Propagation of Thunder

[Hornbein]

Recently there was a thunderstorm here of great power. There would be a bright flash then after ten seconds a rumble that would build up for two seconds, then a sound as though someone were hitting a bass drum hard outside the walls. I would have expected that that shock wave would be the first thing to arrive, without that buildup. How can sound arrive more quickly than that?

 

[Ken Fabian]

A lightning bolt can be many kilometres long as well as reach high altitudes. There is sound transmitted through the ground as well as air - faster through ground than air but more usually felt as ground vibration than heard - and lightning hitting the ground can be (but not always) the first 'sound' perceived following the strike, because of shortest distance to listener.

The nearest part gets heard first and loudest, the furthest part of the lightning will be the last heard and faintest. Ten seconds delay indicates the nearest the lightning was from you was about 8 km 3.5 km. What was heard after that was further away.

The longest lightning bolts can be hundreds of km. At 1.2 km 340m per second for the speed of sound in air that makes thunder that lasts a long time.
(Edit - corrected speed of sound).

 

[Dale]

Nice answer @Ken Fabian

I was just thinking in terms of dispersion, but that makes way more sense.

 

[Hornbein]

faster through ground than air

That could be it. The ground waves produce secondary air waves that arrive earlier. With a lightning bolt of ordinary power these might not be all that audible, so would only be heard with these extreme cases.

 

[kuruman]

At 1.2 km per second for the speed of sound in air . . .

I always thought that the speed of sound in air is about 340 m/s and used the rule "three seconds per kilometer" for a quick and dirty calculation.

 

[Ken Fabian]

I always thought that the speed of sound in air is about 340 m/s and used the rule "three seconds per kilometer" for a quick and dirty calculation.

Ah, good catch. I got that very wrong - thinking 1200 metres per second when it is 1,200 (approx) km per hour. Post has been edited. Thanks.

 

[Orthoceras]

Another possibility: a segment of the lightning path is approximately circular, and the observer is at the center of the circle. The observer would experience the thunder from that arc as a bang.

 

[DaveC426913]

Another possibility: a segment of the lightning path is approximately circular, and the observer is at the center of the circle. The observer would experience the thunder from that arc as a bang.

View attachment 371629

Yes. I've read about his too.

An arc of lightning that runs along your line-of-sight will produce thunder that is long and rumbly.
An arc of lightning that runs perpendicular to your LOS will produce a thunder clap.

 

[tech99]

I also observed similar effects with a sonic boom. When Concorde was flying over the Atlantic, I would often hear the boom when walking in the countryside in England. But the interesting observation was that Pheasants, that are ground birds, would suddenly squawk and rise before I heard anything. Maybe (a) as their ears are low down, they heard a boom travelling fast hrough the ground, so arriving earlier (b) they heard a fast-travelling low frequency component that I could not hear (c) they felt fast-travelling seismic vibrations with their feet. I talked to the UK Geological Survey about it, and they said the boom would be visible on seismographs, so it is possble that the mechanism is (c) and it is interesting that, as Concorde was over water, the wave must have possibly coupled into the ground once it reached land.
Regarding thunder, I have noticed that a cloud-to-cloud dishcarge has a much longer rumble than a cloud-to-ground discharge; maybe its path is longer and more irregular, and the sound travells through media of varying temperature and humidity.

 

[DaveC426913]

Regarding thunder, I have noticed that a cloud-to-cloud dishcarge has a much longer rumble than a cloud-to-ground discharge; maybe its path is longer and more irregular, and the sound travells through media of varying temperature and humidity.

Vertical flashes are restricted by altitude (~10 miles).
Horizontal flashes have been recorded as long as a whopping 515 miles.