What is ball lightning?
Ball lightning, also called globe lightning, is a rare aerial phenomenon in the form of a luminous sphere that is generally several centimetres in diameter. Britannica It usually occurs near the ground during thunderstorms, in close association with cloud-to-ground lightning. Britannica
It may be red, orange, yellow, white, or blue in colour and is often accompanied by a hissing sound and distinct odour. Britannica It normally lasts only a few seconds, usually moving about and then vanishing suddenly, either silently or explosively. Britannica Ball lightning has been reported to cause damage by burning or melting but is usually harmless. Britannica
The Met Office describes ball lightning as an incredibly rare form of lightning in which a persistent and moving luminous white or coloured sphere is seen. Met Office
Ordinary fork lightning, the RMetS article notes, is an atmospheric electrical discharge, typically formed when a negatively charged leader from a cloud is met by a positively charged streamer. RMetS
According to other accounts collected by the RMetS article, ball lightning ranges in size from a golf ball to a small car. RMetS The same accounts say it can hover in the air, roll along the ground or come crashing through a window, lasts between a few seconds and a couple of minutes, and can simply disappear or violently explode. RMetS
Abrahamson and Dinniss, in a 2000 Nature letter, write that the “average” ball lightning appears as a sphere with a diameter of 300 mm, a lifetime of about 10 s, and a luminosity similar to a 100-W lamp. Nature 2000 In their wording it floats freely in the air, and ends either in an explosion, or by simply fading from view, and almost invariably occurs during stormy weather. Nature 2000
Summarising earlier reports, they write that it is usually a sphere or ellipsoid with a diameter between 1 and 100 cm, moving horizontally with speeds of a few meters per second, and that the color can be white, yellow, red, orange, purple, or green. PRL 2014
How big, and for how long?
Good sources do not give one settled typical size or lifetime. Encyclopaedia Britannica says ball lightning is generally several centimetres in diameter and normally lasts only a few seconds. The Royal Meteorological Society, citing other accounts, says it ranges from a golf ball to a small car and lasts between a few seconds and a couple of minutes. Abrahamson and Dinniss, writing of an “average” ball lightning, give a diameter of 300 mm and a lifetime of about 10 s. Cen, Yuan and Xue, summarising earlier reports, give a diameter between 1 and 100 cm, and their own 2012 event lasted 1.64 s. This site sets the figures side by side and does not pick a single typical size or time.
Sometimes bead lightning is mistaken for ball lightning. Britannica Bead lightning is most apparent when the current in a cloud-to-ground flash persists for an appreciable fraction of a second. In those cases the luminosity also persists and the channel may have regions of enhanced luminosity that resemble a string of beads. Britannica
The Met Office also lists pearl-necklace lightning, also termed chain lightning or beaded lightning, in which variations of brightness along the discharge path give rise to a momentary appearance similar to pearls on a string. Met Office
Bead or pearl-necklace lightning is a lingering, beaded flash along a channel. Ball lightning, in the reports, is a separate glowing sphere that can move about. The two are easy to mix up, which is why Britannica and the Met Office mention both.