Why are thunder reverberations louder?

[HMS Irruncible]

I have often noticed that the initial thunderclap is often nowhere nearly as loud as its subsequent reverberations. How can this be? I would think that reflected sounds would not be as energetic as the original sounds.

For example, just now I saw a bright flash of lightning, followed by a moderately loud bang 2 seconds later, then a few seconds later there was an even louder rumbling that lasted 5 seconds or so. Often it sounds like a large ice shelf calving... first the crack of the ice splitting, then the really loud rumbling as it gives way. I don't understand why thunder ought to sound like that.

[Telemark]

Lightning isn't always a single streak, it's often multiple bolts going up and down the same path. The loudest part may not be the first part. I don't think there are "reverberations" in the general sense.

[Musicat]

I have often noticed that the initial thunderclap is often nowhere nearly as loud as its subsequent reverberations. How can this be? I would think that reflected sounds would not be as energetic as the original sounds.Your premise is faulty. You are assuming that the first sound is direct and the following ones are reflections. Upon what do you base this assumption? You do know that most lightning is a bunch of closely-spaced events, even if it appears to be a single strike?

[HMS Irruncible]

Your premise is faulty. You are assuming that the first sound is direct and the following ones are reflections. Upon what do you base this assumption?
I don't believe it's geometrically possible that any reflected sound could reach any given point before the original sound does, unless the direct path is somehow completely blocked.
You do know that most lightning is a bunch of closely-spaced events, even if it appears to be a single strike?
Do we know that? Spaced how closely?

[HMS Irruncible]

Lightning isn't always a single streak, it's often multiple bolts going up and down the same path. The loudest part may not be the first part. I don't think there are "reverberations" in the general sense.
If the multiple bolts occurred in milliseconds, how is it that I hear thunder rumbling with increasing intensity for more than 10 seconds? If the multiple bolts occurred over 10 seconds, why don't I see increasingly intense flashes of light?

[crowmanyclouds]

... a bright flash of lightning, followed by a moderately loud bang 2 seconds later, then a few seconds later there was an even louder rumbling that lasted 5 seconds or so. ...Leader formation and the return stroke (http://en.wikipedia.org/wiki/Lightning#Leader_formation_and_the_return_stroke)

As a thundercloud moves over the surface of the Earth, an electric charge equal to but opposite the charge of the base of the thundercloud is induced in the Earth below the cloud. The induced ground charge follows the movement of the cloud, remaining underneath it.

An initial bipolar discharge, or path of ionized air, starts from a negatively charged mixed water and ice region in the thundercloud. Discharge ionized channels are known as leaders.
...
Once a channel of ionized air is established between the cloud and ground this becomes a path of least resistance and allows for a much greater current to propagate from the Earth back up the leader into the cloud. This is the return stroke and it is the most luminous and noticeable part of the lightning discharge.
...
The electrical discharge rapidly superheats the discharge channel, causing the air to expand rapidly and produce a shock wave heard as thunder. The rolling and gradually dissipating rumble of thunder is caused by the time delay of sound coming from different portions of a long stroke. ...So, a SEWAG, the first bit of thunder is the formation of the leader and the louder thunder following is the return stroke?

CMC fnord!

[Musicat]

I don't believe it's geometrically possible that any reflected sound could reach any given point before the original sound does, unless the direct path is somehow completely blocked.How do you know that ANY sounds are reflections?

[Squink]

If the multiple bolts occurred in milliseconds, how is it that I hear thunder rumbling with increasing intensity for more than 10 seconds? Because the bolt is more than two miles long. With sound moving at a mile every five seconds, you'll hear thunder from the far end of the bolt seconds after you hear it from the near end. Additionally, if for some part of the bolt's path, it remains at the same distance from you, you'll hear the thunder from that section all at the same time. That'll make it louder.

[Musicat]

You do know that most lightning is a bunch of closely-spaced events, even if it appears to be a single strike?Do we know that?Yes, we do. See below.Spaced how closely?
From http://en.wikipedia.org/wiki/Lightning
High speed videos (examined frame-by frame) show that most lightning strikes are made up of multiple individual strokes. A typical strike is made of 3 to 4 strokes. There may be more.

Each re-strike is separated by a relatively large amount of time, typically 40 to 50 milliseconds. Re-strikes can cause a noticeable "strobe light" effect.
I myself have examined videos taken of a thunderstorm, and what may appear to be a single bolt can be shown in slow-motion to be many separate ones often following the same or similar path.

[HMS Irruncible]

How do you know that ANY sounds are reflections?
1: I know sound can be reflected
2: In this case, I see the flash of lightning associated with the thunderclap. If it's in fact multiple events, they can only be milliseconds apart at most.
3: The initial clap, and the subsequent booms and rumbles I hear are not spread over milliseconds, they are spread over 10-ish seconds. It's not possible that one sound outraced the others if they were all traveling in air, hence some of it must be reflected.

'm not necessarily saying I'm right, I'm saying that given the assumptions above, my intuition is logically correct. Show me what's wrong with that logic, don't just roll your eyes and tell me it's obviously wrong.

[HMS Irruncible]

Because the bolt is more than two miles long. With sound moving at a mile every five seconds, you'll hear thunder from the far end of the bolt seconds after you hear it from the near end. Additionally, if for some part of the bolt's path, it remains at the same distance from you, you'll hear the thunder from that section all at the same time. That'll make it louder.
I hadn't thought of the length and geometry of the bolt. Best answer so far, thanks.

[HMS Irruncible]

So in the exact conditions of my OP, it sounds like a bolt that propagates directly away from me, then turns horizontal for some significant distance. That's why I hear what sounds like one well-defined sound, and then a bunch of others jumbled together.

[LSLGuy]

Lightning doesn't ned any special geometry to sound rolling.

Lightning bolts start well up in the clouds. One which was perfectly vertical and landed a couple hundred feet from you would also have a long rolling sound coming from the upper reaches of the bolt 2 3 or 4 miles straight up.

I suspect the initial bang from the close impact would leave you deafened enough that the next few seconds of sound arriving from higher up would be hard to notice. But they'd be there to be recorded.

Also, any sound is strengthened or attenuated a bit while passing through differing air currents and difering densities of rain. If there's one thing which characterizes a thunderstorm, it's a highly aggressive & chaotic pattern of air currents and rain (& sometimes hail) inside the cloud. So the sound is being aggressively modifed as itpasses through the cloud on its way to you.

last of all, the initial leader is much weaker than the return stroke. And each subsequent stroke differs from the others. So we have a complex sound being emitted at different intensities over time and space. Which is then modified by the ambient atmosphere as it radiates. Which also adds and subtracts as you have diffiering distances from each radiating point.

[Napier]

From what I read in Martin Uman's work and conversations with him as well as readings elsewhere, the important bit is that lightning bolts often have longish straightish sections (perhaps the tracings of cosmic rays that ionize paths in the air). From the point of view of a listener they are all instantaneous but at different distances. Sections that are perpendicular to a line to the listener create sound that reaches the listener all at once, creating loud bangs. A short perpendicular close section and a longer perpendicular section further away give a "bang... BANG". Sections arranged more nearly parallel to a line to the listener give a more continuous ripping sound, as the sound from different points on the section arrives at different points in time. There is also a tendency for sound to curve out of a straight line due to the atmospheric density gradient, and a tendency for higher frequencies to be absorbed sooner, so the more powerful bass notes are the ones that predominate at a distance, while the gunshot and crackle may be more stimulating for nearby strikes.

I've always heard about thunder reflecting off of distant hills, coastlines, and so forth, but this part doesn't make sense to me. With rare exceptions, the landscape is just too flat and featureless on a scale of kilometers. I think this may be popular misconception but have never heard from an authoritative source.

[HMS Irruncible]

Thanks Napier... I expected as much... would love to read some more of that cite. Not available online I suppose?

[barbitu8]

Lightning isn't always a single streak, it's often multiple bolts going up and down the same path. The loudest part may not be the first part. I don't think there are "reverberations" in the general sense.

Lightning creates thunder by superheating air. The air is not superheated by just the leader. The return stroke is necessary for the lightning and thunder. A lightning stroke has 4 parts: 2 strokes down and 2 up. In the first part of the stroke sequence, the stepped leader is created: a stream of negative ions looking for a path to the ground. These cause positive charges to gather on what is normally negatively charged ground. The stepped leader, having ionizing the air, smooths the path for the return stroke: a rush of positive charges from the ground. The thunder is caused by the superheated air. When a lightning stroke occurs (i.e., the leader connects to the return stroke), air in a narrow channel around the stroke is heated to 54,000 degs F very suddenly. Enormous pressure gradients are created, pushing air away from the bolt at more than 800 mph. This is supersonic and creates a sonic boom, which we call thunder.

Many lightning sequences involve two strikes in rapid succession because the air channel established by the stepped leader and the return stroke has been ionized and creates a favorable environment for the transmission of current. The second pair of strokes starts with a dart leader, which is similar to the stepped leader. This may be the cause of what sounds like a larger clap of thunder.

So how does the base of the cloud become negatively charged? you may ask. The normally negatively charged ground can induce positive charges in ice crystals in the base of a deep cloud. The small crystals are transported upward by the updraft, whereupon they grow larger due to accumulation of moisture, fall back to the base, and this cycle is repeated. If and when the hailstones become heavy enough, they fall to the ground. However, while in the cloud, collisions between two particles may transfer the negative charges to the hailstones, resulting in a surplus of positive charges at the cloud top and negative charges at the base. The negative charge at the base repels the negative charge in the ground, inducing positive charge in the ground and charge separation.

This was a description of cloud to ground lightning, but cloud to cloud lightning also occurs.

[JWT Kottekoe]

The answer was given already, namely a lightning bolt produces all its sound within a fraction of a second along a complex jagged path. The parts aimed directly at you give a quieter, long duration sound, while the parts perpendicular to you give an abrupt loud bang.

Years ago there was an article (in Scientific American, I think) describing an experiment in which cameras and microphones were set up at multiple locations. From the sounds alone, it was possible to reconstruct the shape of the lightning bolt.

Next time you hear a half a minute of thunder after a single flash, be impressed with how long some of these lightning bolts are.

[Musicat]

1: I know sound can be reflected
2: In this case, I see the flash of lightning associated with the thunderclap. If it's in fact multiple events, they can only be milliseconds apart at most.
3: The initial clap, and the subsequent booms and rumbles I hear are not spread over milliseconds, they are spread over 10-ish seconds. It's not possible that one sound outraced the others if they were all traveling in air, hence some of it must be reflected.

'm not necessarily saying I'm right, I'm saying that given the assumptions above, my intuition is logically correct. Show me what's wrong with that logic, don't just roll your eyes and tell me it's obviously wrong.After reading the comments in this thread since your posting, are you still claiming that the sounds you hear are reflections?

You need something to reflect from to make reflections. In many areas, the only candidates are land or water, and reflections from those won't all bounce towards your ears, but scatter in all directions above the surface of the earth (and maybe below, but you won't hear those).

I'm just saying that because what you hear doesn't meet your self-imposed model of a lightning strike, that doesn't mean acoustic reflections are the cause, and it doesn't define how much is direct and how much is reflected, if any.

If you ask, "Why does it sound like that?" you are asking a different question from "Why does the sound come from reflections?" The first is a fair question, the second starts with unproven assumptions.

[HMS Irruncible]

After reading the comments in this thread since your posting, are you still claiming that the sounds you hear are reflections?
I'm not getting wrapped around the axle with you over this, since several other people were able to infer what I was asking and provide some superior responses. You might have a look at them.

[Musicat]

I'm not getting wrapped around the axle with you over this, since several other people were able to infer what I was asking and provide some superior responses. You might have a look at them.I did, and suggest that you do likewise. Their responses don't make the assumptions you did. Others are suggesting reasons for the sound, but you are claiming they are reflections just because you don't know how else to explain them. All sounds are not reflections.

[ZenBeam]

Cosmic Relief got good answers and realized he hadn't considered the length of the bolt a week ago. Why are you going on about this now?

[Napier]

Cosmic Relief, it sounds like you are pretty happy with your new understanding of thunder, without invoking reflections anymore. I don't know where online to find the books I read. JWT Kottekoe's Scientific American reference sounds familiar - I usually read Sciam - I bet it's an excellent discussion that would take you further, and I've bought digital reprints from their web site for maybe $3 or so. Happy crackling!