OK,
here’s my explanation on how I made the THunderStom image posted here recently.
http://www.selleri.org/Blender/tuts/Lightnings.pdf
I’d bet there are better ways. I should have tweaked more all settings 
Seeya
Stefano
hmm, I thought you were a professor? I thought lightning actually travelled up?
besides that. really usefull tutorial. but that other way was nice too
Roel
Actually i though lightning goes both up and down (obviously not at the same time). I learnt something about charged particles in the attracting opposite charges…or something like that…but thats besides the point. The point is, cool method dude.
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Yup,
lightning direction is not so predetermined. Furthermore it depends if you consider the direction as the direction of the CURRENT, of the ELECTRONS (which is the opposite 
) or the direction going from where the air insulating properties break off first 
Thanx for comments,
Stefano