experiences, where many or all types of weather can be experienced in a single day

Contents

Seasons

Winter in the UK is generally a cool, wet and windy season. Temperatures at night rarely drop below -10°C and in the day rarely rise above 15°C. Precipitation is plentiful throughout the season with occasional snow.

Towards the later part of the season the weather usually stabilises with less wind, precipitation and lower temperatures. This change is particularly pronounced near the coasts mainly due to the fact that the Atlantic is often at its coldest during this time after being cooled throughout the autumn and the winter. The early part of winter however is often unsettled and stormy; often the wettest and windiest time of the year.

Snow can fall occasionally and mainly affects northern and easterly areas and chiefly higher ground, especially the mountains of Scotland where the amount of lying snow may be significant enough on occasions to permit skiing. Snow however rarely lasts more than a week in most areas as the cold air, usually caused by a high pressure system, gives way to North Atlantic depressions. Low pressure systems move in from the Atlantic ocean frequently throughout the season, often bringing strong winds and heavy rain along with mild temperatures.

During periods of light winds and high pressure frost and fog can become a problem and can pose a major hazard for drivers on the roads.

Spring is generally a rather calm, cold and dry season, principally since the Atlantic has lost much of its heat throughout the autumn and winter. However, as the sun rises higher in the sky and the days get longer, temperatures can rise relatively high and thunderstorms / heavy showers can develop.

There is a fair chance of snow earlier in the season when it is cooler.

Summer climatic differences are more influenced by latitude and temperatures are highest in southern and central areas and lowest in the north. Generally, however, summer temperatures rarely go much above 30°C, although temperatures have soared as high as 38°C.

The north west and north east has cool summers, the south west has rather warmer summers (average 17°C) and the south and south east have the warmest summers.

Summer is a rather dry season on average but rainfall totals can have a wide local variation due to localised thunderstorms. These thunderstorms mainly occur in southern, eastern and central areas and are less frequent and severe in the north and west.

North Atlantic depressions are not as frequent or severe in summer but increase both in severity and frequency towards the end of the season.

Autumn in the UK is notorious for being extremely unsettled. As cool polar air moves southwards following the sun in the sky, it meets the warm air of the tropics and produces an area of great disturbance along which the United Kingdom lies. This combined with the warm ocean, which due to heating throughout the spring and summer, produces the unsettled weather of autumn. In addition, when the air is particularly cold it may actually be colder than the ocean and this can result in significant amounts of evaporation, producing clouds which eventually condense and bring rain to the UK.

Atlantic depressions during this time can become intense and sustained winds of hurricane force (greater than 74 MPH) have been reported. One such intense depression was the great “hurricane” of 1987. (see below)

Western areas, being closest to the Atlantic, experience these severe conditions to a significantly greater extent than eastern areas.

As such, autumn, particularly the later part, is often the stormiest time of the year.

Regions

Regional climatic differences in the United Kingdom are dominated by the Atlantic Ocean. The western side of the United Kingdom is, being closest to the Atlantic, generally the mildest, wettest and windiest region of the UK. As its temperatures are most under the moderating effect of the Atlantic, temperature variations here are seldom extreme.

Eastern areas are by contrast drier, cooler, less windy and also experience the greatest daily and seasonal temperature variations.

The various regions of the UK are under the influence of the various air masses to varying degrees:-

• The north east is most under the influence of the continental polar air mass, which brings cold dry air.

• The south east is most under the influence of the continental tropical air air mass, which brings warm, dry air.

• The south west is most under the influence of the maritime tropical air mass, which brings warm moist air.

• The north west is most under the influence of the maritime polar air air mass, which brings cool moist air.

The proximity of a place to these air masses greatly determines the climate of it.

Sunshine and cloud

The average annual amount of sunshine for the United Kingdom is low and most days are either completely or partly cloudy. Southern coasts, however, often have the clearest skies because cumulus cloud formation generally takes place over land, so coastal areas are often cloud free. These south coast areas have annual average totals of 1,750 hours of sunshine a year. Western and mountainous areas are generally the cloudiest areas of the UK, with some mountainous areas receiving less than 1,000 hours of sunshine a year.

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A fine summer's day

Valley areas such as the Welsh valleys, due to their north-south orientation, often receive less sunshine than flat areas because the hills/mountains on either side of the valley obscure the sun in the early morning and late evening.

On occasions blocking anticyclones (high pressure systems) may move over or near the UK and may persist for weeks or even months. The cool dry air often results in clear skies and few clouds, bringing frosty nights in winter and hot days in the summer.

The mountains of the UK can be especially cloudy with extensive mist and fog. Near the coast, sea fog may move in during the nights and fog in other areas of the UK can persist for hours or even days in the winter and can pose a major hazard for drivers and aircraft.

Average annual daily hours of sunshine range from between one and two hours in midwinter to between five and seven in midsummer.

Most sunshine in one month: 383.9 hours at Eastbourne (East Sussex) in July 1911.

The Atlantic Ocean

One of the most important influences on the British climate is the Atlantic Ocean and especially the North Atlantic current which brings warm waters from the Gulf of Mexico to the United Kingdom by means of the global conveyor.

This has a powerful moderating and warming effect on the UK climate. The North Atlantic Drift warms the climate of the UK to such a great extent that temperatures in winter would be about 10°C lower than they are today if it didn't exist and means that England can have vineyards where Canada has polar bears.

These warm ocean currents bring substantial amounts of humidity with them and contributes strongly to the notoriously wet climate the UK experiences.

Depressions are another major product of the Atlantic Ocean and roll in towards the UK throughout the year and are especially intense and frequent in autumn and winter. These depressions can be very severe and often bring in heavy rain and strong winds.

Winds

A high temperate latitude and close proximity to a large ocean on its westerly side means that the United Kingdom is a windy place.

The prevailing wind in the United Kingdom is from the south west but in such a changeable climate it may blow from any direction for sustained periods of time. Winds are strongest near westerly facing coasts and inland areas where there is little topography, such as mountains, to divert the wind.

Gales (which are defined as winds with speeds of 32 to 63 miles per hour) are strongly associated with the passage of deep depressions across or close to the United Kingdom, and both are most frequent in the winter. The Hebrides experience on average 35 days of gale a year (a day where there are gale force winds) while more inland areas in England receive less than 5 days of gale a year.

Areas of high elevation tend to have higher wind speeds than low elevations and Great Dun Fell in Cumbria (at 857 meters) averaged 114 days of gale a year during the period 1963 to 1976.

Highest gust recored at a low level: 103 knots (118 MPH.) at Gwennap Head (Cornwall) on 15 December 1979.

Rainfall

Parts of the United Kingdom are surprisingly dry - London receives less rain annually than Rome, Sydney or New York. In England it typically rains on about 1 day in 3 and even more in winter. The wettest seasons are the winter and autumn.

Rainfall amounts can vary greatly across the United Kingdom and generally the further west and the higher the elevation, the greater the rainfall. The Lake District is one of the wettest places in the UK with an average annual rainfall total that exceeds 2000mm. The mountains of Wales, Scotland, the pennines and the moors of the south west are also particularly wet. In contrast, the south east, east, north east and the midlands receive less than 700mm of rain per year.

The county of Essex is one of the driest in the British Isles, with an average annual rainfall of around 600mm (24 inches), although it typically rains on over 100 days per year. In some years rainfall in Essex can be below 450mm (18 inches) -- less than the average annual rainfall in Jerusalem and Beirut.

The main reasons for high number of rainy days in the UK are it's mid latitude position, its close proximity to the Atlantic ocean and the warm waters the North Atlantic Drift brings.

Most rainfall in the UK comes from North Atlantic depressions which roll into the UK throughout the year and are particularly frequent and intense in the autumn and winter. They can on occasions bring prolonged periods of heavy rain and flooding is not rare.

Precipitation over the mountains is especially high and are some of the wettest places in Europe with an average annual rainfall exceeding 60 inches.

Eastern areas, away from the ocean, are considerably drier than western areas.

Temperature

Generally the UK has mild to cool winters and warm summers with little variation in temperature throughout the year. In England the average annual temperature varies from 8.5°C to 11°C, but over the higher ground this can be several degrees lower. This small annual variation in temperature is to a large extent due to the moderating effect the Atlantic ocean has since water has a much greater heat capacity than air and tends to release it slowly throughout the year. This has a warming influence on coastal areas in winter and a cooling influence in summer.

The ocean is at its coldest in February or early March, thus around coastal areas February is often the coldest month, but inland there is little to choose between February and January as the coldest.

Temperatures tend to drop lowest in late winter nights in the presence of high pressure, clear skies, light winds and when there is snow on the ground. On occasions, cold polar or continental air can be drawn in over the United Kingdom to bring very cold weather.

The floors of inland valleys away from warming influence of the sea can be particularly cold as cold air, being denser than warm air, tends to drain into them. A temperature of -26.1C was recorded under such conditions at Edgmond in Shropshire on 10 January 1982, the coldest temperature recorded in England and Wales. The following day the coldest maximum temperature in England, at -11.3C, was recorded at the same site.

The warmest winter temperatures tend to occur on the lee of high ground and are produced when a moist south or south west wind warms up downwind after the crossing the mountains. Temperatures in these areas can rise as high as 16°C in winter on rare occasions.

July tends to be the warmest month and the highest temperatures tend to occur away from the Atlantic in south eastern and central areas where summer temperatures can soar as high as 33C. It soared to 38.5C in Kent in the summer of 2003, the highest temperature ever recored in the United Kingdom.

Severe weather

While the UK isn't particularly noted for extreme weather, it does occur, and conditions have been known reach extreme levels on occasions. In the winter of 1982 for example, for a few days parts of central and southern England experienced temperatures lower than central Europe and Moscow. In contrast, the summers of 1975 and 1976 experienced temperatures as high as 37C. It was so dry the country suffered drought and water shortages.

There have also been occurrences of severe flash floods caused by intense rainfall, the worst of which was the Lynmouth disaster of 1952 in which 34 people died and some 38 houses and buildings were completely destroyed. Recently in the summer of 2004, a severe flash flood devastated the town of Boscastle in Cornwall.

Extended periods of extreme weather, such as the drought of 1975-1976 and the very cold winter of 1982-1983, are often caused by blocking anti-cyclones which can persist several days or even weeks. In winter they can bring long periods of cold dry weather and in summer long periods of hot dry weather.

Thunderstorms are most common in the south and eastern inland areas, and least common in the north and west. As a result of this, south and eastern inland areas tend to have their wettest months in the summer while western, northern and eastern costs are most likely to have their driest month in the spring and their wettest in late autumn. At London and Birmingham for example, thunderstorms occur on average about 15 days a year, while in the north in west it is more like 8 days a year.

The most rain to fall on a single day was 279 mm at Martinstown (Dorset) on 18 July 1955.

Climate history

The climate of the United Kingdom has not always been the way it is today, and in some periods it was much warmer, and in others it was much colder. One of the greatest climatic events the UK has experienced was the Ice Age. This was a period of extreme cold weather that lasted for tens of thousands of years and ended about 10,000 years ago. During this period the temperature was so extremely low that the much of the surrounding ocean froze and a great ice sheet extended over all but the very southern edge of the UK.

10,000 years ago the UK began warming, resulting in the melting of the ice sheets bringing the interglacial period that were are in today. There have been many glacial and interglacial periods in the geological history of the United Kingdom.

Climate change

Some evidence exists that suggests that the climate of the United Kingdom is undergoing a process of warming as a result of global climate change.

Current research has concluded that the north of the UK is warming faster than the south and this process is expected to continue in the future unless global emissions of greenhouse gases, the prime culprit for global warming, are reduced. Average annual temperatures are expected to rise to by about 2 and 3.5°C by the 2080's and the autumn and winter will see the greatest warming, with summer rises of temperature in south England and south Wales expected to be the highest.

Rises in temperature in the winter of the northwest of Scotland are predicted to be between 1°C and 2°C. The south east of the United Kingdom will experience some of the most dramatic changes in temperature with an increase of 4 or 5°C expected by the 2080's.

Precipitation is expected to increase in all areas of the United Kingdom with increases of 15% to 35%, depending on the area of the country. Summers will become drier and the country as a whole may be anything from 35% to 50% drier by the 2080's with the largest changes expected in the south east of england and the smallest in the north west of Scotland.

Snow will become up to 60% more rare in parts of Scotland and up to 90% more rare in the rest of the UK.

It is expected that due to this warming severe weather events in the UK will become more frequent and severe and rising sea levels from melting ice caps could flood parts of low lying areas of the UK such as London.

The summer of 2003 was the hottest on record, with a temperature of 38.5C being recorded in Faversham, Kent - the highest ever measured in the UK. Some point to this as evidence of global warming.

Some believe there is a small possibility that the UK will be plunged into a new ice age in the near future due to a decrease in salinity in the North Atlantic ocean caused by the melting ice caps of Greenland. This diluting of the salt water could mean that North Atlantic Drift will no longer be able to sink to complete the cycle of the global conveyor, thus shutting off the warm water supply of the North Atlantic Drift to the UK which would result in much colder conditions.

External links

Met Office (http://www.meto.gov.uk/)
BBC weather center (http://www.bbc.co.uk/weather/)
British weather events (http://www.personal.dundee.ac.uk/~taharley/britweather.htm)

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