Rolling thunder

 

Once again, Helen Czerski's book Storm in a Teacup has got me thinking about simple ideas that I'd never considered. She tackles the idea of why thunder is often a prolonged rolling sound. The answer is to do with the distance away. The sound is made from all points on the tall discharge but because sound travels so much slower than light and the distances are big enough to make a difference, the sound from the top of the discharge arrives later than the sound from the bottom. In the example above, if sound travels at 330m/s, it will take 6.1 seconds for the sound to arrive. From the top it would take 6.8 seconds. That is a noticeable difference. However, I think that the further away you get the less the difference will be. Perhaps I'm not understanding this correctly.