Source: BBC News Sci/Tech
February 3, 2000
New Lead for Fireball Riddle
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
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.
"And then, when the lightning strike has finished, the vapour is free
to erupt, to appear above the ground in the form of a ball.
"The jet of hot gas will be very much the same as the jet coming out
of one's mouth when one blows a smoke ring - it forms a little
re-circulating vortex and it's quite self-contained."
The scientists' model predicted that heating above a given starting
temperature would lead to an explosive end for a fluffy ball, whereas
with a lower starting temperature the ball would melt and fade away.
Because the ball would become visible only over the latter part of its
lifetime, it would appear to materialise out of thin air after a
One theoretical umbrella
And Dr Abrahamson believes their theory will even explain how ball
lightning passes through windows and walls.
"Most, especially old, houses have cracks around their windows and
cracks near doors," he says.
"The network of silicon filaments should be very flexible and move
wherever air moves. So if air can get through a crack as a draught,
the ball should be able to squeeze through, and then rearrange itself
on the other side."
Unfortunately, the researchers have not been able to generate ball
lightning in the laboratory. But Dr Graham Hubler, of the United
States Naval Research Laboratory, who has taken a keen interest in
lightning balls, says the research has much promise.
"I have followed the theories that have come out over the years, and
few can explain all of the features of ball lightning. This one,
however, unifies an awful lot of the properties of ball lightning
under one theoretical umbrella, so I think it stands a very good
chance of perhaps being correct.
"It's not necessarily the whole story, or even the story, but the nice
thing about it is that it can be tested experimentally. Then we'll
know whether this theory has any merit in the future."
Both Dr Hubler (hubler@...
) and Dr Abrahamson
) are keen to hear from anyone who
has pictures or video footage of ball lightning.
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