We had a thunderstorm toady and I noticed a couple of things:
First, why does the sound of thunder go CrrrrrrrrrraaaaACK-BOOM. Is that the sound of the lightning propagating? Does it take that long? It seems like I’m hearing something approaching, but even when it’s cloud-to-cloud, there’s still the big Boom at the end. Not CrrrraaACK-BOOM-CRAaaack. What am I hearing?
Secondly, we had one real close hit that made the dogs jump; that was one huge BOOM, without any warning. The weird thing with that one was that the smoke alarms went of for a second before the lightning strike. What’s up with that?
Unless you’re in a very open area, the thunder you hear is composed of a superposition of sound that travelled more or less directly to you, plus echoes from other objects such as buildings and terrain. This spreads out and duplicates some of the sounds you hear.
The old theory was that lightning briefly produced a vacuum. The “crack” was the propagation of the lighning itself; the “boom” was masses of air collapsing into the vacuum.
We now know this isn’t what’s going on. The enormously high temperartures cause a sudden thermal expansion of the plasma in the lightening bolt. This causes it to expand outwards at faster than the speed of sound, causing a shock wave similar to a sonic boom. That’s the “crack”. The “boom”, as Mangetout says, is reflections and echoes of the same sound.
As lightning connects to the ground from the clouds, a second stroke of lightning will return from the ground to the clouds, following the same channel as the first strike. The heat from the electricity of this return stroke raises the temperature of the surrounding air to around 27,000 C° (48,632 F°). Since the lightning takes so little time to go from point A to point B, the heated air has no time to expand. The heated air is compressed, raising the air from 10 to 100 times the normal atmospheric pressure. The compressed air explodes outward from the channel, forming a shock wave of compressed particles in every direction. Like an explosion, the rapidly expanding waves of compressed air create a loud, booming burst of noise
*Because electricity follows the shortest route, most lightning bolts are close to vertical. The shock waves nearer to the ground reach your ear first, followed by the crashing of the shock waves from higher up. Vertical lightning is often heard in one long rumble. However, if a lightning bolt is forked, the sounds change. The shock waves from the different forks of lightning bounce off each other, the low hanging clouds, and nearby hills to create a series of lower, continuous grumbles of thunder. *
I’m not precisely sure of the mechanism but I’d guess you were scary close to this one. I’m unsure if I may be barking up the wrong tree, but it does occur to me that around a lightning strike the air is ionised. And most smoke alarms work off detecting a change in ionised air within the detector.
Now, my understanding is that smoke detectors contain a small chamber in which air is ionised, allowing current to flow between two charged plates despite an air gap. When smoke enters the chamber it de-ionises the air in the gap, which causes the current to cease to flow, which triggers the alarm.
Now maybe the ionised air caused by the atmospheric charge which was about to cause a lightning strike caused a change in the smoke chamber of your detectors which somehow triggered the alarms.
I’ve heard single Booms from lightning. This is the simple lighting bolt from a low cloud…
when you hear multiple it can be because there is multiple lightning bolts, which means its at various heights, distances and times …
This part here gives a crackle, this one gives a crack, this one gives a boom…
And then you can add in reflections…
The charged cloud can cause electrical charging of the surface of the earth.
This interfered with the operation of the smoke detector.
Believe the smoke detector can tell the difference between air and smoke because the smoke does become more IONISED and CONDUCTS.
The particles from the Americurium are used to test if the air in the gap will ionise…
That’s what I thought, but Princhester appears to be right - apparently it’s the other way around - the air is more readily ionised than the smoke - so the smoke breaks the circuit.
On one rainy day I was sitting in my parked car and saw a flash of lightning and a split second later heard the thunder–one of the loudest sounds I’ve ever heard! Scared the daylights out of me. :eek:
an ionization smoke detector compares two current paths, one open to the air and one not.
smoke diminishes the current flow in the open current path, the two current paths are unequal triggering the alarm.
if the air is strongly ionized in a lightning strike then you will smell ozone.
if you are close to lightning (deafening boom and blinding flash are simultaneous) but not struck, then you might feel the ionization effects.
the smoke detector is very sensitive to ionization. as the ionization effect travels it may imbalance the two current paths. not all my alarms get triggered when that has happened, it will depend on each detector and its orientation to the ionization.
I was halfway to France during a recent electrical storm - there were lightning flashes every couple of seconds for pretty much the whole night, but hardly any thunder - a few rumbles, but most of the lightning flashes were not followed by any significant sound. Some of them were close enough that I could see them hitting the water - and a few, judging by the shadows they threw, must have struck the ship itself.
Not only that, but the lightning bolt itself can be kilometers long, so even with the path that is directly to you, the time the sound takes to reach you can be spread over several seconds.
I used to think that all the rumbling was echos, until I read an article about this in Scientific American many years ago. That article explained that the crackle or rumble comes from sections of the bolt that are traveling toward you or away from you, so the sound reaches you over a period of time. The loud booms occur when a large section of the bolt is traveling nearly perpendicular to your direction, since the sound all hits your ear at the same time. Using multiple microphones, it is possible to reconstruct the shape of the discharge.