If I understand correctly, lightning is the discharge of electricity from the atmosphere into the planet. However, if I switch on a lamp, the wires are not causing thunder (or any audible sound).
I've also heard that the thunder comes from lightning breaking the sound barrier. This sounds weird to me since I would assume that lightning would be traveling at, well, light speed, so I'm not sure how the threshold could be crossed.
How does lightning cause thunder?
Answer
The sound barrier theory is complete nuts. However, lightning does not travel at the speed of light. In fact, when the voltage between the cloud and the Earth reaches a critical level, many small low-intensity lightning bolts propagate more or less randomly downwards, ionizing the air along the way, forming a channel of conductive ionized air. Once one of those bolts reaches the ground, the actual bolt is formed from the bottom up into the sky. This can be observed in this video I found on Youtube.
So what does cause the thunder. When the lightning strikes, the air along its path rapidly heats up and expands. This pressure wave is perceived as sound.
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