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?


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 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.

Sound travels like a wave. Think ripple in water. The further the wave moves away from the source, quieter it gets.

The Boom is what you are hearing. The slow fade off, is the echo of the boom off of everything around you. The sound you hear immediately after the boom, is the sound bouncing off houses around you, then after that it is the sound from buildings down the street, and it keeps going further until the distance gets too far to hear.

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.

The sound you hear is a combination of the sound following different paths to your ears.

Imagine if the entire world was made of mirror. You’d see thousands of reflections of the same object, coming from all directions and distances. Every blade of grass, another tiny reflection of the world.

That’s the world for sound. Every blade of grass, leaf, tree, building, and rock reflects a bit of sound. Normally you can’t hear that reflection, but with a loud BANG you hear all of those little reflections adding together. The more distant reflections are weaker and reach you later, so you hear the initial noise and then a fading stretched-out echo of it.

The sound waves bounce off of objects and as they get further away, the amplitude of those sound waves bouncing back towards you drops off as they dissipate through resistance.

Similar to how someone’s voice gets lower and lower the further they get from you, the sound waves from the thunder bouncing off of the ground and objects getting further and further away from you drops off too.

Often referred to as reflections. For the sound to just stop would require absolutely no reflective surfaces which is something that does not occur in nature. Outside of nature we have things like anechoic chambers which absorb almost all sound reflections. So you can’t hear someone unless their mouth is facing you (for the most part). If you were to recreate a small thunder strike sound in there, chances are it would be extremely brief. And it would likely sound nothing like thunder since most of the sound of thunder is the sound reflecting off of the ground and objects. The original sound I would venture to guess would only be milliseconds in length.

High frequency sound (the crack) is filtered more by the atmosphere across distance than the lower frequency (rumble).

See Zahorik, Brungart, & Bronkhorst, 2005 for an in-depth review of Auditory Distance Perception research

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.

With close thunder, you mostly get a loud sound with reverb (or minor, overlapping echoes that sort of extend out the original sound). This fades away over time.

Distant thunder, however, gets some really complicated stuff with how over very large distances and with very loud sounds different sounds will have a different reverb spread that will fade in and out and almost all of the sound energy will be reverb. Suffice to say, you’ll get most of the early part of what you hear being a bit higher pitch which doesn’t stay as loud after traveling before the low end that will keep its energy over vast distances hits.

Correct me if I’m wrong ng but it’s like a car. Close your eyes when a car is passing by you. While it’s close it’s louder and it fades away as it gets further

The same reason why the ripples in a pond fade away when you throw a rock in it. The traveling wave dissipates over time as it expends energy moving outwards.

Same reason a car that’s driving away gets quieter, it’s travelling away from you.

If it stopped instantly, it would be travelling so fast that you’d be dead from the air being so hot, due to friction.

Not an answer so hopefully this doesn’t get removed but I live in Alpine NZ and when we get thunder it echoes through the hills, sometimes you can hear it for a very very long time.

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).

It’s like ripples in a pond the sound waves get bigger the further it gets away from its initial point

Thunderstorms are one of the most special experiences I’ve had. Like a unique blend of fear, awe, humility, respect, and curiosity, and excitement. It’s truly a natural shut the fuck up and pay attention experience for me.