Climate of the United Kingdom

The climate of all of the United Kingdom is classified is a mid-latitude oceanic climate (Koppen climate classification Cfb), 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 relatively small.

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

File:Low pressure near Iceland, 2000.jpg

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.

Seasons

Winter

Winter in the UK is generally a cool, wet and windy season. Temperatures at night rarely drop below -10 °C (14 °F) and in the day rarely rise above 15 °C (59 °F). Precipitation is plentiful throughout the season, though snow is relatively infrequent despite the nations' high latitude.

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 falls intermittently and mainly affects northern and eastern 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 at one of the five Scottish ski resorts. Snow however rarely lasts more than a week in most areas as the cold air brought by northerly or easterly winds, or sustained under high pressure system, gives way to mild southerly or westerly winds (normally introduced by 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. However, on rare occasions some potent depressions may move in from the north in the form of 'Polar Lows', introducing heavy snow and often blizzard-like conditions to parts of the UK, particularly Scotland.

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

Spring is generally a calm, cool 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 and heavy showers can develop.

There is a fair chance of snow earlier in the season when it is colder. Some of the country's heaviest snowfalls of recent years have happened in the first half of March and snow showers can occur infrequently until mid-April, even in southern areas.

Summer

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 35 °C (95 °F); but it is not unusual to record temperatures of over 32°C during a particular summer. The record maximum is 38.5 °C (101 °F) recorded in Kent, in August 2003 - due to its proximity to the European land mass, the South East usually experiences the highest summer temperatures in the UK.

The north west and north east have cooler summers (average 14 - 15 °C (58 °F)), the south west has rather warmer summers (average 16 °C (61 °F)) and the south and south east have the warmest summers (average 17 - 18 °C (63 - 64 °F)).

Summer can often be a dry season 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. Summer often sees high pressure systems from the Azores dominate.

Autumn

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 condensation, 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 latter part, is often the stormiest time of the year.

However, the UK often experiences an 'Indian Summer', where temperatures particularly by night can be very mild and rarely fall below 10 °C (50 °F). Such events are aided by the surrounding Atlantic Ocean and seas being at their warmest, keeping the UK in warm air, despite the relatively weak Sun. Examples of this were in 2005 and 2006, where September and, especially, October saw above average temperatures which felt more like a continuation of summer, than autumn. Autumn, especially September, has been very mild or warm in recent years with notable extremes of precipitation as the UK has seen some of its wettest and driest autumns since the turn of the millennium.

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 northeast is most under the influence of the continental polar air mass, which brings cold dry air.

The south and southeast is most under the influence of the continental tropical 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 mass, which brings cool moist air.

The proximity of a place to these air masses greatly determines itsclimate.

Sunshine and cloud

The average annual amount of sunshine for the United Kingdom is relatively low and around 2/3 of days are partly cloudy, occasionally with little sun at all. Sunshine is usually recorded between 30% and 55% of the maximum possible annually. Southern coasts, however, often have the clearest skies because cumulus cloud formation generally takes place over land, so coastal areas are often cloud free. The south coast counties of Dorset, Hampshire, Sussex and Kent have annual average totals of between 1,750 and 2100 hours of sunshine a year. North-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.

Valley areas such as the South Wales 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. This is particularly noticeable in winter as there are only a few hours of sunshine, with the sun low in the sky, even on a clear day.

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, with some coastal areas achieving near maximum possible sunshine for periods of weeks.

The mountains of the UK can be especially cloudy with extensive mist and fog. Near the coast, sea fog may move in during winter 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 daily hours of sunshine in midwinter range from 1 hour or less in some mountainous areas up to 2 to 5 hours on the South Coast midwinter, whilst average daily hours of sunshine in midsummer range from 4 to 8 in high western areas to between 6 and 10 hours a day on the South Coast.

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 at the same latitude where Canada has polar bears.

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

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 and Wales 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 recorded at a low level: 103 knots (118 mph, 191 km/h) at Gwennap Head (Cornwall) on 15 December 1979.

Rainfall

Parts of the United Kingdom are surprisingly dry (contrary to stereotype) - London receives less rain annually than Rome, Sydney or New York. In Eastern England it typically rains on about 1 day in 4 and slightly more in winter.

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 2000 mm. The mountains of Wales, Scotland, the Pennines and the moors of the south west are also particularly wet. In contrast, the south, south east, East Anglia, Lincolnshire and the southern Midlands receive less than 700 mm of rain per year.

The counties of Essex and Cambridgeshire are amongst the driest in the British Isles, with an average annual rainfall of around 600 mm (24 inches), although it typically rains on around 90 days per year. In some years rainfall in Essex can be below 450 mm (18 inches) — less than the average annual rainfall in Jerusalem and Beirut. On average the driest part of the UK is that part of south Essex and North Kent closest to the Thames Estuary, which experience a semi-arid climate.

The main reasons for high number of rainy days in the UK are its 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 in the north and flooding is not rare.

Precipitation over the mountains of the north is especially high and are some of the wettest places in Europe with an average annual rainfall exceeding 60 inches (1,500 mm).

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

The UK has had severe drought problems in recent years, particularly in the South East of England, which has experienced the driest period on record.^[1] Fires broke out, even across the normally damp higher ground of North West England and Wales. The driest areas since 2003 have been in Sussex, Kent, Surrey and London but the drought temporarily spread to many other parts of the country, during the summer of 2006. The landscape in much of England and south east Wales turned very parched and lost the green colour often associated with the British Isles, even near coasts. Water restrictions were in place in some areas.

July 2006 was the hottest month on record for the UK and much of Europe.^[2] However England has had warmer spells of 31 days which did not coincide with a calendar month; in 1976 and 1995. As well as low rainfall, drought problems are made worse by the fact that the driest parts of the UK also have the highest population density, and therefore highest water consumption, and the population in these areas is also seeing the biggest growth. The drought problems are now coming to an end as the period from October 2006 - January 2007 had well above average rainfall.

Temperature

Generally the UK has mild to cool winters and warm to very warm summers with moderate variation in temperature throughout the year. In England the average annual temperature varies from 8.5 °C in the north to 12 °C in the south, but over the higher ground this can be several degrees lower. This small 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 inland, 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 -26.1 °C 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 -11.3 °C, was recorded at the same site.

The warmest winter temperatures tend to occur on the south and west coasts, however, warm temperatures occasionally occur to 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 18 °C in winter on rare occasions. This is a particularly amazing event in Northern Scotland, mainly Aberdeenshire, where these high temperatures can occur in midwinter with just a couple of hours of sunlight.

July tends to be the warmest month and the highest temperatures tend to occur away from the Atlantic in southern, eastern and central areas where summer temperatures can rise as high as 35 °C (95 °F). It soared to 38.5 °C (101 °F) in Kent in the summer of 2003, the highest temperature ever recorded in the United Kingdom. North Kent, in the shadow of the North Downs now seems to be established as the warmest part of the UK, as well as the next driest after South Essex, the other side of the Thames.

2006 has seen yet more unprecedented warmth, with many more records falling. While the year started off average and even fell well below average in early-March, the period from mid-April onwards has seen an astonishing lack of any cooler than average weather. The weather conditions quickly changed from wintry to summery in the middle of Spring. Early-May and June saw temperatures 10-12 °C above average at times. July was the hottest month on record, with records stretching back hundreds of years; the absolute maximum temperature for July was also broken that month. September was the warmest September on record and October was one of the warmest on record. November was also extremely mild, making it the warmest Autumn on record by some margin. May through October was also the warmest consecutive 6-months on record. 2006 looks almost certain to be the warmest year on record, as December keeps up the mild weather.

Severe weather

While the UK is not particularly noted for extreme weather, it does occur, and conditions have been known to reach extreme levels on occasions. In the winter of 1982, 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 34 to 35 °C. It was so dry the country suffered drought and water shortages.^[3]

Extended periods of extreme weather, such as the drought of 1975-1976 and the very cold winters of 1962-1963, 1978-79 and 1981-1982, 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.

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 38 houses and buildings were completely destroyed. Recently in the summer of 2004, a severe flash flood devastated the town of Boscastle in Cornwall. However, the worst floods in the United Kingdom in modern times occurred in 1953. A powerful storm from the Atlantic moved around Scotland and then down the east coast of England. As it moved south it produced a storm surge which was magnified as the North Sea became narrower the further south the storm moved. By the time the storm affected south-east England and the Netherlands the surge had reached heights of 12 feet. Over 300 people were killed by the floods in eastern England.

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 coasts are most likely to have their driest month in the spring and their wettest in late autumn. In London and Birmingham, thunderstorms occur on average about 15 days a year, while in the north in west it is closer to 8 days a year.

Strong winds occur mainly in the autumn and winter months associated with low pressure systems. The "Great" storm of 1987 (23 fatalities) and the Burns Day storm of 1990 (97 fatalities) are particularly severe examples.

Relative to its land area, the United Kingdom has more reported tornados than any other country (around 33 per year), excluding the Netherlands, although most are minor.

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. During 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 exists today. There have been many glacial and interglacial periods in the geological history of the United Kingdom.

The temperature records in England are continuous back to the mid 17th century. The most quoted record is the Central England temperature (CET) record which is a compound source of cross-correlated records from several locations in central England.

External links

References

^ "South East England still in drought". BBC Weather. Retrieved 2006-11-05.
^ McKie, Robin (October 15, 2006). "Official: this summer is the longest, hottest ever". The Observer. Retrieved 2006-11-05.
^ "The 1976 Drought". http://www.bbc.co.uk/weather/features/understanding/1976_drought.shtml. {{cite web}}: External link in |work= (help); Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)

[1] "South East England still in drought". BBC Weather. Retrieved 2006-11-05.

[2] McKie, Robin (October 15, 2006). "Official: this summer is the longest, hottest ever". The Observer. Retrieved 2006-11-05.

[1976_Drought-3] "The 1976 Drought". http://www.bbc.co.uk/weather/features/understanding/1976_drought.shtml. {{cite web}}: External link in |work= (help); Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)

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