The King of Storms: Supercells
It's twister season out in the US Midwest, with the long, straight roads of northern Texas, Oklahoma and surrounding statesÌýonce againÌýthe annualÌýhunting ground ofÌýhundreds of storm chasers and weather enthusiastsÌýfrom around the globe.
Be they professional orÌýamateur, they're all pursuing some truly awesome and rareÌýthunderstorms of a very specific variety: Supercells.
The colossus of theÌýCumulonimbus family, supercells are characterised by a persistently-rotating updraft (called a ) thatÌýensures the longevity of the storm cell; influences it's ground trackÌýand fuelsÌýit's propensity to deliver a variety of severe weather.
Sadly, the devastating nature ofÌýsuch extremeÌýweather on lives, homes andÌýbusinesses , as a supercell storm spawned a very powerful tornado - estimated at nearly a mile wide -Ìýwhich tracked acrossÌýChoctaw, Yazoo and HolmesÌýCounties, killingÌýten people, includingÌýthree children. Elsewhere, further tornados struck across neighbouring Louisiana, Arkansas and Alabama.
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But what causes such violent storms - and are we at risk of themÌýhere in the British Isles?
Well, thunderstorms are commonplace worldwide, but severe ones - in the strict meteorological sense - are not. Approximately 2,000 thunderstormsÌýare occuringÌýaround the world at any one time, with some 100,000 recorded globally every year.ÌýHowever, onlyÌý10% of these can be classified asÌýsevere. And of these, supercells are much rarer still.
To , some common factors must exist: deep instability in the atmosphere; moisture at low levels; plus a 'trigger' mechanism to force otherwise innocuous cumulus clouds to evolve skywards into giant storm cells. For example, this forcing mightÌýarise from the passage of a cold front; or when low-level winds converge and air isÌýsqueezed upwards; or from the lifting generatedÌýwhere air rises over high ground.
But for the atmosphere to cook-up supercell storms,Ìýa vital 'special'Ìýingredient is also required:Ìýstrong .
Rather than replicate hereÌýsome very good web examplesÌýillustrating this process, , which graphically shows how the critical influence of vertical shear can create a supercell storm, versus a 'normal' thunderstorm.
A colourful lexicon of meteorological and storm-chaser terminology has been developed to dissect this King of Storms into identifiable features. If you're after Overshooting Tops; Beaver's Tails, Wall Clouds or Flanking Lines, they're all here... See the graphics above andÌýbelow, courtesy of theÌýexcellent website hosted by theÌý...
I've been under supercells in the USA andÌýeven for the most ardent storm aficionado, they can prove decidedlyÌýnerve-wracking experiences.
About ten years ago - on holiday just outside Nashville, Tennessee -ÌýIÌýsaw a tornado warning issued on local TV and I ventured out to videoÌýthe weather, taking shelterÌýunder the eaves ofÌýa domestic garage.
The ink-black, turbulentÌýcloudbase beneath a massive supercell delivered torrential rain; wild, gusty winds from the storm's outflow and frequent flashes of lightning. In the comparative clutter of this suburban environmentÌý- with many trees and houses blockingÌýany clear, expansive view of the horizon - I appreciated just how difficult it would be to spot an approachingÌýtornado in such circumstances during daylight, let alone after dark.Ìý It was frankly aÌýclaustrophobic experience.
The has been developed over many decades, combining local coupled toÌýstate-of-the-artÌýmeteorological analysis; a network of trained ; an immediate media response to broadcast warnings and a generally high level of public safety awareness.
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A video of a tornadic supercell in Poland, August 2008. Three people were killed and several injured asÌýsevere storms lashed the country.
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What about closer to home? Supercells occur across many parts of Europe and in 2008 and 2009, caused some deadly weather in parts of France, the Netherlands, Germany and Poland. , but are exceptionally rare here. Nonethless, it's worth noting that the unique dynamics of these storms - with their rotating updraft - were first described not in the USA, butÌýin England,Ìýby notedÌýmeteorologists Keith Browning and Frank Ludlam. Their landmark study ()Ìýused radar to observe the characteristics ofÌýa spectacular hailstorm that tracked above Wokingham on 9 July 1959.
Last summer, I remember a couple ofÌýtimes - while on duty at the weatherdesk - watching supercells on radar and satellite formingÌýin various parts of Europe, including just across the Channel over France and the Netherlands. I'm not aware of any that formed across the British Isles in 2009, but what will 2010 yield? We shall see...
Have you experienced any European or British supercells? Share your experiences on the blog and feel free to send me any photos (instructions areÌýhere)Ìý- I'llÌýshow the best ones below.
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I'm Ian Fergusson, a Â鶹¹ÙÍøÊ×Ò³Èë¿Ú Weather Presenter based in the West Country. From benign anticyclones to raging supercell storms, my blog discusses all manner of weather-related issues. I also provide updated race weekend forecasts tied to our Â鶹¹ÙÍøÊ×Ò³Èë¿Ú coverage of Formula One. You can follow me on , too.
Comment number 1.
At 29th Apr 2010, Dan H wrote:I remember the supercell that developed off the south coast near Hastings on the evening of Sunday 15th July 2007. It then drifted slowly northeastwards across much of east Kent. It allegedly spawned a tornado somewhere between Fairlight and Rye (both in East Sussex), but I'm not sure if this was confirmed or not. To my knowledge this was the last supercell recorded in this country?
I was located in central-west Kent at the time and remember seeing an anvil in the distance drift across the eastern horizon as the sun set in the west. The anvil was lit up spectacularly with constant bright orange lightning - it was very eerie because there was no thunder (it was some 30 miles away from me) but just this constant lightning.
I managed to grab some footage of the lightning during the latter stages, but it's difficult to pick out the anvil etc because it was almost dark (around 9.30pm) when I filmed it:
There's a news report from the day here: (though the lightning strike from Edenbridge was actually caused be a separate storm some 12 hours beforehand)
Another viewer's footage, much closer to the storm than mine:
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Comment number 2.
At 1st May 2010, Swindon Queso wrote:Ian - what's the weather going to be like for the election on Thursday?
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