Saturday’s squally weather and reports of tornadoes

27 01 2014

On Saturday we saw a number of heavy rain showers group together in what’s known as a ‘squall line’ – a narrow band of thunderstorms, intense rain, hail, and frequent lightning accompanied by brief but very strong gusts of wind and possibly tornadoes.

Radar image showing the narrow band of showers moving across the UK.

Radar image showing the narrow band of showers moving across the UK.

This squall line swept across Wales and then moved south east across southern parts of England – bringing about 6mm of rain to places in a very short period of time with gusts of wind of around 60mph or more in places.

It  also had the characteristics of a cold front, with the temperature ahead of it being around 11°C, falling to 7°C once the squall line had passed.

There have been reports of possible weak tornadoes from some locations, however it’s hard to verify them without pictures or footage because these features are generally too small to be picked up by satellites or weather observation equipment.

It’s also worth noting that squally winds can often be mistaken for tornadoes because these gusts can be sudden and strong – potentially causing very localised damage.

You can see more information on tornadoes and how they form on our website.





Stormy weather in the Mediterranean

15 11 2013

The central and western Mediterranean will experience very unsettled conditions through the weekend and next week.

Very heavy rain is expected to affect the northeast of Spain, southern France, the Balearic Isles, Corsica, Sardinia, Italy and the Adriatic facing Balkan nations as the very unsettled conditions move slowly east through the region.

Rainfall totals could be as high as 250mm in places, with a risk of up to 200mm in 24 hours. The average rainfall for November in this region is between 50mm and 100mm.

The rain will be associated with thunderstorms which could also produce hailstorms, very strong gusty winds and the possibility of tornadoes in a few places.

Storms developing over the western Mediterranean

Storms developing over the western Mediterranean

In addition to the rain, very strong winds are expected through the central and western Mediterranean, with widespread gales and a risk of storm force winds for a time. This will lead to rough seas that could pose a threat to shipping in the region.

There is also the risk of strong or gale force southeasterly winds affecting the Adriatic during Tuesday and Wednesday next week. These strong winds, combined with very heavy rainfall across the Venice region over the next few days could bring the risk of flooding in Venice.





Five things you might not know about thunderstorms

20 03 2013

1. Lighting can strike twice. The empire state building in New York has been struck by lightning as many as 48 times in one day.
2. The average flash of lightning would light a 100 watt light bulb for three months.
3. Lightning can strike in volcanic ash clouds. Not much is known about volcanic lightning, but we’re using it to help track ash clouds.

Volcanic lightning

Volcanic lightning

4. Thunderstorms can trigger asthma. The Met Office has worked with the NHS and Asthma UK to try and understand why.
5. The most thundery part of the earth is the island of Java where the annual frequency of thunderstorms is about 220 days per year.





Evolution of Thursday’s thunderstorms

29 06 2012

A series of intense thunderstorms brought exceptionally severe weather across parts of the UK yesterday, causing flash flooding and disruption in many places.

As the storms tracked across the country our observation sites picked up some very heavy hourly rainfall totals, with Scampton in Lincolnshire seeing 28.4 mm falling in an hour.

Several other sites saw hourly totals in excess of 20 mm. This led to flash flooding of properties, roads, and landslides in places.

More than 111,000 lightning strokes were also detected across Europe, with more than 1,000 detected over the UK in a 5 minute period at the peak of activity yesterday.

Hail stones ‘the size of golf balls’ also caused damage in Leicestershire, according to media reports.

The storms were borne out of hot, humid air which had tracked up from the Azores far to the south in the Atlantic. This air mass tracked up on southerly winds, moving over Spain before reaching the UK.

As a result, much of the country saw warm and muggy conditions, with the temperature reaching 28.4 C at St James’s Park, Central London.

The heat and moisture in the air were enough to cause thunderstorms, but the really intense storms were formed as an Atlantic weather front moved in from the west.

As it ‘collided’ with the warm and humid air mass, air rapidly rose to create towering cumulonimbus storm clouds which were laden with water, and ripe for developing hail, thunder and lightning.

This led to several distinct lines of thunderstorms developing along the boundary where the two air masses met.

As shown in the radar sequences below, one line originated in the Cardiff area of south Wales in the early morning. This moved in an east-north-east direction across Worcestershire, Shropshire, the West Midlands and Leicestershire to clear Lincolnshire by late afternoon.

A second line of thunderstorms reached the Lancashire coast around late morning and moved in a NE direction to reach the Newcastle area later in the day, clearing the north east coast by late evening.

There were also torrential downpours across parts of Northern Ireland and western Scotland. Southern parts of England and Wales saw relatively little rain and periods of warm sunshine.





Why are we getting thunder and lightning?

11 04 2012

With the weather in April being distinctly showery so far, what exactly causes this changeable weather and why do some showers give thunder and lightning?

 

Thunderstorms are normally associated with convective clouds which form from rising air warmed by the Sun. At this time of year we have longer days and therefore more heat reaches the Earth’s surface giving a greater chance for convective clouds to form. The air is continuously moving within the cloud in a very disorderly fashion, allowing the cloud to grow and water droplets or ice crystals to form. Given enough time and growth, the cloud may develop into a Cumulonimbus cloud and give quite heavy bursts of rain or hail for short periods of time, and possibly thunder and lightning.

Hail forms when ice crystals or frozen raindrops within the cloud get thrown about with the rapidly circulating air. As they ascend they grow as water freezes on the surface of the droplet or crystal. Eventually the droplets will become too heavy to be supported by the updraughts of air and they fall to Earth as hail.

 As hail moves through the cloud it picks up a negative charge as it rubs against smaller positively charged ice crystals. A negative charge collects at the bottom of the cloud where the heavy hail collects, while the lighter ice crystals remain near the top of the cloud and create a positive charge. The negative charge is attracted to the Earth’s surface and other clouds and objects and when the attraction becomes too strong, the positive and negative charges come together, or discharge, to balance the difference in a flash of lightning. The rapid expansion and heating of air caused by lightning produces the accompanying loud clap of thunder.

 Thunder and lightning facts:

  • A bolt of lightning lasts on average for about one 10,000th of a second.
  • The average speed at which the lightning cuts through the air is 270,000 mph.
  • There are several types of lightning, the most common being “sheet lightning” in which the discharge of positive and negative charges occurs within the cloud.
  • The risk of being struck by lightning is minimal and ninety percent of lightning travels from cloud to cloud. Lightning takes the shortest and quickest route to the ground, usually via a high object standing alone.
  • The average annual frequency of lightning is less than 5 days in western coastal areas of the United Kingdom and over most of central and northern Scotland, and 15 to 20 days over the east Midlands and parts of southeast England

Get more facts from our thunderstorms fact sheet.

See lightning observations for the last three hours on our observations map.





Volcanic lightning could help monitor volcanic ash

11 12 2010
Volcanic material thrust high into the atmosph...

Volcanic lightning (Source: Wikipedia)

Met Office researchers have published new findings in ERL suggesting that the amount of lightning produced near a volcano each hour is roughly proportional to the plume height. They believe that the technique could ultimately be used to monitor volcanoes in remote locations or those that are obscured by rain.

To assess the number of lightning strokes the team used ATDnet, the UK Met Office‘s long-range lightning-location network, which has recently been updated.

“ATDnet is used to monitor thunderstorms over a vast area of the world extending from the US to China and the Arctic to the South Atlantic, centred over the UK,” siad Alec Bennett of the Met Office. “Lightning emits powerful electromagnetic pulses over a broad frequency range creating both the bright flash seen by an observer and the crackles on a nearby radio. The peak energy of this emission is found in the very low frequency (VLF) radio band at about 10 kHz, and it is this part of the spectrum that ATDnet uses to locate lightning, utilizing sensors positioned across Europe and beyond.”

The system has previously located lightning from volcanic plumes in southern Chile and earlier eruptions in Iceland. “The availability of quantitative observations of volcanic lightning is sparse, so our results are hoped to add new and detailed observational data to this research field,” said Bennett. “The eruption of Eyjafjallajökull in April–May 2010 provided an excellent opportunity for us to study both the performance of ATDnet after recent network upgrades and to use the lightning data to learn more about the relationship between volcanic activity and plume electrification, a process recognised for decades but still subject to ongoing scientific debate.”

Bennett and colleagues from the UK Met Office and Icelandic Meteorological Office found that plumes from Eyjafjallajökull that reached about 5 km above sea level generated lightning strong enough to be detected by ATDnet. Above this threshold, the rate of lightning production was approximately proportional to the plume height.

“Perhaps most importantly, it was found that this relationship did not exist all of the time, with some plumes higher than 5 km producing no lightning capable of being detected by ATDnet at all,” said Bennett. “This presents us with a unique case study whereby we have several days where a tall plume was not strongly electrified followed immediately by several days of strong electrification, enabling more research to be undertaken on the cause of vivid volcanic lightning displays.”

An eruption of Iceland’s Eyjafjallajökull volcano in late-March 2010 produced a large plume of ash and considerable disruption to flights across Europe.

The team used C-band radar data from the Icelandic Meteorological Office to quantify the plume height above the volcano. This allowed the scientists to combine detailed observations on lightning and plume height as well as modelled atmospheric parameters, such as wind speed, wind direction and ambient temperature. “With these data we can begin to quantify the effect of plume and atmospheric conditions on the generation of volcanic lightning,” said Bennett.

“By comparing the distance between the vent and lightning strokes it was also possible to gain evidence that a charging mechanism was present away from the vent, with many lightning strokes produced several kilometres downwind from the vent,” he added. “This finding suggests that two different charging mechanisms may be present in the plume, one immediately adjacent to the vent – producing numerous weak lightning strokes visible to an observer nearby but not strong enough to be detected by ATDnet – and a further strong charging mechanism operating within the plume, generating lightning with peak currents of at least 3 kA and detected by ATDnet.”








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