I learned all about lightning when I was presenting a science show called “Lightning Strikes” using a MASSIVE tesla coil in a big cage which I got to strike a small plane, a car and other stuff with! So hopefully I get the answer right to this one:
Lightning moves at the speed of light (300,000,000 m/s really quick) whereas the sound from it travels at the speed of sound (340 m/s = much slower). Lightning is usually either between clouds or down to the earth, and it travels a big distance while it strikes (or discharges…).
The “rumble” sound starts when the sound from the nearest part of the ‘strike’ reaches you and lasts until the sound from the farthest part reaches you, which can be many hundreds of metres or even kilometers from where the lightning started. That’s why the sound can last for a few seconds.
I hope that answers your question, I love talking about lightning so just ask if you have any more questions!
Where as the lightning bolt is very fast, the sound wave it produces is much slower – lightning bolts further away from you means that the sound wave has more time to spread out before it reaches you – lightning bolts closer to you don’t make as long a sound.
Well what you see is the electricity of the lightning strike ionising the air and making it glow – what you hear is the sound waves produced by changes in pressure and temperature around the lightning. This echoes and changes as it moves across the landscape.