Answer. Thunder is caused by the rapid expansion of the air surrounding the path of a lightning bolt. ... However, the grumbles and growls we hear in thunderstorms actually come from the rapid expansion of the air surrounding the lightning bolt.

What does it mean when thunder is really loud?

If the thunder sounds more like a rumble, the lightning was at least several miles away. The loud boom that you sometimes hear is created by the main lightning channel as it reaches the ground. ... Remember, if you can hear thunder, chances are that you're within striking distance of the storm.

What does it mean when you hear thunder?

If you can hear thunder, you are close enough to the storm to be struck by lightning. Lightning can strike as far away as 10 miles from the area where it is raining. If you can see lightning flashes, count the seconds after a flash until you hear thunder.