A stroke of lightning may take 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. Whichever the ease, the visible flash is an incandescent channel of air, no more than an inch or two in diameter, but heated by the electricity passing through it to a temperature of 30,000 degrees Celsius or more.
Meteorologists are not entirely sure of the precise mechanisms in a thundercloud that result in lightning. We do know that even in very fine weather, electrical charges build up in the atmosphere, some regions becoming positively charged, and others negatively so indeed we experience the phenomenon in a harmless, albeit sometimes painful, way when we receive a slight shock from static electricity on making contact with a metal object. And we also know the process occurs most spectacularly in the large towering cumulonimbus clouds that we associate with thundery conditions.
It may be that the build up of electricity is the result of raindrops moving up and down in currents of air through a small initial electrical charge. Or it may be that the cause is friction between ice crystals in the cloud. Whatever the reason, very large electrical charges of opposite polarity develop in different parts of a cumulonimbus cloud, and there is a strong tendency for an electric current to flow from one to the other, to redress the balance. If this electrical tension becomes high enough, the pent up energy is released in a sudden surge and bums a path through the intervening air.
We see the surge as an almost instantaneous stroke of lightning, which lasts for a mere hundredth of a second. Indeed Juliet, you may remember, bewitched but not befuddled by the callow youth mooning near her balcony, found it a useful simile with which to berate the hapless Romeo on the brevity of his midnight trysts:
It is too rash, too unadvised, too sudden,
Too like the lightning, which doth cease to be
Ere one can say It lightens'.
The lightning stroke is often "jagged" in appearance, as the electricity zig zags its way to find the electrical path of least resistance, and sometimes forks in two or more directions. The popular distinction between "forked" and "sheet" lightning, however, is a distinction without a difference the former is a stroke of lightning whose path is clearly visible, while the latter is merely the diffuse glow that can be seen if the flash of lightning itself is obscured by clouds of rain.