WEATHER PHENOMENA

Two New Zealand scientists think they can explain one of the great mysteries of the natural world - ball lightning.

These bright, hovering spheres of light seen during thunderstorms have been reported as far back as the Middle Ages. They can be as small as tennis balls or as big as beach balls.

They can be white, yellow, orange or blue and have been said to pass through windows and walls. Some people have even claimed to see ball lightning pass through aeroplanes.

But despite numerous theories and attempts to recreate the phenomenon in a laboratory, ball lightning has remained an enigma to modern science.

Now, John Abrahamson and James Dinniss, from the University of Canterbury, NZ, have put forward the theory that these ethereal objects are nothing more than burning particles of silicon.

Their experiments show that when ordinary forked lightning hits the ground, mineral grains in the soil can be converted into tiny particles of silicon and its compounds with oxygen and carbon.

Burning fluff balls

These particles, less than a tenth of a micrometre (millionth of a metre) in size, then link up into chains.

The filaments then cluster together into light, fluffy balls that are carried aloft by air currents. The silicon particles are very reactive and burn relatively slowly, emitting light as they do so.

"Lightning penetrates below the surface of the soil and heats a certain portion of it to quite high temperatures, so that it vaporises," Dr Abrahamson told the BBC.

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