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Thread: Thunder - what causes preceeding echos?

  1. #1 Thunder - what causes preceeding echos? 
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    We had a thunderstorm the other day, and I noticed that the main boom of a clap of thunder is both followed by and preceeded by echos - how can that happen?

    Shouldn't the loudest boom of the sound shockwave be heard first? How can echoes, which must have been reflected at least once and so travelled further arrive before the main boom?


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  3. #2  
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    Well maybe the loudest boom was an echo. You see if you have a whole lot of echos (sound waves) you can have constructive interference which is when the sound waves hit and complement each other. That loud boom could have been a wave that co-incided with a lot of the other waves.

    You may have heard the actual thunder at first as a small sound.


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    hmm, my limited understanding of waves and acoustics makes me think that it would be very unlikely that a large number of random echos would conspire to all be in phase and produce such a huge main boom. Or could they?

    The only alternative explanation I can come up with is that the pre-echos are sound waves travelling through the clouds. Waves travel faster in denser materials and if they are being scattered by the denser-than-air clouds they would reach you before the main air-borne shock wave.

    Anyone got a better explanation?
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    You are assuming that there is only a single lightning bolt, and therefore only a single thunder clap. This is a false assumption. All (practically all) lightning involves a complex sequence of current flows, generating multiple shock waves.
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    Quote Originally Posted by Ophiolite
    You are assuming that there is only a single lightning bolt, and therefore only a single thunder clap. This is a false assumption. All (practically all) lightning involves a complex sequence of current flows, generating multiple shock waves.
    Yes I can see that I made a wrong assumption there.

    You can have a roll of thunder lasting several seconds from the very first initial sounds to the main clap yet I've never seen lightning that last longer than a fraction of a second, or a second at most.

    Can the current flows take place over several seconds leading up to the main discharge? Ie 'invisible' lightning before the main bolt that generates sound but not enough energy to cause light?

    If so then that would answer my question!
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    Don't forget that lightning bolts are usually hundreds of meters (or more) in length. So even if there is only one bolt that lasts a fraction of a second the sound doesn't come from a single point. One end of a lightning bolt may be closer to you than the other.
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  8. #7  
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    I think the shockwave generated is supersonic, this would be your first arrival and would sound as a loud "crack", but they can't travel very far so you only hear them if you're near to the event. After that you hear the normal pressure-waves which travel at the speed of sound, the later arrivals have bounced off multiple reflectors so you get a stream of arrivals that sounds like a rumble.

    Incidentally, the denser air is normally closer to the surface and not in the clouds - this fact alone ensures that once rays have obtained an upward velocity, they will never come back down to the surface, and nobody will hear them (unless they're above the clouds of course).
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  9. #8 Re: Thunder - what causes preceeding echos? 
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    Quote Originally Posted by gruff
    We had a thunderstorm the other day, and I noticed that the main boom of a clap of thunder is both followed by and preceeded by echos - how can that happen?

    Shouldn't the loudest boom of the sound shockwave be heard first? How can echoes, which must have been reflected at least once and so travelled further arrive before the main boom?

    This I can answer with some authority.
    1)
    You will probably know that Lightning can be several miles/K's in length. imagine that A 'bolt with one end, a kilometre above your head and the other 10 Km away due south. IMagine there is a 1KM 'dog leg' feature where all parts of this feature are the same distance from you, ie the 'dog leg' forms part of the circumference of a circle (radius 5K) of which you are standing at the origin. Now approx 3.2 seconds after the flash the first sound wave will hit you, a crack - it's loud because it is close, then the sound starts to arrive from further along the bolt so after 6.4 seconds you are hearing the noise from 2 K away, the sound will begin to fade as the source point recedes from you. After approximately 17 seconds the sound will suddenly and dramatically increase as the sound from a 1KM length of the bolt (the dog leg) gets to you simultaneously, you will hear what appears as a second 'loud bang' after which the sound will continue to fade. The shape and direction of 'bolts' are of course far more complex and therefore there is simply more variation. if the 'dog leg' was say 2K away it would account for your perceiving it as the 'main clap'

    2) Layers of different thermal temperatures and pressures form 'walls' in the atmosphere from which sound waves bounce off, which add another dimension to the cocophany. And there are plenty of theses 'walls' in a thundrestorm - afterall it's these 'wall's that generated the storm in the first place. I have used 'wall's ' here to illustrate a principle it is a 'little' more complex than that.

    These are the two main reasons for lightening lasting a fraction of a second but the thunder rumbling on for many [sometimes] tens of seconds. Ground echoes by comparison are negligible unless you are standing close to a sheer granite cliff face.
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  10. #9 Re: Thunder - what causes preceeding echos? 
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    Quote Originally Posted by billco
    This I can answer with some authority.
    1)
    You will probably know that Lightning can be several miles/K's in length. imagine that A 'bolt with one end, a kilometre above your head and the other 10 Km away due south. IMagine there is a 1KM 'dog leg' feature where all parts of this feature are the same distance from you, ie the 'dog leg' forms part of the circumference of a circle (radius 5K) of which you are standing at the origin. Now approx 3.2 seconds after the flash the first sound wave will hit you, a crack - it's loud because it is close, then the sound starts to arrive from further along the bolt so after 6.4 seconds you are hearing the noise from 2 K away, the sound will begin to fade as the source point recedes from you. After approximately 17 seconds the sound will suddenly and dramatically increase as the sound from a 1KM length of the bolt (the dog leg) gets to you simultaneously, you will hear what appears as a second 'loud bang' after which the sound will continue to fade. The shape and direction of 'bolts' are of course far more complex and therefore there is simply more variation. if the 'dog leg' was say 2K away it would account for your perceiving it as the 'main clap'

    2) Layers of different thermal temperatures and pressures form 'walls' in the atmosphere from which sound waves bounce off, which add another dimension to the cocophany. And there are plenty of theses 'walls' in a thundrestorm - afterall it's these 'wall's that generated the storm in the first place. I have used 'wall's ' here to illustrate a principle it is a 'little' more complex than that.

    These are the two main reasons for lightening lasting a fraction of a second but the thunder rumbling on for many [sometimes] tens of seconds. Ground echoes by comparison are negligible unless you are standing close to a sheer granite cliff face.
    After consulting my meteorology text book I feel I maust challenge your self imposed authority.
    Okay, you're definitely right about the sound emanating from different areas along the the stroke to give a rumbling effect, I'll give you that. However your "walls" in the atmosphere are sheer fantasy, you just don't get sound channels in the lower atmosphere under normal thunderstorm conditions. Sound travels faster in warm air. Thunderstorms form in a "conditionally unstable atmosphere" the warm air is at the surface, this means that sound will be refracted away from the surface (according to Snell's law). Also, the ground echoes are not negligible, relections from buildings, hills, trees and cliffs will all be audible if you're in the right place to hear them.
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  11. #10  
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    The reflections from buildings, tress etc is only a factor if they are very near. Since they are very near any echoes will be almost indiscernable in the time domain. Just go outside, and at the top of your voice yell "ECHO" and listen. I did make the point that Cliffs are an exception, If they are far enough away and big enough you may hear an echo. if they are close they will effectively amplify the noise rather than extend it's duration. I believe the question was about duration and source which I feel I covered correctly. I may have placed a little more emphasis on variations in atmospheric temperature and pressure variations but, sound nevertheless DOES reflect off these. In a similar way submarines can hide behind oceanic thermal 'wall's which can completely defeat their detection by passive or active sound ranging. It is the second largest factor unless you live in a built up area or the bottom of the grand canyon.
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