Ball lightning

Now that we’re finally starting to see some hot weather, it won’t be long before we begin to see something else: thunderstorms and lightning (very, very frightening me! Galileo, Galileo…sorry, just a little Queen flashback).

It’s the lightning, of course, that makes thunderstorms thunder. If I may quote myself from a previous column, lightning “is a massive but short-lived electrical discharge in the atmosphere, usually several kilometres long.

“Lightning arises because of a charge separation in a cloud. A ‘charge separation’ just means that there are more electrons in one place than another. Cloud-to-ground lightning occurs when there are lots of free electrons in the base of the cloud. These electrons are discharged in what is called a stepped leader: ‘stepped’ because it descends from the cloud in discrete steps, each about 50 metres long (which is what gives lightning its jagged appearance), and ‘leader’ because that’s what it is–the precursor for the main bolt.

“Since electrons are negatively charged, this stepped leader has a very strong negative charge. When it gets within 100 metres or less of the positively charged ground, another leader moves up to meet it, often through handy protruding objects like buildings, trees and golfers.

“What we think is the main stroke of lightning is actually the return stroke, which propagates upward from the ground along the path formed by the leader and the stepped leader. Several subsequent strokes usually follow along the same path…because the stepped leader knocked electrons loose from molecules of atmospheric gas along the way, creating a channel of positively charged air, a path of least resistance…All these strokes come and go with the space of a second, traversing the distance between cloud and sky at up to half the speed of light, until the surplus of electrons in the lower part of the cloud is eliminated.”

That’s the lightning we all know and, um, love.

But there’s another kind of lightning whose nature is far more obscure: ball lighting.

Ball lightning, reported by thousands of people, consists of luminous spheres, typically the size of a grapefruit, said to last for a few seconds or even minutes, sometimes hovering, sometimes bouncing along the ground, sometimes doing weird things like burning through screen doors or bouncing on someone’s head.

In 2007, New Scientist noted that a theory by John Abrahamson and James Dinniss of the University of Canterbury in Christchurch, New Zealand, suggesting ball lightning could form when lightning strikes soil, vaporizing any silica in it, had been supported by experimental work in Brazil.

Abrahamson and Dinniss suggested that as the silicon vapour cools, it condenses into a floating aerosol, bound into a ball by charges that gather on its surface and glowing from the heat of the silicon recombining with oxygen.

To test the idea, a team at the Federal University of Pernambuco in Brazil vaporized wafers of silicon in an arc between two electrons. Sure enough, the arc occasionally spat out luminous orbs the size of ping-pong balls that lasted for up to eight seconds, moved randomly as their surfaces emitted little jets, emitted smoke trails, and were hot enough to melt plastic.

In the last couple of weeks, though, another explanation has come along: Joseph Peer and Alexander Kendl at the University of Innsbruck in Austria suggest that at least some “ball lightning” is actually a hallucination.

Noting that transcranial magnetic stimulation—stimulating the brain with a rapidly changing magnetic field–can cause subjects to see glowing discs and lines, they pointed out that the rapidly changing magnetic fields associated with repeated lightning strikes are powerful enough to cause a similar phenomenon in humans within 200 metres.

The strike has to be of a particular type in which there are multiple return strokes at the same point over a period of a few seconds, and of course the observer has to be uninjured, sitting inside a house, perhaps, which, sure enough, is where ball lightning is often experienced. Peer and Kendle estimate that roughly one percent of those who experience lightning close at hand without being harmed are likely to experience magnetic fields strong enough to induce hallucinations.

So, is ball lighting floating spheres of silicon gas, or all in the observers’ heads?

Since investigating either requires you to be in close proximity to powerful lightning strikes, I’m content to let the scientists figure it out.

Although if Ben Franklin were alive today, I’m sure he’d be right on it.

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