Why does the thunder from close lightning produce a BOOM and then a slow fade off? Why does it fade off instead of just stop? Conversely, why is thunder from far away a slow fade-on instead of an instant but quieter boom?

Question

Why does the thunder from close lightning produce a BOOM and then a slow fade off? Why does it fade off instead of just stop? Conversely, why is thunder from far away a slow fade-on instead of an instant but quieter boom?

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Answer 1

The shock-wave (thunder) forms along the entire lightning bolt, which can be several miles long. The initial crack comes from the closest part of the bolt. The rollng continues as the rest of the shockwave reaches you.

A shockwave is literally a wave of compressed air that travels at sonic speed. As the shockwave proceeds, it weakens and spreads out, lengthening the loud crack into a sinister roll.