Ball lightning is very unusual event. The eye-witness reports have certain aura of medieval witchcraft or magic about them. Luminous balls have been reported entering homes. One report said a ball went right through a stationary tram car drilling huge holes at the points of entry and exit. Another spectacular report was of a luminous sphere rolling down the inside of an all-metal aircraft in flight. In the past, sceptics have dismissed the accounts as figments of the imagination. However, genuine sightings have been turning up and the phenomenon is aquiring a scientific status.
The spheres have been seen in daylight. They have varied in diameter from one to 100 cms, and have ranged in colour from blue to red. Some have been reported moving horizontally, some vertically, some erratically and some have even been stationary. The most enigmatic part of the phenomenon is, how the balls remain coherent, stable and luminous for durations upto, apparently, some minutes. The balls dissappear as suddenly as they appear, without trace, and often explosively.
The observed properties of the balls are so puzzling and bizarre that none of the several theories proposed have been able to provide a satisfactory explanation. The theories proposed can be broadly classified into two types: (1) those postulating an energy source within the ball, and (2) those postulating an external energy source that keeps the object stable througout its life.
The early theories in the first category, based on ionic recombination, failed to explain the observed long lifetimes of the balls (the ions and electrons were assumed to be produced in the vicinity of a lightning stroke). Various plasma theories have been published, but these run into problems of stability. Even nuclear theories have been tried, but in such a case, the observer would receive enough radiation dose to cause radiation sickness or even death, but this has not been reported so far.
The external energy source in the second category has been ascribed to a very high-frequency electromagnetic field or a steady current flow from cloud to ground. One of the most promising theories is that electromagnetic waves are generated after a lightning stroke and a standing wave system is set up by the interference effect between these waves and the reflections from conducting surfaces. At the antinodes, where the intensity of the field is greater, ionisation is caused by the absorption of energy from the field creating a lightning ball which continues to absorb energy and later moves to a node. The theory may account for many of the enigmatic properties of luminous balls, but there is no evidence that the required amount of ultra high frequency (UHF) electromagnetic waves are generated by thunder produced by an ordinary lightning stroke.