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What's Making A Weird Glow At The Centre Of A Twister?

New Scientist

The last place most people would try to start a fire is inside a tornado. But two researchers who have pulled off a similar trick in a lab in New Zealand say their experiment may explain enigmatic weather phenomena such as ball lightning.

At first glance, tornadoes don't appear fire-friendly. Even at the calm centre of the whirlwind, there is enough of an updraft to make any flame tenuous, and the fast winds at its edge would blow out any blaze.

Yet fireballs have been reported in some tornadoes, such as the twister that struck Dorset in Britain in 1989. Vortices have also been associated with floating spheres of ball lightning, which sometimes disappear with a loud explosion, suggesting they, too, contain combustible material.

So John Abrahamson, a chemical engineer at Canterbury University in Christ- church, was intrigued when his former student Peter Coleman proposed trying to create a fireball in a mini tornado. They reckoned one might form in the vortex breakdown region, where air moves relatively slowly. "If it was coloured, you'd see this doughnut of air," says Abrahamson.

Intriguingly, the vortex breakdown region is used in "vortex burners", in which a flame burns in a closed, horizontal cylinder. A horizontal vortex mixes and contains hot gases so that the fuel burns efficiently. But it was unclear if the combustion would be stable in a free-standing, vertical vortex.

To find out, Abrahamson and Coleman built a circular chamber about a metre wide. Slats at the base allowed air to enter at various angles and an extraction fan pulled air upwards from above. This created a vortex 10 centimetres wide. Liquefied petroleum gas was introduced into the breakdown region through a pipe and was ignited with a spark plug.

The vortex produced a stable fireball if the air entered at an angle of 66 degrees. Whether the fuel pipe was above, below or to the side of the vortex breakdown region, the fuel was drawn into the doughnut of air and burnt as a sphere.

Abrahamson concludes that if a natural vortex swept up fuel from the ground, and if something like a lightning strike or power line ignited it, this could form a stable fireball. His experiment will be described next month at a meeting of the American Geophysical Union in Boston.

"There are many theories about ball lightning, but not many of them can be studied so thoroughly in a lab as this one," notes Stanley Singer, president of the International Committee on Ball Lightning in Pasadena, California. However, he adds that some reports suggest ball lightning can pass through solid objects-something that is hard to reconcile with a combustion theory.

Abrahamson points out that many of those reports have been called into question. Even if his experiment fails to explain meteorological mysteries, he believes it may find uses in industry. "I don't think anyone has ever created a vortex to control an open flame before. It could be useful." The experiment could even explain some UFOs, adds Coleman. "Some pictures of supposed UFOs I've seen look like classic fireballs."

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Author: Philip Cohen
New Scientist issue 22 May 1999

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