Hard Light Productions Forums
Off-Topic Discussion => General Discussion => Topic started by: Retsof on June 23, 2011, 08:37:10 pm
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Just something I was curious about. Since all thunder is produced by a relatively constant source, lightning, why is it sometimes a harsh crack and other times a drawn out rumble?
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What's your mathematical background? How well are you familiar with Fourier analysis?
I can explain this with mathematics or as an analogy, depending which way you see it better.
Oh, my understanding is that the charges before the breakthrough are constant, but the current itself is rather short and I wouldn't call it constant.
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Ehm, I nearly failed Algebra (may have been Algebra II, can't remember), so not much math here. Although I've always had a fairly good grasp on scientific subjects. I could tell you about black holes and time dilation and space-time distortion, just not the math behind it.
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Can't you hear, can't you hear the thunder?
You better run, you better take cover.
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You've been...THUNDERSTRUUUUUCK!!!
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I'm not an expert but the fairly basic explanation as I see it is that between you and the lightning bolt are a lot of factors including the air density, clouds and lots of moisture, other objects, etc. So between that and because not all lightning is the same in shape or in the direction that it's traveling... you're going to hear quite a few different types of cracks of lightning. One that appears nearby and is relatively unimpeded by objects and clouds that hits the ground is going to produce a much sharper crack than one that is 10 kilometers away on the other side of a storm that went between clouds or the same cloud...
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That makes sense, but it is still strange when you get a low boom that threatens to knock out your windows. Could the shape of the bolt have much of an effect? Making the sound waves interfere with each other. Or is it in the intensity of the strike?
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The principle behind thunder is actually really closely related to lightning processes. If you're familiar with basic circuit principles, that's a really good way to explain it. (I'm an electrical engineering student.) What happens is this: Lightning occurs when two "masses" of opposite charge are connected by a path through air. When that happens, the result is similar to the behavior of a capacitor. When you touch the two leads of a charged cap together, the cap dumps the charge. The same thing happens with the charge "masses". The important thing with caps though, is their governing relationship, which states that the faster the voltage between the two charge "masses" changes (drops, in this case), the more current flows, which makes sense, because current is just quantity of charge per second.
When current passes though any medium, it sheds power as heat. In this case, it sheds its heat so fast though, that it superheats the air to a plasma, causing a rapid expansion, followed by a cooling contraction. That sudden pressure change is what you hear as thunder. The more current that gets dumped in a lightning strike, the sharper the crack. Next time there's a storm, pay attention to the flash duration. Generally, longer flashes are slower current dumps, which yield a long, low rumble. Fast flashes are fast dumps, which give you a sharper crack.
Hope that helps.
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The length of rumbling has little to do with those characteristics of the lightning strike itself. The pitch and volume certainly, but regardless of how long the actual strike lasts, it is nearly instantaneous as seen from a human perspective. The protracted rumbling you sometimes hear stems from a different mechanic: Lightning isn't a point source of sound. Even a perfectly vertical lightning bolt is still several kilometres long, and horizontal lightning can get much longer than that.
Now, knowing this, remember to consider the speed of sound. The nearest part of the lightning may be 5km away, while the furthest could be 10-12km away. As such, the sound from the different parts of the lightning bolt won't reach your ears at the same time, resulting in a rumble rather than a sharp bang. Environmental factors and the terrain do also affect it some, but the main mechanic is simply the sheer length of the lightning itself coupled with the limited speed of sound.
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You know, that makes a lot of sense. I knew about the whole "different charges basically making a giant static spark superheating the air" thing, but never put much thought into the duration of the strike and the speed of the charge dump. Thanks.
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Ah, that makes sense too. The length of the bolt and it's orientation would have a rather large effect.
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As others pointed out, the source of the sound is not actually a point, but a line several kilometers long (and sometimes more like an area). There are also other factors at play, when lightning hits something close to you, sound is very loud, but if you listen carefully, you keep on hearing more after the bang. Some of that are echoes if the environment allows, and some of that sound just comes later from more distant parts of the bolt.
Also, I think the lower sound frequencies carry the largest amount of energy and are attenuated less. Think of a speaker cone, the thing literally pushes air to form the pressure differences you hear. Given a fixed size of the cone, lower vibration frequency allows a larger movement of the cone; when you think of higher vibration frequencies, the cone has to start moving back earlier since it needs to vibrate faster. Then you get less power for higher frequencies. Now when that sound wave is moving through a medium like air or water, the damping of higher frequencies is greater - those molecules need to transfer energy by colliding with others, and that tends to diminish the less powered components in the sound wave.
It's similar to being outside the disco and hearing the drum beat, but no singing or other instruments. You can guess they are playing some disco stuff, but have no idea of what that song is.
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You will also find that any lightning that produces multiple crackling sounds come from a bolt or bolts that feature multiple forking.
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Acoustics probably have a lot to do with it as well :)
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thunderbolts and lightning very very frightening
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what did ever happen to ole' lord Kalfireth?
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He was lurking about an hour ago :)