Ball lightning - a slow-moving ball of light that is occasionally seen at ground level during thunderstorms - has puzzled scientists for centuries. There have also been reports of ball lightning in aircraft, but the origins of this phenomenon have remained a mystery. Now John Gilman of the University of California at Los Angeles has suggested that one of the properties of ball lightning - the cohesion that keeps the ball together for periods of tens of seconds - can be explained in terms of Rydberg atoms (J Gilman 2003 Appl. Phys. Lett. 83 2283). However, other researchers in the field disagree. It is generally believed that ball lightning is a ball of plasma, and some scientists believe that it is similar to the highly luminous plasma disks that are formed when explosives are detonated. The ball can typically can measure 30 centimetres across. Many theories on ball lightning have been put forward over the years but none can explain all of the characteristics observed."A good theory should also be able to account for the generation of the ball lightning, the constant light output for about one second at about the 60 watt level, the colour - usually red or yellow - and a variety of other factors," says Martin Uman of the University of Florida. None of the existing theories - including Gilman's - can do this, says Uman.
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