During thunderstorm conditions the turbulence in the cloud
causes the charges to
separate in such a way that the negative charges concentrate
in the base of the cloud. Since like charges repel, some of
the negative charges on the ground are pushed down away from
the surface, leaving a net positive charge on the
surface.
Opposite charges attract, so the positive and negative
charges are pulled toward each other. since the negative
charges (electrons) are many thousands of times smaller than
the positive charges (ions--charged atoms) they move much
more easily and cover most of the distance. This first,
invisible stroke is called a stepped leader.
As soon as the negative and positive parts of the stepped
leader connect there is a conductive
path from the cloud to the ground
and the negative charges rush down it causing the visible
stroke. The channel created by the stepped leader is full of
relatively static charge, like a line of cars at a red
light. When the two parts join, it is like that light
turning green, and just as the cars near the light start
moving first, so do the charges near the join. Since it is
the fast-moving charges that create the light, the visible
stroke actually travels upwards, even though the charges are
moving downward!
