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Why does the sound of thunder last several seconds even when lightning lasts for only fraction of a second?

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3 Answers

up vote 4 down vote accepted

This is because the different parts of a stroke are be very different in distance.

This

Reverberations and echoes of sound last longer than those of light.

is obviously wrong. Which walls or mountains are the reflectors of those echos? Which rooms do the reverberations?

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This seems unlikely. When you're close to the lightning, the thunderclap is very sharp, short, and percussive. When you're far away, it's rolling. However, when you're far away, the difference in distance to different parts of the stroke decreases, so the prediction of this explanation is the opposite of what's observed. –  Mark Eichenlaub May 30 '11 at 10:08
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When You are "close", You are close to the point where the stroke hits the ground, and the sound from the closest part obscures the fainter part. And for "far away" look at some lightning fotos, typically You have strokes down to earth, but at the same time strokes in the cloud, extending horizontally over big distances. –  Georg May 30 '11 at 12:06
    
How could the sound from the closer part obscure the sound from the part further away? You mean it damages my hearing so that I can't hear the rest of the sound? –  Mark Eichenlaub May 30 '11 at 12:20
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Since lightening bolts typically span kilometers, unless the bolt is oriented perpendicularly to the observer, you will get quite a range of arrival times. Obviously if the bolt passes close to the observer, the closest portions will generate much much higher amplitudes than the far reaches of the bolt. At greater distances statistical filtering out of the weaker/shorter bolts also contributes to the impression. –  Omega Centauri May 30 '11 at 15:14
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If a bomb (or cracker) exlodes at a good distance, we usually hear a bang with slight echo, not an extended thunder-like sound. For the thunder, even though the echo and refraction factor can't be ruled out, but the primary factor still looks to be the shape of lightning bolt. The secondary factors (echo, refraction) should only be giving a slight "quality" to the sound rather than altering the sound; that's why accepting Georg's answer. –  pongapundit Jun 15 '11 at 18:23
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I have always assumed it could be put down to a small set of causes:

  • The stroke may have a physical extent and geometry that leads to a perceptible duration between the arrival of the sound due to the near part of the strike and that due to the far part. That is, what Georg said.
  • If the geometry of the terrain is right you may be hearing echos off of cliff faces, hill-sides, buildings, etc. These obviously come by a longer path and therefore arrive later.
  • An active storm has masses of air at different temperatures and humidities. The speed of sound will differ slightly between these regions, leading to refraction at the boundaries. As such there may be lensing going on in the air and again you hear the same sound arriving by multiple different paths at different times.

Add to that the fact that most strokes are in fact multiple discharges, and you can hear sounds that last much longer than the observed light.

I have not gone to the trouble of taking precise data, but my impression is that the rumble tends to be longer for strikes from further away, suggesting that the last two points are the dominate effect.

Other things to look for include

  • Frequency dispersion. But if this was important you would expect the pitch of the sound to have a characteristic dependence on time, which I have never noticed.
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I have been trying to find a link that describes a sort of acoustic wave guide effect that can happen between two layers of clouds, unsuccessfully. I am sure I saw this within the last year. Such a wave guide could transmit sound for longer distances and times and also reverberate. There is an anecdotal story from the greek revolution in 1821 when an ambush in the mountains was betrayed because the discussion of how it would happen was transmitted booming through the cloud layers and people were warned. :). –  anna v May 30 '11 at 14:56
    
@anna: The last section of the wikipedia article may be relevant, and a earlier section points to a case in the US Civil war where Richmond could have been taken but was not because a General didn't hear the guns (though the likely mechanism was different). –  dmckee May 30 '11 at 15:02
    
""If the geometry of the terrain is right you may be hearing echos off of cliff faces, hill-sides, building, etc."" If! Tell that somone in the great plains, that the rolling sound comes from mountains hundreds of miles away :=) –  Georg May 30 '11 at 15:35
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@Georg there need not be only one answer, and for something as complicated and varied as the sound of thunder it would be a little surprising if there was. I have lived in places where echo were a part of the deal and you could sometimes here two or more boom-rumble patterns from a single stroke, and now I live on the plains and thunder has a different character. –  dmckee May 30 '11 at 15:48
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I recall sitting in a lecture about sound propagation from explosives at one time. I don't recall the details but there was a significant impact from the type of ground over which the sound was moving. Hard surfaces (concrete was the example) lead to a much sharper shorter sound at the listening point, while softer soil resulting in a more extended rolling noise. The experimental set-up was on military ground (of course) with flat fields, no clouds and very little nearby to give echoes. –  Peter May 30 '11 at 23:16
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The 3D shape of the lightning channel over many kilometers of extent can be teased out of three simultaneous sound recordings made from three different spots. The coding is similar to that which produces CAT scans, very abstruse, lots of matrix algebra. Similar systems are used to localize gun shots in urban areas using microphone networks.

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Starting in WW I up to the 70ties (at least, about today I don't know) there were sound locating batteries which used to determine the location of artillery sites by measuring sound from 3 or more positions. –  Georg May 30 '11 at 16:37
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