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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The loudness of something is the pressure difference in compression/ratification of molecules in air, and as that pressure moves out from the source, its energy spreads out. The sound right next to a lighting strike is very high because most of the energy is not all that spread out yet, but at a great distance, a small fraction of the energy reaches your ears.
The “fade” is *reflection,* or reverberation, off of surfaces. Sound waves that moving away from you relative to the lighting strike take time to hit something hard, bounce, and come back toward you, so they come later, and the spreading out of the energy makes them quieter.
If it was just one thing it bounced off of, it would sound like a repeat, or echo, but because the sound is bouncing off of *everything,* it is more of a “smeared” sound, a rumble rather than a crack. Think of it as thousands of small, quiet echos at random and mixing with each other.
That’s also why far-off-thunder is less distinct, because lots of different echos mess with each other by the time any of the sound reaches you. Technically, if there’s a direct line from you and the lighting with nothing in between, it won’t “fade in” and still start with a boom, but most far-off lighting strikes have so many things in the way you actually only hear different echoes that are so mashed together it seems like it’s fading in first.
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