Å®ÉúСÊÓƵ

A novel chemical theory for ball lightning has been proposed by a British scientist. He claims the theory explains two of the phenomenon’s most puzzling characteristics – its stability and its tendency to stay near the ground despite a natural buoyancy.

Ball lightning has been reported thousands of times over the past few centuries. Because of the remarkable consistency in the descriptions, many scientists are convinced the phenomenon is real.

Almost always sighted during thundery weather, ball lightning appears as a free-floating globe of glowing gas, usually between the size of an orange and a grapefruit. The balls have exhibited a range of colours and brightness although they rarely give out much heat. Often, they move across the ground at a height of about a metre and at at speeds up to 10 metres per second. The phenomenon rarely persists for more than a minute before the ball explodes with a bang or simply disappears.

David Turner, a scientist at the University of Bristol, has submitted a new theory of ball lightning to the Royal Society. In common with the models proposed by other scientists, Turner’s reflects his scientific interest. ‘Physicists apply physics to the problem,’ he says. ‘Well, I’m a chemist so I have applied chemical thermodynamics.’

Turner’s speciality is the behaviour of salts in steam. Working at the Central Electricity Generating Board before it was privatised, he advised modellers on the thermodynamics of what would happen in an accident at a pressurised water reactor. Such a nuclear accident would undoubtedly release a large amount of steam, and this could influence how much radioactive caesium iodide would be released.

Very similar conditions prevail in a thunderstorm. The air is very humid, and ions may be created when intense electric fields cause the air to ‘break down’.

Turner does not address the question of how such fields are created, saying that many other researchers have tackled this problem. Instead, he starts by assuming that the air is already broken down to create a localised ‘plasma’ of ions at a temperature of anything up to several thousand degrees. He then uses his knowledge of chemistry to model what happens around an ion escaping into the surrounding cool humid air, which is electrically charged.

Turner finds that a structure similar to an onion’s is set up around the central plasma, with successive spherical shells dominated by different chemical processes. For instance, immediately outside the core of plasma is a cooler zone in which most of the high-energy ions recombine rapidly, releasing heat. Outside this is the ‘hydration zone’ in which the temperature is low enough for ions to accumulate large numbers of water molecules. It is the thermodynamics of this zone which is crucial to Turner’s model, and which he says no one else has considered.

Turner has calculated what would happen to common ions, such as H3O+ and NO₂ ^–, when they combine at the edge of the ball. He has found that if the ions are carrying more than about five water molecules, heat is actually removed from the surroundings. ‘It’s as if water is evaporating off the ions,’ says Turner. ‘Evaporation naturally causes cooling.’ The effect is to refrigerate the outer surface of the ball. Indeed, there have been some reports that ball lightning is unusually cool.

‘The surface refrigeration allows the condensation of water in quantities sufficient to counteract the buoyancy of the hot plasma,’ says Turner. In short, water droplets weigh the ball down.

Water droplets are also involved in Turner’s explanation of the stability of ball lightning. Turner points out that the formation of nitric acid is likely in ball lightning. This would require a steady supply of oxygen and nitrogen from outside the ball.

Nitrate ions (NO₃ ^–) would form and provide nuclei for the condensation of nitric acid droplets. These droplets in turn would impede the flow of oxygen and nitrogen into the ball. ‘The droplets would feel an inward pressure which balances the outward pressure of the hot plasma,’ says Turner.

In effect, the inward flow of gas past the droplets would provide the ball with a surface tension. ‘The surface tension is roughly equivalent to that of a child’s rubber balloon,’ says Turner. This would explain the baffling stability of the ball and also the reported ability of ball lightning to bounce along the ground.

Turner has submitted a paper for publication in the Philosophical Transactions of the Royal Society, having been encouraged to do so by Brian Pippard, a physicist at the University of Cambridge. Pippard became interested in the subject after a bright ball was seen inside the Cavendish Laboratory, where he works. Pippard says Turner’s ideas are ‘interesting’, though, being a physicist rather than a chemist, he is unable to judge them in detail.

This highlights a problem faced by people proposing theories of ball lightning. Theories cross scientific disciplines so journals are unable to find referees to judge whether they are flawed. ‘In these circumstances’, says Pippard, ‘there is always a danger that a good idea is discarded because nobody can say that it is good.’

Brian Burrows of the lightning test and technology group at Culham Laboratory near Oxford says Turner’s ideas ‘sound more promising than anything I’ve heard heard of’. But Burrows points out that Turner’s theory may not be able to explain sightings of ball lightning inside high-altitude aircraft. ‘The air inside such aircraft is very dry indeed,’ he says.