Thunderstorms contain very strong electrical charges. Through processes I won't go
into (because it will take way too long!) the ground acquires a strong positive charge
and the lower section of the cloud acquires a strong negative charge. This is the case
for the majority of thunderstorms. There are other differing charges in other parts of the
cloud, but for this description we'll only need these. As you probably know, opposite
charges attract. The storm is constantly generating more charge, and when the force of
attraction between ground and cloud becomes large enough, the air between them is
vulnerable to electrical breakdown. Air is not a good conductor, but when electrical
breakdown occurs the 'broken down' air is a much better conductor. The 'broken
down' air is called ionized air. The air 'breaks down' or ionizes in a chain-reaction type
fashion starting at the bottom of the cloud and moving downward. The air ionizes in
narrow paths that split apart and branch out as the 'chain reaction' moves toward the
ground. The 'chain reaction' moves downward in steps, instead of in a smooth,
constant flow. Picture the 'paths' as sticks being laid end-to-end and every moment
adding another stick to make the 'path' longer. These downward-moving 'paths' of
ionized air are collectively called the stepped leader. The stepped leader dimly lights
up every time a new 'step' is formed, but we do not see this because the stepped leader
moves downward very rapidly and because the actual 'bright' lightning we see occurs
so close in time after the stepped leader. Since the bottom of the storm cloud has a
strong negative charge, the stepped leader that is propagating (meaning moving out
from) down from the cloud also has a strong negative charge. When this stepped leader
gets close to the ground, (around 100 yards or so up) there is electrical breakdown that
is initiated from the ground up moving upward in paths similar to the stepped leader
except that the upward-moving 'paths' are positively charged and they do not move up
in 'steps'. These upward moving 'paths' are called leaders. There may be many leaders
initiated by one stepped leader. By the time the stepped leader gets that close to the
ground, it has many branches, so there is sort of a 'race' for which branch will reach
first. Whichever downward-moving branch touches an upward-moving leader first,
'wins'! When there is a 'winner', there is now a complete path of ionized air connecting
the ground and the cloud, like a big, long wire. When this connection is made, the
opposing charges equalize themselves rapidly by flowing through this 'wire'. Even
though the ionized air path is a better conductor than air, it is, in effect, overloaded by
the intense current flowing through it, so it and all of the 'branches' light up brilliantly and
heat up violently, like a big, long 'instant' heating element! This flow of current is called
the first return stroke, and that is the 'lightning' that we see.
The path of air heats to a temperature greater than the surface of the sun, and it heats
up that much in a split second! When air is heated, it expands, and when it is heated
that fast and that violently, it expands outward explosively! This 'explosive' expansion
generates loud sound waves moving outward from the 'path' in all directions, and those
sound waves are the thunder we hear. Sometimes more charge is deposited at the
bottom of the cloud by other discharges within the cloud immediately after the first
return stroke. Since there is already a path of ionized air from the original stepped
leader, the 'new' negative charges flow down the same path as the first return stroke in
a smooth (not stepped) motion towards the ground and 're-ionize' the path of air. This
smoothly-moving negative charge is called the dart leader, and like the stepped leader,
it moves too fast for our eyes to see. When the dart leader reaches the ground, there is
another flow of current and another return stroke, which is not as bright as the first
return stroke, and which does not illuminate the 'branches'. This 'dart leader process' is
often repeated several times, making the lightning seem to 'flicker' with each new return
stroke. The return strokes are roughly 1/30th of a second apart, so it is impossible to
see the dart leaders.