Pressure – What exactly is it?

You will have heard a million times forecasters talking of high or low pressure systems, but what exactly are they, and how do they affect our weather?  A huge amount of air is pushing down on us at all times, the gases in the air are made of tiny molecules that although we can’t see them, are taking up space and have weight.  The reason we can’t feel this is because our bodies also have air, which pushes outwards and this creates a balance between the two.  Warm air molecules move quickly and push each other apart, meaning the air gets lighter and rises.  Cooler air molecules move slowly, and they take up less space than the warm air molecules, and as the air becomes denser and heavier it sinks.  Air pressure is the weight of the air pushing down on the earths surface, when air warms up it rises, when air cools and sinks there is high pressure.  Air pressure varies all over the planet as different areas receive different amounts of the suns warmth.  Pressure is measured with a barometer, and is recorded as millibars (mb).  There is a difference between absolute pressure and sea level pressure.  You can find your sea level pressure with a quick google search by typing in your location.  My weather station records the actual pressure where I am, and will be different to those nearer the sea.  Where there are hurricanes/typhoons the pressure at sea will drop very low, as these storms from from low weather systems.

High Pressure

As stated, when the air cools it sinks, and this means there is more air pushing down on the earths surface, thus the air pressure rises.  As the air sink it becomes warmer and drier and the clouds disappear, so areas of high pressure usually bring warm dry weather with clear skies.  In weather maps, high pressure will be depicted as red.

Low Pressure

When the air warms it rises, leaving less air pushing down on the earths surface, and this leads to the pressure going down.  Areas of low pressure, which are usually described as a depression, bring rain, wind and storms, and changeable temperatures.  As high pressure is shown as red on weather maps, low pressure is shown as blue.

So generally, high pressure = good, low pressure = bad.  Here in Britain, when we have typical weather for the seasons, we will have high pressure in spring/summer, and low pressure in autumn/winter.  The reason we haven’t had the typical weather patterns lately is because of the jet stream, because it has been sat to the south of the UK for much of the past 18 months it has allowed low pressure to sweep right across the UK, usually in summer it sits above us, therefore drawing away low pressure above us and allowing high pressure to build.  This is the kind of weather we have been seeing lately, the jet stream is in its more typical place for this type of year so the low pressure is being dragged north of us.  Let’s all hope it will stay there throughout July and August!

 

 

Summer ready to arrive?

After much debate over whether we will have to endure another washout summer, and it did actually look like a bit of a damp squib until recently, it is now fairly certain that from the end of this week at least we shall experience some glorious summer weather.  All charts now seem to be in agreement that from around Friday temperatures will rise and blue skies and sunshine are on they way, and could actually stick around for a while.  Some say we could have the hottest July temperatures since 2006, which I have to say would be very welcome after last years washout.  The chart below gives a brilliant indication of what is to come:

A brilliant sight to see :)

 

That chart is for the 10th July, so you can see how well the UK is looking for this time.  Let’s just hope it continues until autumn, and is followed by a nice snowy winter!

The reason the weather is set to improve is again down to the position of the jet stream, which is set to head well north of the UK, allowing high pressure to build from the continent.  This picture is taken from Matt Hugo, and shows the position of the jet stream and how it allows the high pressure to reach us:

In summer, high pressure from the continent brings warmth and summery weather, whereas in winter it brings cold and snow.  Weather systems from the Atlantic (low pressure), regardless of season bring with them  wind and rain, and while we would expect these kind of weather systems to dominate through autumn or springtime, the position of the jet stream over the past few years has meant we have had Atlantic weather a lot more than usual, and this caused a lot of the flooding throughout last year.  In short, if everyone blows north we may be able to keep the jet stream well clear of us, and the summery weather will remain!

Atlantic Hurricane Season

We take a look back at the North Atlantic Hurricane season (it is classed as being 1st June-30th November)

JUNE

Or rather, MAY! This years season was kicked off early as Tropical Storm Alberto (min pressure of 995mb)formed on the 19th May off the coast of South Carolina, followed by Tropical Storm Beryl min pressure 992mb).  Beryl began life as a Sub Tropical Storm over the north-western Caribbean sea on May 26th, before reaching peak winds of 70mph the following day when she became a full Tropical Storm.  Beryl made landfall in Jacksonville, Florida where she weakened into a Tropical Depression as she headed towards Georgia and South Carolina.

Hurricane Chris (min pressure 987mb) was next, beginning life as a Tropical Storm on the 19th June, and becoming a Hurricane on the 21st.  As the storm hit colder waters 6 hours after becoming a hurricane it weakened back to a Tropical Storm and was absorbed by a larger non-tropical storm on the 22nd June.

Following quickly on Chris’s heels came Debby (min pressure 990mb), this Tropical Storm formed on the 23rd June in the Gulf of Mexico, and made landfall in Steinhatchee, Florida, on the evening of the 26th June.  The remnants of Debby dissipated on June 30th.

JULY

Hurricane Ernesto (min pressure 980mb) started life as a tropical wave off the west coast of Africa on the 26th of July, although did not gain Tropical status during this month.

AUGUST

Tropical Depression 5 was officially formed on the 1st August (this was the storm mentioned in July) and became Tropical Storm Ernesto the following day.  The storm moved over the Carribean, bringing with it high winds and heavy rain.  By the 7th August the storm was officially declared a Hurricane, and made landfall on the 8th August as a Catagory 1 Hurricane with winds of 85mph.   After weakening into a tropical storm and moving into the Bay Of Campeche, the storm made landfall again near Coatzacoalcos, Mexico where it dumped heavy rain before moving into the Eastern Pacific where it contributed to the formation of Tropical Storm Hector.

Tropical Storm Florence  (min pressure 1000mb) began life as a vigourous tropical wave off the coast of Africa on the 1st August, and by the 4th had became Tropical Depression 6.  By the next day it had strengthened to a Tropical Storm, although it soon weakened and the last advisory was issued that day by the NHC.

Tropical Storm Helene (min pressure 1004mb) began to form as a tropical wave near Cape Verde on August 6, and by the 9th had became organised enough to be named Tropical Depression 7.  The storm degenerated and on August 14th the storm moved inland.  By the 17th August the storm re-emerged into the Bay of Campeche and after rapid intensification it was immedietly named Tropical Storm Helen.  Helene moved northwest and made landfall near Tampico, Mexico on the following day.  Later that day, the storm was downgraded to a tropical depression, before dissipating early on August 19. The remnant tropical wave produced heavy rainfall in Trinidad and Tobago, and caused flooding and landslides on the Islands.

Another one that developed off the coast of Africa was Hurricane Gordan (min pressure 965mb), the low developed on the 10th August, and on the 15th August Tropical Depression 8 was born, and within 12 hours it intesified to become Tropical Storm Gordon.  After passing over the Azores Gordon developed more convection around its centre and was classed as a Hurricane on the 18th August.  Although passing directly over parts of the Azores damage was limited to power cuts and torrential rain caused some flooding.

The next storm yet again was born near the coast of Africa, a tropical wave developed over the 15/16th and headed west where it became Tropical Depression 9 on the 21st of the month.  Despite being quite disorganised, this storm was also named within 12 hours, just as Gordon was, and Tropical Storm Isaac (min pressure 968mb) was formed.  An eye formed on the 25th, and winds reached 70mph as the storm hit Haiti, causing flooding and damage to many places.  After moving back over water Isaac next made landfall in Cuba, although major damage was not expected.  Yet again Isaac was over open water, this time strenghtening into a Hurricane on the 28th August, and all eyes were suddenly focused on the Gulf of Mexico, moreso along the coasts of Louisiana and Mississippi as projected tracks had the storm making landfall in either state.  As it happened Isaac made landfall in Louisiana, near Port Fourchon on the 29th August, 7 years to the day since the much larger and more destructive Katrina hit the state.  Much was made of the dates, but thankfully although Isaac caused an awful lot of damage with flooding, it came nowhere near to what Katrina had done years before.

The next storm to form was Tropical Depression 10, this was a tropical wave that started producing disorganised showers between Africa and the Cape Verdi Islands on the 20th August.  2 days later it was classed as organised enough to be a Tropical Depression, and 10 was born.  On the 23rd August TD10 became Tropical Storm Joyce (min pressure 1006mb), although significant deelopment was neer a chance as the storm had such a strong wind shear to the south of the centre of circulation, and it soon dropped back to a depression, before the last advisory by the NHC was given out on the 24th August.

Hurricane Kirk was named as Tropical Depression 11 on the 28th August, and conditions at first appeared unfavourable for further development of this storm. However, the next advisory from the NHC upgraded the system to Tropical Storm Kirk, and after a further 2 days became a hurricane.  Hurricane Kirk (min pressure 970mb) reached catagory 2 status with winds of 105mph before hitting cooler waters and taking on Post Tropical status on the 2nd September.

The last storm to form during August was Hurricane Leslie (min pressure 968mb) which started as a tropical wave that moved off the west coast of Africa into the Atlantic on the 27th August, taking on a westerly track.  Deep convection began around the centre on the 30th and Tropical Depression 12 was born.  Due to the warm sea surface temperatures it very quickly intensified, and became Tropical Stomr Leslie the same day.  Despite being somewhat disorganised an eye like feature was noted on September 1st, and Leslie was upgraded to a Hurricane on September 5th.  After a journey up the east coast it made landfall in Canada on the 11th September, with 70mph winds where it became extra-tropical.

SEPTEMBER

Hurricane Michael (min pressure 964mb) formed northwest of the Cape Verde Islands, and on the 3rd September it was deemed organised enough to become a Tropical Depression, before becoming a Tropical Storm the next day.  It didn’t really intensify much until the %th September when it was declared a Hurricane, and within 2 hours it reached category 2.  4 hours later it reached category 3, with peak winds of 115mph.  After maintaining its intensity the storm finally succumbed to high wind shear from nearby Tropical Storm Leslie and became a convection less vortex by September 11, when it was declared a post-tropical cyclone by the National Hurricane Center.

Hurricane Nadine (min pressure 978mb) was next up, the system was declared a Tropical Depression on the 10th September, and was upgraded to Tropical Storm status the following day.  and was then declared a hurricane on the 15th September.  After convection weakened, an upper-level low developed, and the wind field became larger than average, Nadine transitioned into a subtropical cyclone on September 21, and then declared a post-tropical cyclone the day after.  However, by the 23rd September the system became again a Tropical Cyclone, and held this intensity until the 28th at which time it was declared a Hurricane yet again.  Due to wind shear the following day, Nadine weakened back into a tropical storm, but briefly restrengthened into a hurricane for the third time hours later. It reached its peak intensity of 90 mph on September 30, holding on to its strength for 18 hours until it again started to weaken due to increasing wind shear and cooler waters. Nadine weakened to a minimal tropical storm on October 3, when it briefly reentered warm waters, thus dropping its pressure. On October 3, Nadine was picked up by the jet stream and eventually merged with a non-tropical system.

OCTOBER

October 3rd saw the birth of Tropical Depression 15, and later that day this was upgraded to Tropical Storm Oscar (min pressure 994mb).  The storm reached peak intensity of 50mph winds on the 5th October, and early on the 6th the system was absorbed by a cold front.

Tropical Depression 16 originated North East of the Bahamas in early October, however it did not become organised enough to be declared a depression until the 11th of October, and later that same day was named Tropical Storm Patty (min pressure 994mb).  Wind shear inside the system soon weakened and the following day Patty was downgraded to  a depression, where soon after it fizzled out.

October 12th saw Tropical Storm Rafael (min pressure 969mb) organise enough to become the next named system of the year, and 3 days later was classed as a Hurricane.  By the 17th of October the system had become a strong extratropical cyclone with no threat.

The next storm to be named was the most talked about for years, Sandy (min pressure 940mb) was born on the 22nd October as Tropical Depression 18 about 320 miles south of Jamaica, and later that same day was upgraded and named Tropical Storm Sandy.  The NHC noted that “remaining nearly stationary over the warm waters of southwestern Caribbean Sea is never a good sign for this time of year.”  Sandy first made landfall in Jamaica on the 24th October as a Category 1 Hurricane, and as it continued its journey towards Cuba it was upgraded to a category 2 Hurricane with 110mph winds.  After striking Cuba the system weakened slightly before again becoming a Hurricane.  The convection organized further early on October 29,and around the same time, Sandy began transitioning into an extratropical storm early on October 29 after the western periphery of the circulation began interacting with a cold front.  Around this time much was made in the media of the “frankenstorm” that was going to hit mainland USA (the name as Halloween was right around the corner).  Sandy became post tropical and it began its journey towards the New Jersey area.  The intensity at landfall was estimated at 90 mph (150 km/h), although the strongest winds were located east and southeast of the center.  After moving ashore, Sandy continued to the west, weakening below hurricane force by the time it reached Pennsylvania. The hurricane caused widespread power outages and also dumped record snowfall on West Virginia. By 0900 UTC on October 31, the circulation degenerated into a trough of low pressure, with no discernible circulation.   Later that day, the remnants of Sandy spread into the Great Lakes, and the HPC issued its last advisory.

Tropical Storm Tony (min pressure 1000mb)  was the last of the 2012 Atlantic season, and began as a tropical wave on the 19th of October.  After meandering for a while it became a tropical Depression on the 20th, although it was expected to dissipate within days.  Tropical Depression Nineteen moved northward and became Tropical Storm Tony  October 24.  After a brief interlude as a named storm this was declared post tropical on the 25th October, when it was located south-west of the Azores.

Info from Wiki

 

 

Wet and mild gives way to cold and snow?

Well we began the new year the same as we left the old one, damp, mild and quite frankly a bit depressing!  All that is set to change over the coming days as we are set for quite a cold spell.  The hold the Atlantic has had over us for quite some time has finally given way to colder air from the east, which will result in freezing temperatures, and snow is possible for most regions.  I think as this winter has been mild for the majority we will really feel the difference as the temperatures begin to drop.  Already the past 2 days have shown lower temperature readings on my weather station, after balmy temperatures 10-12c that we have regularly saw over the past weeks we are now seeing 5c, and that is set to drop to minus figures before long.  Saturday is looking to be the first chance of snow, with more possible on Sunday, and then a higher chance again on Monday.  Remember though, snow is really hard to predict, as the model runs nearer the time give us their output I will be able to predict a bit better where it will fall and how much.  Because the cold air is coming from the east/north-east it will interact with the relative warmth of the North Sea (I say relative warmth, you wouldn’t want to go for a dip!!) which can cause significant snow showers along the east coast, so i think a lot of radar watching will be taking place to see where exactly the moisture is and whether or not it will blow as far inland as to hit us.  You may have heard or read recently about the “ssw event”, ssw stands for Sudden Stratospheric Warming, and is a factor in the cold weather, a good explanation on this can be found here:

http://metofficenews.wordpress.com/2013/01/08/what-is-a-sudden-stratospheric-warming-ssw/

Also, for anybody who would like to know what exactly “the models” are that forecasts are based on, a really good explanation can be found in the info bible that is Wikipedia:

http://en.wikipedia.org/wiki/Ensemble_forecasting

For now I am personally getting the sledges and windscreen scrapers out, and looking forward to hopefully out first real snow in quite a while.  I will leave the weather station on as much as I can over the next few days so the temperatures etc are available for anybody who would like to view them, but I am still having problems with the laptop over heating so I won’t be able to leave it on 24/7

 

Jet Stream!!!

Soooo, I’m guessing most people have heard of the jet stream, but aren’t entirely sure of what it does.  As mentioned previously on this site the jet stream that we mention is the one that sits over the Atlantic and beyond.  There are actually 4 jet streams around the globe, 2 in the northern hemisphere, and 2 in the south.  They flow east to west, and as has been shown recently they can stray from their more “usual” path.  The jet stream affecting us would normally at this time of year be sitting above the UK, dragging the low pressure systems above our shores and allowing high pressure (warmer and sunnier in summer) to build over us.  As the jet stream has been sat to the south of the UK the low pressure systems, bringing with them rain, and basically doom and gloom, have been dragged right across the UK, with many of these slow-moving systems dumping massive, massive amounts of rainfall, hence this ridiculously wet weather we have been having.  The jet stream moving north means England will experience much better weather as of next week, with temperatures climbing and rainfall dropping significantly.  The North West of Scotland however can expect cooler temperatures and wet weather as the jet will bring low pressure over the top of the country.

It isn’t just the UK that has been unduly affected so far this year by the jet stream, North America are currently experiencing a drought, and have had record-breaking temperatures over the past few weeks, as the position of the jet stream has allowed high pressure to build over many states, bringing severe storms with it and extremely high temperatures. Russia has experienced devastating effects from flooding after large parts inundated with rain, with many deaths reported and devastating loss of infrastructure.

Back to us, and southern England will bask in better weather to begin with as the jet shifts, fingers crossed we won’t be too far behind and summer will finally kick in just in time for the summer holidays.  Watch this space!!

Info taken from Paul Hudson @bbc

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