Extreme Weather – UK

While the United Kingdom 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 35 °C (95 °F). It was so dry the country suffered drought and water shortages.  Extended periods of extreme weather, such as the drought of 1975–1976 and the very cold winters of 1946–1947, 1962–1963, 1978–79, 1981–1982, 2009–2010 and 2010–2011, are often caused by blocking anti-cyclones, which can persist for 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 most severe was the Lynmouth disaster of 1952 in which 34 people died and 38 houses and buildings were completely destroyed. In the summer of 2004, a severe flash flood devastated the town of Boscastle in cornwall.  Hoewever, the worst floods in the United Kingdom in modern times occurred in the North Sea flood of 1953. A powerful storm from the Atlantic moved around Scotland and down the east coast of England. As it moved south it produced a storm surge which was magnified as the North Sea became narrower further south. By the time the storm affected south-east England and the Netherlands, the surge had reached the height of 3.6 metres (12 ft). Over 300 people were killed by the floods in eastern England.

Thunderstorms are most common in southern and eastern England, and least common in the north and west.  As a result of this, inland areas in the south and east 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, thunderstorms occur on average 14–19 days a year, while in most of Northern Ireland and the west of Scotland thunderstorms occur on around 3 days a year.  Occasionally, thunderstorms can be severe and produce large hailstones as seen in ottery St Mary, Devon, in October 2008, where drifts reached 1.8 metres (5 ft 11 in).

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. The United Kingdom has around 33 tornados per year, which is the second highest amount per land area in the world.

The most rain recorded to fall on a single day was 279 mm at Martinstown, Dorset on 18 July 1955, but also 243 mm fell at Bruton, Somerset on 28 June 1917.  Heavy rain also fell between 20 and 25 June in 2007; some areas experienced a months rainfall in one day. Four people died in the flooding and over £1.5 billion of damage to businesses and properties was caused.

Tropical Cyclones themselves do not affect the UK due to the seas being too cold, they need temperatures above 26.5 °C (79.7 °F) to remain active. The waters near the UK, the Atlantic Ocean, only have temperatures of 10 to 15 °C (50 to 59 °F), so any Tropical Cyclone that does come anywhere near the UK has said to have undergone a process called extratropical transition. This now means it is an extratropical cyclone, which the UK frequently experiences. The Great Storm of 1987 was a very deep depression which formed in the Bay of Biscay, which also contained the remnants of Hurricance Floyd.  Hurricane Lili of 1996 and Hurricane Gordon of 2006 both crossed the UK as strong extratropical cyclones with tropical storm-force winds, causing transport closures, power-cuts and flooding in large parts of the UK.

Facts and info from Wikipedia

 

Thunder and Lightning

Some people say that they can tell when a storm is coming.  The air “feels” different, animals quiten down, birds disappear until the storm has passed. 

There are actually several types of thunderstorm, although here in Britain we are most likely to experience one in the warmer months.  In summer months, the earths surface is heated by the sun, and with this warm air rising, and cooler air sinking it creates conditions for a storm.  The 3 more common storms are an Orographic storm, a frontal storm, and an Air mass thunderstorm.

OROGRAPHIC STORMS

These kind of storms are caused by the lifting of air over a mountain or a hillside.  These storms can be accompanied by large volumes of rainfall.

FRONTAL STORMS

Frontal storms occur when different air masses meet, ie, when a cold front meets a warm front.  Cold air is denser than warm air, so as a cold front approaches warm air the warm air is lifted, which then creates unstable conditions in the troposphere (the lowest portion of earths atmosphere).  This unstable air can create massive thunderstorms, and can bring a lot of rain.  You can often see one of these storms developing, they appear as big, dark cumulonimbus clouds.  If there is enough cold air this kind of storm can appear day or night, although often a nightime storm can appear far more impressive as the lightning is more visible in the dark sky.

AIR MASS THUNDERSTORM

An air mass thunderstorm typically lasts less than an hour, and can be very localised.  It is caused when a large mass of warm, moist air interaacts with even a small pocket of cold air.  When these storms hit you can quickly find yourself drenched one minute, then drying out the next as the sky clears and the sun comes back out.

SUPERCELL THUNDERSTORM

What sets this kind of storm apart from the others is the rotation in the cloud.  A supercell is usually found in the warm part of a low pressure system, and can be one of the most dangerous types of storm.  It is basically a huge rotating thunderstorm, the area of rotation withhin the storm is called a mesocyclone that can spawn a tornado. The storm itself can rotate when winds at different levels of the atmosphere come from different directions. If the winds are lined up just right, with just enough strength, the storm turns like a top. Air circulations within the storm combined with a strong updraft contribute to tornado formation.

Info from Wikipedia

British Cimate

British weather is divided into 4 seasons – Spring, Summer, Autumn and Winter.

SPRING

Climatologically The Met Office classes the first day of spring as the 1st day of March, but because of the variation in the days on which the equinox and solstice falls, it is more convenient to use whole months. The Met Office therefore classifies the spring months as being March, April and May.  Traditionally the first day of spring is Astronomically derived, and is March 20th.  At the time of the equinox the Sun will cross directly over the Earth’s equator. This moment is known as the vernal equinox in the Northern Hemisphere. For the Southern Hemisphere, this is the moment of the autumnal equinox.  Traditionally spring is thought to be about new beginnings, the weather gets warmer, the days get longer…..In spring, the axis of the Earth is increasing its tilt toward the Sun and the length of daylight rapidly increases for the relevant hemisphere. The hemisphere begins to warm significantly causing new plant growth to “spring forth,” giving the season its name.  Spring is generally a calm, cool and dry season, particularly because 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, but often tend to drop off again at night due to the cool oceans and the warm weather dependent solely on the sun.  Thunderstorms and heavy showers can develop occasionally particularly towards the end of the season.

There is a fair chance of snow earlier in the season when temperatures are 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.

Mean temperatures in Spring are markedly influenced by latitude. Most of Scotland and the mountains of Wales and northern England are the coolest areas of the UK, with average temperatures ranging from -0.6 to 5.8 °C (30.9 to 42.4 °F).  The southern half of England experiences the warmest spring temperatures of between 8.8 and 10.3 °C (47.8 and 50.5 °F)

 

SUMMER

Summer lasts from June to September and is the warmest season.  Rainfall totals can have a wide local variation due to localised thunderstorms. These thunderstorms mainly occur in southern, eastern, and central England and are less frequent and severe in the north and west. North Atlantic depressions are not as severe in summer but increase both in severity and frequency towards the end of the season. Summer can see high pressure systems from the Azores High.

Climatic differences at this time of year are more influenced by latitude and temperatures are highest in southern and central areas and lowest in the north. Generally, summer temperatures seldom go above 30 °C (86 °F), which happens more frequently in London and the South East than other parts of the country. Scotland and northern England have the coolest summers, while Wales and the south-west of England have warmer summers, and the south and south-east of England have the warmest summers.

Autumn

Autumn in the United Kingdom lasts from October to November. The season is notorious for being unsettled—as cool polar air moves southwards following the sun, it meets the warm air of the tropics and produces an area of great disturbance along which the country lies. This combined with the warm ocean due to heating throughout the spring and summer, produces the unsettled weather of autumn. In addition, when the air is particularly cold temperatures on land may be colder than the ocean, resulting in significant amounts of condensation and clouds which bring rain to the country.

Atlantic depressions during this time can become intense and winds of hurricane force (greater than 74 mph) can be recorded. 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. One particularly intense depression was the Great Storm of 1987.

However, the United Kingdom sometimes 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 country in warm air, despite the relatively weak sun.

Winter

Winter in the UK lasts from December to February. The season is generally cool, wet and windy.  Precipitation is plentiful throughout the season, though snow is relatively infrequent despite the country’s high latitude: The only areas with significant snowfall are the Scottish Highlands and the Pennines.  Towards the later part of the season the weather usually stabilises with less wind, less precipitation and lower temperatures. This change is particularly pronounced near the coasts mainly because the Atlantic Ocean 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.  Mean winter temperatures in the UK are most influenced by proximity to the sea. The coldest areas are the mountains of Wales and northern England, and inland areas of Scotland.  In the 1990s and 2000s, most of the winters were milder and usually wetter than average with daytime temperatures going below freezing a rare occurrence. In fact, the winter of 1995/1996 was the only one which was defined as below average in terms of the UK as a whole. The winters of 2008/09, 2009/10 and 2010/11 have however seen a different pattern with these three winters being defined as below or well below average with large snowfall amounts widespread and very low temperatures; this was the first time three consecutive cold winters in the UK have occurred since the 1960s.

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. The last glacial period 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 low that much of the surrounding ocean froze and a great ice sheet extended over all of the United Kingdom except the south of England.  We think we are cold when temperatures reach 0, imagine being around back then!

The cold period from the 16th to the mid-19th centuries is known as the Little Ice Age.

Dewpoint and Heat Index explained

What is the “dew point”

The dewpoint temperature is the temperature at which the air can no longer hold all of its water vapor, and some of the water vapor must turn into liquid water. The dew point is always lower than, or equal to, the air temperature.

If the air temperature cools to the dew point, or if the dew point rises to equal the air temperature, then dew, fog or clouds begin to form. At this point where the dew point temperature equals the air temperature, the relative humidity is 100%.

If there is then further cooling of the air, more water vapor must condense out as even more dew, fog, or cloud, so that the dew point temperature then falls along with the air temperature.

While relative humidity is (as its name suggests) a relative measure of how humid the air is, the dewpoint temperature is an absolute measure of how much water vapor is in the air.  During the summer, it is the dewpoint temperature, not the relative humidity, that gives a better indication of how humid it feels outside. It is also a good measure of how much “fuel” is available to showers and thunderstorms, with a higher dewpoint representing more water vapor available for conversion to rain

HEAT INDEX

The heat index given in weather readings is an index that combines air temperature and relative humidity, to give an indication of the temperature we are actually feeling.  This can also be known as the “apparant temperature”.  For example, if it is a hot day, say 30c, and there is a very high humidity reading, the actual temperature that you feel can be far more than 30c.

Rainfall

How does it rain?

Almost all the air around us is moist.  We all know that clouds are made from water, but how exactly? The water condensing in the clouds has to become heavy enough to fall to Earth. The tiny droplets alone just aren’t heavy enough to fall.  To fall from the sky, these droplets need to become heavier, and they do this by colliding with other droplets, growing bigger and heavier, and they are able to fall as rain.  Some may get caught in upward blowing winds and get blown back into the clouds for a while, but once they are heavy enough to overcome the force of the wind, they will fall to earth. It will keep raining as long as the conditions are right to make the clouds and let the water droplets grow heavy enough to fall.

There are 3 types of rainfall:

Convectional Rain

On a warm day the ground is heated by the sun, and the air above the ground also warms, and begins to rise.  The higher you go the cooler it gets, so as this warm air rises it cools because of the colder air around it.  Eventually the air reaches a height where the temperature forces the water vapour in the warm air to start condensing. This is called the condensation point, and is where the clouds begin to form. The typical cloud formed this way is called a cumulus cloud, or a cumulonimbus cloud when it has a grey bottom, these are the type of clouds that people often look at the sky and say it ‘looks like rain’.  The rain forming process starts, and usually leads to very heavy rain, perhaps with thunder and lightening. Convectional rain is often experienced at the end of a hot summer day and associated with torrential downpours with large drops and towering dark grey clouds

Frontal Rain

Both warm and cold air are needed for this kind of rain. The warmer air is less dense, so when it meets the cooler air it rises up over the cooler air mass. The cooler air stays where it is, and lies underneath the warmer air.  As the warmer air rises over the cold air it starts to cool down, and as it cools water vapour is precipitated and the cloud forming process begins, leading to rain. The sky will generally be grey, and will cover almost all of the sky.

Relief Rain

Relief rain is common in upland and mountainous areas where is can lead to extraordinary local rainfall patterns. It’s common for one side of a mountain to be in warm sunshine, yet only a few hundred meters away it’s raining on the other side of the mountain ridge.  Relief rain needs a physical obstruction of some kind, so that warm moist air is forced to rise up over it. Mountain ranges, big hills and even cliffs along the coast can be large enough to force the air to rise.  As the warm air rises over the obstruction it cools and clouds form. Rain falls from the clouds, or if the droplets don’t grow large enough, fog may form over the hill tops. Fog is basically just clouds at ground level.  The air passes over the obstruction and can sink again, gaining warmth as it does so. This air is drier than it was before it lost water as rain, so any clouds left will evaporate again, leaving clear skies. This area has only a little rain because the cloud making process isn’t working

Clouds

There are several types of clouds, and they are split into 3 catagories, Low Clouds, Middle Clouds and High Clouds.

LOW CLOUDS

Stratus clouds are a kind of grayish cloud that often cover the entire sky. They resemble fog that does not reach the ground. Usually no rain falls from stratus clouds, but sometimes they may drizzle.

Nimbostratus clouds form a dark gray, “wet” looking cloudy layer associated with continuously falling rain or snow. They often bring light/moderate rain.

MIDDLE CLOUDS

“Alto” Clouds

Alto Clouds are middle level clouds that have bases between 2000 and 7000 m (6500 to 23,000 ft.).

Altocumulus clouds are made of droplets of water,  and appear as gray, puffy clouds which can sort of be rolled out in parallel waves or bands. These clouds can result in thunderstorms.

Altostratus clouds are gray or blue-gray that are made of both ice crystals and water droplets. They usually cover the entire sky. In the thinner areas of the cloud, the sun may occasionally be seen. Altostratus clouds often form ahead of storms that will produce a lot of rainfall.

HIGH CLOUDS

Cirrus Clouds

Cirrus clouds are thin, wispy clouds that are blown by high winds into long streamers. These are high clouds, which can be found over 6000 m (20,000 ft). Cirrus clouds usually move across the sky from west to east. They generally mean quite nice weather

Cirrostratus clouds are very thin, high clouds that can cover the sky. They are so thin that the sun and moon can be seen through them

Cirrocumulus clouds appear as small, rounded white puffs.

Cumulus clouds are puffy clouds that sometimes look like pieces of cotton wool or candyfloss. The base of each cloud is often flat and may be only 1000 m (330 ft) above the ground. The top of the cloud has rounded towers. When the top of the cumulus resembles the head of a cauliflower, it is called cumulus congestus or towering cumulus. These clouds grow upward, and they can develop into a giant cumulonimbus, which is a thunderstorm cloud.

Info taken from Wikipedia