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

Throw a brick into a pond. One very sudden impact, and over before you know it, but the waves ripple out and about for minutes. Something doesn’t have to be happening all the time for its consequences to be happening all the time.

As for the fading, notice how the waves are taller close to where the brick hit, and flatten out as they move further away from the brick and stretch out.

The same happens with lightning. It stirs up the atmosphere in a moment, but the waves in the atmosphere behave almost exactly like the waves in water: be nearby and you hear a tall wave (a lot of compressed air), be further away and you hear a short wave (less compressed air, because it spread out and evened out more before reaching you).