First images of thunder: Acoustic imaging of triggered lightning [Full Text]

[Dan Robinson]

First images of thunder: Acoustic imaging of triggered lightning

An acoustic camera comprising a linear microphone array is used to image the thunder signature of triggered lightning.

This paper suggests that the portion of the lightning channel closest to the ground produces the most intense thunder compared to the channel higher up.

 

[James K]

The above link didn't work for me, but I think this may be the same article?
repository.lib.fit.edu/handle/11141/749
(was able to get a PDF there)

 

[Robert Edmonds]

Hmmm... I could do this in 2d, I should set this up.

 

[Marshall Stoner]

This paper suggests that the portion of the lightning channel closest to the ground produces the most intense thunder compared to the channel higher up.

Cool. This is what I always thought as well, but never really understood why. CG strikes are almost always heard the most easily from the longest distance, but big IC flashes can be just as bright without seeming to produce a lot of thunder.

 

[Chris Kwinta]

Cool. This is what I always thought as well, but never really understood why. CG strikes are almost always heard the most easily from the longest distance, but big IC flashes can be just as bright without seeming to produce a lot of thunder.

A lot of it has to do with the brief luminosity of return strokes (a lightning flash's short-circuit with the ground). If you look at high-speed images of lightning, return strokes are by far the brightest events that you'll see. Some return strokes are less bright than others-- and I've found that the thunder that follows those strokes is much quieter. I captured a positive CG flash (notoriously loud) this past September with a low peak current and the thunder sounded nothing like a ground stroke, let alone a positive strike.

 

[Marshall Stoner]

A lot of it has to do with the brief luminosity of return strokes (a lightning flash's short-circuit with the ground). If you look at high-speed images of lightning, return strokes are by far the brightest events that you'll see. Some return strokes are less bright than others-- and I've found that the thunder that follows those strokes is much quieter. I captured a positive CG flash (notoriously loud) this past September with a low peak current and the thunder sounded nothing like a ground stroke, let alone a positive strike.

That's interesting. Another thing I don't quite understand though is how the perceived sharpness of the initial report from a major return stroke decreases with distance. If a return stroke contacts the ground, say 0.5 km away, yet the channel extends vertically up to 5 km, I'd logically expect to hear a gradually diminishing crash lasting several seconds. Instead I often hear a single very sharp "pop", with comparatively little sound arriving after the first second or so (unless there were other more distant return strokes linked to the same leader burst). The sound from the return stroke is very unidirectional such that not much sound seems to arrive from above if you are very close. A return stroke that is 1.5 km or more away OTOH produces the logically expected sound, a heavy tearing crash that gradually diminishes over several seconds.

 

[Chris Kwinta]

That's interesting. Another thing I don't quite understand though is how the perceived sharpness of the initial report from a major return stroke decreases with distance. If a return stroke contacts the ground, say 0.5 km away, yet the channel extends vertically up to 5 km, I'd logically expect to hear a gradually diminishing crash lasting several seconds. Instead I often hear a single very sharp "pop", with comparatively little sound arriving after the first second or so (unless there were other more distant return strokes linked to the same leader burst). The sound from the return stroke is very unidirectional such that not much sound seems to arrive from above if you are very close. A return stroke that is 1.5 km or more away OTOH produces the logically expected sound, a heavy tearing crash that gradually diminishes over several seconds. I have many audio recordings of thunder claps from return strokes at various distances that I can share if you're interested.

It all has to do with the morphology (shape) of the flash. Basic CG flashes that have just one ground termination and minimal post-return stroke development can produce a 2-3 second rumble before a rapid decay in sound which obviously becomes a loud crash if it terminates within a kilometer of your position. I've had thunder from horizontally-extensive flashes go on for well over a minute. Flash polarity also influences the thunder signature due to different branching behavior and different sound produced by leaders of different polarity.

With any close high-current return stroke, the sound will peak instantly and decay rapidly due to higher sections of the stroke having much more relative distance from your position than the lower section. If the flash is far away, the distance from the lower section is relatively similar to the higher section, if that makes sense.

 

[Marshall Stoner]

After searching youtube and listening to my own audio of close lighting strikes, it seems that what you say is true. The perceived sudden drop off in sound with very close return strokes might just be due to brief hearing loss. The phenomenon is more pronounced in person than when listening to recordings. Microphone recordings may be clipping the initial shock the reaches the ear though. You can't get the full dynamic range of a close strike unless you can put the microphone on a very low manual setting. I've probably damaged my hearing listening to storms lol...