Thursday, 4 April 2002
A New Zealand scientist may have finally explained the mechanism behind the extraordinary phenomenon of ball lightning.
Associate Professor John Abrahamson, a chemical engineer at the University of Canterbury in Christchurch, explains his theory in the April issue of Physics World Digest.
Ball lightning appears as a glowing, hovering ball of light that moves slowly near the ground before disappearing or exploding. The ball usually measures about 30 cm in diameter, although two park rangers in the Australian outback reported seeing one in 1987 that was 100 metres wide.
There are hundreds of theories about why ball lightning occurs, said Associate Professor Abrahamson — but according to his theory, there are several basic requirements.
The first is regular lightning. Second, the lightning must hit a structure such as a building, soil, or a tree. The struck object must have a metallic or an oxide component.
"Soil contains silicon oxide and carbon," Associate Professor Abrahamson explained. "The lightning reduces this to silicon metal, which is how we make silicon industrially."
The silicon vapour condenses to form silicon nanospheres, which collect together in long strings.
The third requirement is the presence of 'fulgerides' — long sausage-like holes in the soil full of hot vapour. These are created by lightning hitting the ground.
(Pic: Copyright Ern Mainka Photography. Used with permission.)
"The lightning strike on soil digs a hole in the ground, forming a very hot channel," said Associate Professor Abrahamson.
"Geologists have dug them up afterwards and found them to be made of frozen molten glass oxides, often in the form of tubes."
The silicon vapour is then ejected back out of the soil, forming a vortex ring — "like a smoker's puff" — which forms a sphere.
Once in this shape, the ball can move long distances, said Associate Professor Abrahamson.
"You have quite a robust structure, which continues to oxidise, and stays hot and visible," he explained.
The layer of oxide on the surface of each of the particles slows the process down, until eventually each particle runs out of metal. At this point the ball either fades away or explodes.
Ball lightning is not as dangerous as regular lightning, but it does carry very high levels of energy.
"People have been killed by ball lightning," Associate Professor Abrahamson warned.
"The metal in the high-energy balls will react with any water-containing substance, including flesh. If you touch ball lightning you could be severely burnt."
Understanding ball lightning is useful because its chemistry relates to that of ordinary lightning, he added.
"People haven't thought about the chemistry of ordinary lightning very much — this fills in that part of it."
A special issue of the Philosophical Transactions of the Royal Society in January 2002, edited by Associate Professor Abrahamson, added to more than 10,000 scientific reports on the subject of ball lightning.
Danny Kingsley – ABC Science Online