The Delicate Sound of Thunder

[Cris Schroeder]

This may be an unusual question, but what can affect the sound of thunder? Two nights ago I woke up around 1:45 (am) and noticed lightning flashes. I was surprised at how bright and nearly continuous they were with no thunder at all. I’m used to hearing the low rumble from the distant discharges, and the occasional ka-BOOM-rumble-rumble-rumble from the closer ones. I didn’t hear any thunder until we took a REALLY close strike, and then it sounded more like a WHUMP than a rumble. For ten or fifteen minutes we took some close shots and none of the thunder sounded normal. If anything, it reminded me of artillery fire.

At first I thought maybe there was a really intense rain core that was damping out the sound of the strikes, but the really close strikes (less than a quarter-mile away) sounded funny and there wasn’t any rain at the house. It’s hard to explain the sound with words. Normally a close shot will sound like crackle-BOOM-rumble-rumble or something like that, with a sharp sound leading up to the main boom, and then 5-10 seconds of post-strike rumbling. This lightning had absolutely no sound prior to the initial boom, and less than a half-second before it was dead quiet again. We probably took 20-30 shots like this over the 10-15 minute period, then it was back to the bright strobe-like lightning with no thunder.

Am I going crazy, or has anyone ever heard something like this? I understand the physics involved with thunder but can’t think of a really good reason why the sound was so different. I’ve almost started suspecting it was a dream, except I took some screen grabs from GRLevel3 during the storm so I know it was real.

 

[Skip Talbot]

Am I going crazy, or has anyone ever heard something like this? I understand the physics involved with thunder but can’t think of a really good reason why the sound was so different. I’ve almost started suspecting it was a dream, except I took some screen grabs from GRLevel3 during the storm so I know it was real.

Definitely Cris!

Check out my April 24 video: Nickerson Supercell and Tornado (18MB WMV)

Near the end of the video you'll hear a distinct WHUMP like you described. I even commented that it sounded like a mortar round. We were experiencing some inflow winds at the time, and these winds can dampen the sound of thunder quite well. A more common occurence is to be in the outflow of a storm and have the elongated rumblings of thunder carried right to you.

 

[Cris Schroeder]

Thanks for the clip, Skip! That was exactly what I heard, over and over.

I'll cancel the appointment with the shrink.

 

[Doug Lee]

Just speculation on my part, so if someone suggests something more plausible, go with what they say.

The flashes without thunder could be high-altitude CC and the sound decays before reaching the ground due to erosive effects of turbulance within the storm. This would be similar to the effect of a car muffler.

As for the WHUMP vs. crackle-BOOM sounds, the length of the lightning path and the path betweent the strike and the listener may be the deciding factors. A quick, nearly vertical strike might produce a whump because of the short path. A longer, forked bolt might produce the crackle-boom. The tendrils probably crackle and the main bolt booms.

Turbulant air near the surface could be another factor serving to muffle the sound. Also, strong surface winds blowing towards the strike could reduce the strength of the sound.

Also, sound has a natural tendency to decay due to the inertia of the air in front of the pressure wave.

 

[John Diel]

Like it's stated above, some of the things that will affect sound waves (Thunder).

Air Density
Air Temperature
Amount of moisture or contanimants
Wind speed/direction
Rain
Distance
Ground Objects

I know with the Lahoma storm there was a LOT of lightning. CC and CG. As we pulled away from the storm, we found the OFB and just beyond that we found a HUGE difference in temp/humidity. I mean the difference of 10 degrees in less than a mile. It was enough to immediately fog up the windshield as you drove into the boundry. That was a first for me. I've never had that big of a difference unless I was walking out of an air conditined house into the summer air, it was that drastic of a difference.

We also noticed a definite change in the sound of the Thunder as we moved into the boundry area. What was sharp cracks became muffled rumbles or was lost altogether.

 

[Gerrit Gulden]

Ive heard some storms referred to as "Electric storms" and basically what you end with continous Lightning with maybe a distant rumble every other minute or so. Basically just a light display without the full effect of thunder. I can say I questioned the cause of lightning and no thunder or barely thunder and the causes you all mentioned make perfect sense.

-gerrit

 

[Dan Robinson]

Smaller lightning discharges in the higher levels of thunderstorms produce low-intensity thunder that is typically inaudible at the ground level simply because of its low initial volume.

The volume of thunder is a factor of how violent the expansion of the channel is, which is determined by how quickly the current in the lightning channel 'ramps up' during a return stroke. Many ground-to-cloud tower strikes, for instance, feature an upward-propagating leader that carries a slowly increasing current load as the leader moves deeper into the cloud. A CG leader channel, by constrast, carries little current until the ground connection is made. A CG, therefore, has a very rapid current spike at ground level compared to upward-moving and most intracloud discharges, and therefore a more explosive and much louder sound wave to an observer on the ground.

Tower lightning is great for observing and documenting thunder. I've been several hundred yards away from some very bright upward-moving tower strikes that only produced soft thunder. They were as bright as a CG strike, but the current ramped up slowly enough to prevent an explosive shock wave. I've also witnessed tower strikes where the first return stroke had soft thunder, but the subsequent return strokes were much louder.

You can hear these effects in the following video:

http://stormhighway.com/video-tower1.shtml

With the first discharge on the video, notice the thunder starts out soft, then gets suddenly louder. You can visibly see the first return stroke is slowly ramping up, lasting much longer than a normal return stroke - and lasts long enough to visibly drift sideways with the wind. The following return strokes are much more rapid and 'sudden', resulting in louder, sharper thunder.

Later in the video, there are a few discharges that have much softer thunder, as they did not contain any of the rapid-current spike return strokes.

 

[Donna Burkhart]

If you've ever been in convective thundersnow, it sounds exactly as you describe. There of course you have cold, dense air and extremely large snowflakes muffling the sound.

Delicate Sound of Thunder - a great Pink Floyd album btw.

 

[Doug Lee]

I forgot to mention earlier, rumbles are usually echoes of the original thunder bouncing off hillsides.

My house sits to the west of a ridgeline that arcs from north to suthwest. Another ridgeline sits to the north and it runs east-west. A couple of hills sit to my southeast. The rumbles I hear sometimes will rattle the windows!

 

[Joe Nield]

On a similar note, I was lying in bed awake late one night a few years ago listening to a nearby storm. Even though the lightning was not all that frequent, the thunder was nearly continuous, echoing for many seconds after each strike. It was really quite impressive.

 

[Cris Schroeder]

All good info. I thought about the differences between CC And CG, but I never considered how the current ramp-up time could affect the sound.

Thanks, everyone.

 

[David Draun]

Also, I've noticed rapidly growing updrafts, especially supercells, have a nearly continuous rumbling sometimes. It's almost like the sound you are hearing is the updraft growing. That's kinda spooky.

 

[Skip Talbot]

I love that continuous rumbling. The best supercells I've been on have featured that effect and I know I've really got something when I encounter it.

Here are some more phenomena that could contribute to the sound of thunder, forwarded to me by a non-member:

See the 2nd & 3rd panels on this page
http://hyperphysics.phy-astr.gsu.edu/hbase/sound/refrac.html#c2

This page has the same data, and adds the phenomena of absorbtion of sound in air.
http://www.sfu.ca/sonic-studio/handbook/Sound_Propagation.html

To make it more interesting, both phenomena vary with frequency. With absorption, lower frequencies can travel farther than high frequencies. With refraction, higher frequencies are bent more than lower frequencies, so the "treble" can end up taking a different path than "bass".

Ever notice the sound of an approaching jet plane at medium-to-low altitudes? Often the volume doesn't just gradually increase; there is silence and them suddenly it's quite audible -- and often the pitch of the sound immediately falls, making it sound as if the plane is decelerating rapidly. It's got to do with different frequencies of sound being refracted/absorbed to differing degrees. (The Doppler effect can't account for the frequency drop in this situation.)

So, with thunder, part of the deal is the shape, orientaiton and proximity of the bolt. In the video you posted, I heard the "mortar" sound. Two things strike me -- the lack of the splitting, cracking sound, and lack of rumbling. Sounds like the higher frequencies of the thunderclap were diverted and/or absorbed before they reached you. (Doesn't necessarily explain the missing rumble, though.