Over the years, meteorologists have succeeded in explaining almost every atmospheric phenomenon you might think of. Two, however, have eluded them. There is still no convincing explanation for ball lightning, nor is it known why lightning, very occasionally, flashes from a clear sky.
Of the two, ball lightning is the more common. It usually appears as a red or orange "ball of fire", often described as six to 12 inches in diameter, drifting along two or three feet above the ground. It lasts only for a few seconds, and has an affinity for buildings which it enters through open doors and windows, or down chimneys. But although it has been seen on hundreds of occasions, scientists are still perplexed about its origins. The vast majority of sightings take place in thundery conditions when ordinary lightning has been seen as well, but the occurrence of ball lightning is so unpredictable that systematic observations of the phenomenon have so far proved impossible for scientists.
But even more rare is a flash of lightning from a clear sky. Lightning is almost invariably associated with a thundercloud, taking place from one part of a single cloud to another, from cloud to cloud or, most commonly, from a cloud downwards to the ground below.
It occurs when very large electrical charges of opposite polarity develop in different parts of a thundercloud, and there is a strong tendency for an electric current to flow from one to the other, to redress the balance. When this electrical tension becomes high enough, the pent-up energy is released in a sudden surge and burns a path through the intervening air, which we see as a stroke of lightning.
Essential to this theory, however, is the presence of a thunderstorm. Yet, for example, in September 1966 a lightning flash from an apparently clear sky felled 30 workers picking peppers in Alfrida, Arizona, killing three of them - a bolt, quite literally, from the blue.
And in another well-documented case, a single, blinding flash of lightning at Myrtleford, Victoria, Australia, on June 2nd, 1976, burned three football sized marks into the ground, and fused a highvoltage power-line, all on a perfectly clear and cloudless night.
In theory, this just could not happen: yet apparently it did. Should we treat such sightings as a sort of Hamlet's ghost: "Horatio says, 'Tis but our phantasy, and will not let belief take hold of him"? Or should we regard lightning from a clear sky as an accepted scientific fact? It remains one of the great unsolved mysteries of meteorology; a riddle wrapped in a mystery inside an enigma.