Why does lightning fork?

[neonsignal]

Why does lightning fork?

 

In images of lightning, you see a multibranching, dendritic looking structure. Why doesn't it just follow the path of least resistance in a straight line? Is it really branching, or is that the afterimage of multiple strikes?

[swansont]

Forgive the anthropomorphizing, but how would lightning "know" the path of least resistance?

[StringJunky]

Lightning does follow the path of least resistance but it may not be by the shortest route. The atmosphere between the thundercloud and the ground will have varying and localized levels of conductivity or resistance which will influence the lightning's journey hence its non linear path. This is one theory on how it works:

 

As a thundercloud moves over the surface of the Earth, an electric charge equal to but opposite the charge of the base of the thundercloud is induced in the Earth below the cloud. The induced ground charge follows the movement of the cloud, remaining underneath it.

 

An initial bipolar discharge, or path of ionized air, starts from a negatively charged mixed water and ice region in the thundercloud. Discharge ionized channels are known as leaders. The negatively charged leaders, generally a "stepped leader", proceed downward in a number of quick jumps (steps). Each step is on the order of 50 to 100 ft (15 to 30 metres) long but may be up to 165 ft (50 m).[21] As it continues to descend, the stepped leader may branch into a number of paths. The progression of stepped leaders takes a comparatively long time (hundreds of milliseconds) to approach the ground. This initial phase involves a relatively small electric current (tens or hundreds of amperes), and the leader is almost invisible when compared with the subsequent lightning channel.

 

When a stepped leader approaches the ground, the presence of opposite charges on the ground enhances the strength of the electric field. The electric field is strongest on ground-connected objects whose tops are closest to the base of the thundercloud, such as trees and tall buildings. If the electric field is strong enough, a conductive discharge (called a positive streamer) can develop from these points. This was first theorized by Heinz Kasemir. As the field increases, the positive streamer may evolve into a hotter, higher current leader which eventually connects to the descending stepped leader from the cloud. It is also possible for many streamers to develop from many different objects simultaneously, with only one connecting with the leader and forming the main discharge path. Photographs have been taken on which non-connected streamers are clearly visible.[22]

 

Once a channel of ionized air is established between the cloud and ground this becomes a path of least resistance and allows for a much greater current to propagate from the Earth back up the leader into the cloud. This is the return stroke and it is the most luminous and noticeable part of the lightning discharge.

 

http://en.wikipedia.org/wiki/Lightning

[neonsignal]

As it continues to descend, the stepped leader may branch into a number of paths.

 

Perhaps I could put the question in another way: what causes the spreading behaviour of the stepped leader? Indeed, what are the dynamics of the stepping?