What causes the sound when lightning strikes?

[i.mehrzad]

Well I was just looking out in the sky one thundering night and was just wondering whar is the cause of the flashy sound when lightning does occur. Is it the sound that is generated when the Dielectric breaks down?
If so why??
Or is it that when lightning strikes the friction between the two clouds are at their highest level so therefore the sound generated then is most. If this is so then what is the cause of maximum friction only when lightning strikes?
Well i may be speaking in very vague terms but just to add, I was looking out in the sky one thundering night when my vacations were on.

 

[Astronuc]

Thunder arises from the propagation of a pressure wave which is cause by the rapid heating of air by the lightning. The electrical discharge causes the air to heat to about 30000K, within fractions of one second (1 s), and the air then rapidly expands at the speed of sound.
http://hypertextbook.com/facts/1999/DavidFriedman.shtml

 

[DaveC426913]

What is also interesting is that you can recreate (roughly) the direction and shape of the lightning bolt merely from the sound.

A crack is heard when a bolt passes transverse to the observer; a rumble is heard when the bolt is traveling toward or away from the observer.

So, 'crack rumble crack' is a bolt that passed transverse to the observer, then turned toward or away from the observer, then back to transverse.