How a whirlwind works.
IF YOU WERE TRYING TO LOCATE the magical Land of Oz from the 1930's Hollywood film, you might want to start looking about five kilometres above the plains of Kansas. It's here that the tornado which picked up Dorothy and Toto for their whirlwind ride would probably have begun spinning its powerful vortex at the heart of a vicious storm, which, 20 years after The Wizard Of Oz, a British meteorologist christened a supercell. But in 1959 when Keith Browning pieced together the first accurate picture of tornadic storms he was working out of the Home Counties rather than Tornado Alley in the US. His breakthrough came from the analysis of a storm over (wait for it) Wokingham.
Making a twister
To create a supercell, take a storm where wind speed increases with height, while wind direction veers; a situation in which updraughts and downdraughts within the thunderstorm can support each other's existence rather than cancel each other out. It is as winds blow into this turbulent region from three to five kilometres up that a low-pressure section of the storm may begin to rotate.
The rotation of this part of the storm (known as a mesocyclone) causes the air pressure to fall some more, prompting wind lower down to flow into the storm and accelerate upwards. This creates a spinning updraught (christened the `suck zone' in the film Twister) which high-level winds in the storm can boost in the same way that wind blowing across the top of a chimney does wonders for drawing up an open fire.
You're not yet looking at a tornado, though if you're watching this particular storm develop you might start looking for a getaway car -- especially if the storm begins to change shape. When mid- to upper-level winds upwind of the storm encounter the supercell, some are forced to detour round it. They converge again downwind, moulding the storm clouds into an ominous anvil-shape in the process. But while some wind goes round the mesocyclone, some runs full square into this meteorological brick wall and is forced downward, creating a `rear flank downdraught' (RFD) which many experts believe is what makes or breaks a tornadic storm.
It's when an RFD tries to swing around the base of the storm, narrowing the area of wind flowing into the updraught and increasing its spin (in the same way figure skaters turn faster when their arms are pulled in) that you might want to get into your getaway car. If you're anywhere beneath this whirling piece of meteorological give and take -- a funnel cloud -- you are in a bad, dangerous place known to stormchasers as `the bear cage'. It's where, if the funnel cloud sticks around long enough for the updraught to touchdown on terra firma, you will find yourself on the inside of a tornado.
Only in Hollywood fantasy would you expect to walk out of this, but for a scientist wanting to get inside a twister there is the `turtle'. Named for its shape, this 20-kilogram instrument is a front-line tool in America's VORTEX project (Verification of the Origins of Rotation in Tornadoes Experiment). Designed to survive wind speeds of up to 480 kilometres per hour, turtles are placed at 90-metre intervals on the ground where a tornado may pass, leaving human measurement-takers free to probe from a distance, checking on things from temperature gradients close to a tornado to wind speeds inside a twister. The latter are measured with Doppler radars, similar instruments to the ones police use in speed traps. They use microwave pulses bounced off raindrops or ice particles within the storm.
But this all takes place on the other side of the Atlantic, right? Wrong. Measuring tornado frequency in relation to geographic area, the country with the highest frequency of tornados in the world is the UK.
"People forget that the USA is 39 times larger than the UK," says David Reynolds of Britain's Tornado and Storm Research Organisation (TORRO). "What might happen 39 times a year in the US [in terms of a serious tornado] only happens once here." So while the average number of tornados in Britain is in the 30s, it's nearer a thousand in the US. But this is just an average and the number of tornados in any one year can vary enormously. On just one day in November 1981, for example, an incredible 104 tornados broke out across England and Wales. No wonder British experts hate it when UK twisters are labelled `freak' events.
The misconception that tornados are just `Yankee devils' is fed, says Reynolds, by two factors. The greater use of wood in US building construction makes it likely that a twister dancing through a Midwest town is going to do more damage than if it tried the same trick against good old British brick or stone. The misguided US decision to adopt a scale of tornado measurement (the Fujita Scale) based on damage caused rather than the much less ambiguous scale used elsewhere in the world, the Tornado Intensity Scale (TIS), sometimes known as the TORRO scale, adds to the puffing up of US tornados in the public eye. Devised by British expert G Terence Meaden, the TIS is a natural extension of the Beaufort scale familiar to anyone who has listened in to the Shipping Forecast ("... Viking Forties Cromarty Gale Force 7 ..."). Directly related to wind speed, it's far less prone to the ambiguity and over-estimation of tornado strength that mars Fujita's effort.
While the Americans over-estimate their `naders, we often underestimate ours thanks to problems with visibility caused by them being wrapped in rain or hidden by haze rather than whirling across the wide open plains of Texas. But, don't panic, most tornados (wherever you live) are more likely to knock a few tiles off your roof than relocate your entire house. Which is not to say that the best (or perhaps worst) of Europe can't compete with anything America has to offer.
The worst recorded Stateside tornado touched down in Missouri on 18 March 1925, a `Super Tornado' (T1O-11 on the TORRO scale) that cut a 350-kilometre swathe of destruction into Illinois and Indiana, killing 695 people in three hours. But similar T10-11s have also struck France (Montville, 19 August 1845) and Italy (Treviso, 24 July 1930), though causing less loss of life. Britain's two worst tornados were T8s (file under `severely devastating'), the first striking the fear of God into locals by ripping apart the church at St Mary Le Bow, London, on 23 October 1091, and the second tearing through Hampshire on 22 September 1810.
Yet TORRO warns against complacency. "Our concern is for when, not if, the next violent tornado occurs in the UK," says Dave Reynolds. "Improved public awareness about severe weather that occurs in the UK [could] stop panic to a tornado warning and save lives." Perhaps you should think about another slice of American popular culture next time it gets a bit windy. "Let's twist again, like we did last summer ..."
WANT TO KNOW MORE?
* You can find TORRO, Britain's Tornado and Storm Research Organisation at www.torro.org.uk
* Alternatively, USA Today has excellent links to many tornado and storm-chasing sites. Find it at: www.usatoday.com/weather/tornado/
* If weather interests you, our Internet round-up on page 80 features climate-related sites.