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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Because it’s a wave of energy moving past you. A close wave has no ability to roll on; you’re too close to it’s propagation.
Edit: National Weather Service has the best answer. It’s from the heat of the lightning causing rapid expansion of molecules at each little point of the lightning as it shoots to the ground and branches off from the main ‘trunk’ of lightning –
>Regardless of whether lightning is positive or negative, thunder is produced the same way. Thunder is the acoustic shock wave resulting from the extreme heat generated by a lightning flash.
>Lightning can be as hot as 54,000°F (30,000°C), a temperature that is five times hotter than the surface of the sun! When lightning occurs, it heats the air surrounding its channel to that same incredible temperature in a fraction of a second.
>Like all gases, when air molecules are heated, they expand. The faster they are heated, the faster their rate of expansion. But when air is heated to 54,000°F (30,000°C) in a fraction of a second, a phenomenon known as “explosive expansion” occurs. This is where air expands so rapidly that it compresses the air in front of it, forming a shock wave similar to a sonic boom. Exploding fireworks produce a similar result.
So a firework, as they describe, only has one point of propagation of the sound wave. [Lightning, however, propagates sonic booms / shock waves over and over and over as it races to the ground and branches off](https://www.weather.gov/images/jetstream/lightning/thunder1.png).
They also added –
>In addition, the temperature of the atmosphere affects the thunder sound you hear as well as how far away you can hear it.
>Sound waves move faster in warm air than they do in cool air. Typically, the air temperature decreases with height. When this occurs, thunder will normally have an audible range up to 10 miles (16 km).
>However, when the air temperature increases with height, called an inversion, sound waves are refracted (bent back toward the earth) as they move due to their faster motion in the warmer air. Normally, only the direct sound of thunder is heard. But refraction can add some additional sound, effectively amplifying the thunder and making it sound louder.
>**How warm and cool air affect the sound of thunder**
>This is more common in the winter as thunderstorms develop in the warm air above a cooler surface air mass.
>If the lightning in these “elevated thunderstorms” remains above the inversion, then most of the thunder sound also remains above the inversion. However, many of the sound waves from cloud-to-ground strikes remain below the inversion giving thunder a much louder impact.
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