How many strokes are in a cloud-to-ground flash?
Some flashes end after a single return stroke, but more often than not, there are sequels. Negative charge close to the top of the channel takes advantage of the already-created path, descending as a dart leader. This is a continuous and usually non-branching pulse traveling about ten times faster than the stepped leader. Each dart leader is discharged by a subsequent return stroke that carries perhaps half as much current as the initial stroke (or even less). A typical flash has four strokes; occasionally, more than ten are observed. The time between strokes is on the order of a twentieth of a second. Since this is just within the range of human perception, a set of multiple strokes appears to flicker. A multi-stroke flash may continue for as long as a second.
