As a thunderstorm builds, the difference in potential between the cloud base charge and the ground charge increases. The lightning strike begins with the formation of stepped leaders branching downward from the cloud charge. These stepped leaders propagate in jumps of about one hundred and fifty feet, working their way downward toward the ground charge. Stepped leaders are the tendril-like branches visible in a photograph of a lightning strike. We see a lightning strike in two dimensions, but it has depth too, so there are an entire field of stepped leaders.
When the stepped leaders after traveling several kilometers reach to within a hundred meters or so of the ground, the attraction becomes so intense that objects and structures on the ground begin to break down electrically and respond by shooting off streamers of ground charge upward toward the stepped leaders. Whichever streamer reaches a stepped leader first “wins” the competition and completes the ionized channel that becomes the lightning strike. Occasionally, two or more will meet simultaneously, and forked or branched lightning will occur. The other streamers and stepped leaders simply never mature and dissipate.
Once the ionized path is completed, the lightning discharge occurs. Although a strike may appear to be a single flash, it is actually a series of flashes. The lightning discharge occurs for approximately one thousandth of a second, shuts off for approximately two hundredth of a second, on for one thousandth, off for a couple of hundredth, repeating the process multiple times. When the difference in potential between the cloud charge and ground charge is no longer sufficient to continue the discharge, the event ends.
A lightning transports a massive amount of energy, typically resulting in currents of hundreds of kiloampere and voltages of hundreds of kilovolt at the structure where it strikes.