Ordinary lightning is familiar to most people. It is an electrical discharge from cloud to earth or cloud to cloud. The former is ‘fork lightning’ and is both common and dangerous. The latter is ”sheet lightning’ and can be seen from miles around when it seems to light up that whole part of the sky. All lightning flashes are very brief, of course. Believe it or not, sheet lightning has been misinterpreted as a UAP on at least one occasion I investigated. One of the rarer forms of lightning is known as “ball lightning”. We still do not fully understand its nature, and it seems that it is not absolutely necessary for thunderstorms to be around and about when it occurs (although they most often are).

Here is a description which clearly illustrates its properties:
“A young teenager was walking to see his girlfriend one frosty autumn night in 1973. Approaching the main Manchester to Birmingham railway line at a bridge in rural Staffordshire he heard a buzzing noise, and looking up observed a blue ball of light about one third of a mile ahead and climbing down an embankment towards the railway bridge parapet. He stopped and watched in amazement as this fuzzy sphere (which he estimated as one foot in diameter when it came closer) followed all the contours of the landscape in its descent. It then climbed up and over the bridge and followed the course of the railway’s overhead electricity wires, heading off at a moderate speed in a southerly direction. In all it was observed for about one and a half minutes. The weather was cold, but the sky clear and there was no local thunderstorms.”

One might easily be tempted to classify this as a “controlled” miniature UAP. In fact it classically illustrates most of the features of ball lightning. Following contours – or electrical sources – is common, but it can also move about erratically and explode with a pop or crack (or even silently). It is of course very dangerous indeed. It is normally spherical or oval, and blue or orange in colour. The size estimated in the above example is perhaps an upper limit, as is the duration. Size is no more than a few centimetres, and duration is normally but a few seconds. Not surprisingly, from all these factors, it is very commonly regarded as a UAP.
Indeed in some senses it still is a UAP, and it is by no means valueless to collect accounts of such observations. Scientists are interested in these stories, and Nature and New Scientist have carried reports of them.

Telltale signs: Spherical shape and colouration as das described, very small size, characteristic motions, attractions to electrical or metallic sources, duration usually in the order of ten to thirty seconds (although up to about two minutes is known), probable existent weather conditions.

Resources:
Tornado and Storm Research Organisation, Ball lightning research:
http://www.torro.org.uk/site/ball_info.php