Thunder: Lightning's Sound Waves
A bright flash happens outside. You brace your ears for what inevitably follows. No, it's not old men bowling in the sky. It's thunder- a fearful, audible reminder of the mighty power of lightning. Lightning is the source of thunder. But what causes it? Why does it crackle, rumble and roll? What can thunder tell you about the lightning that made it? Read on to find out!
What is thunder, and how does lightning generate it?
Thunder is the name given to the loud sound waves created by lightning. The lightning channel
heats and expands quickly and explosively, causing a violent disturbance in the air surrounding the strike that radiates outward for a short distance as a supersonic (faster than sound) shock wave
. The shock wave
eventually slows to a normal sound wave a short distance later:
All lightning, and sparks, create thunder. That little 'crack' when a spark jumps from your finger to the doorknob is a miniature version of thunder!
Why does thunder sound the way it does?
Thunder crackles, rumbles, peals and rolls. It makes an uneven, jumbled sound, because the lightning that created it is jagged and uneven (see image at right)
Thunder also sounds different depending on how far away you are from the lightning, what type of discharge it is (intracloud, cloud-to-ground), and which part of the lightning you hear first. Take the following example:
You see a bright flash, followed seconds later by a soft, crackling sound, then a loud, startling crash. This is followed by a gradually softening series of rumbles and rolls.
That's how the typical cloud-to-ground lightning
strike sounds. That soft crackling is the thunder from the dimly lit secondary branches. Some of the branches extending from the main channel will be closer to you than the main bolt, so you'll hear the branches' thunder
first. The loud crash, of course, is the part of the bright main channel
closest to you. The rumbles and rolls are the parts of the main channel
that are farther away, along with some of the in-cloud dendritic branches (meaning branching upward and outward like a tree) at the top of the bolt. (See diagram below) Most of the branching at the top of the lightning bolt will be inside of the cloud, so most of the time you won't be able to see it.
Thunder from in-cloud (intracloud) lightning
Listen to the first few seconds of this audio clip, recorded near Charleston, WV. Most lightning occurs inside of the storm cloud. Since this type of lightning tends to be less powerful than cloud-to-ground lightning and much higher up in the sky, the thunder is generally quieter to observers on the ground. Sometimes thunder from intracloud lightning and 'anvil crawlers' can be so quiet that it almost has a calm, soothing sound rather
than being loud and startling. You've probably heard this type of thunder during early spring and late fall rain showers.
How can I use thunder to tell how far away lightning struck?
You can use thunder to calculate how far away a lightning strike is. When you see the flash, count the seconds until you hear the thunder. Divide the seconds by five (5), and that will be the distance in miles to the strike. For instance, thunder heard 10 seconds after the lightning means the lightning struck 2 miles away.
Use this distance calculator to do the math for you:
|For each calculator: Type a number in the first space, then click the 'Calculate' button. The answer will appear in the second space.|
Keep in mind that this method doesn't always tell the whole story about the lightning flash you just saw. Read the next section to find out why.
Can thunder tell me what the lightning bolt looked like?
Different types of lightning produce different-sounding thunder. You can often tell what the lightning may have looked like by listening to the thunder.
Most cloud-to-ground lightning flashes consist of long in-cloud branches that
extend outward. So, if lightning strikes the ground 5 miles away, you may hear thunder directly above you after five seconds, meaning that the branches are one mile away (straight upwards), not the actual main bolt. (See diagram below)
In this case, the thunder from the main bolt will come a little later- first, you'll hear the soft thunder from the branches. Then, you'll hear a loud boom from the main channel. So, for a more accurate calculation, keep counting seconds until you hear the loud boom.
If you keep counting until the thunder stops, you can calculate how far the lightning extended. For example: If you counted 10 seconds until the thunder started and 10 more seconds until the thunder ended, that means that the closest part of the lightning was 2 miles away and the lightning extended 2 more miles beyond that (4 miles away from you).
Most lightning occurs inside of the storm cloud. This type of lightning tends to be less powerful than cloud-to-ground lightning, and therefore the thunder is generally quieter. Sometimes thunder from intracloud lightning can be so quiet that it almost has a calm, soothing sound rather than being loud and startling. You've probably heard this type of thunder during early spring and late fall rain showers.
Make your own observations: Next time a storm moves through your area, listen carefully to the thunder. See if you can identify the different sounds. Count the seconds beween the flash and the thunder- then count how many
seconds the thunder lasts. Can you tell how long each lightning flash is? See if you can tell if a lightning flash was intracloud
(inside the cloud only) or cloud-to-ground just by listening to the thunder it produced. Try to come up with more
of your own theories and conclusions. If thunder and lightning frighten you, this may even be a way to calm your fears!
Thunder sound clips
These are actual, unedited recordings in MP3 or RealAudio format of thunder made during lightning photography sessions. All of the recordings were made with a simple Panasonic mono tape recorder (at right).
Files are in MP3 format. (Files are copyrighted and may not be used without a license).
*CLOSE* Cloud-to-ground flash (MP3, 122KB): June 29, 1998 - distance: around 500 feet away (Camera pointing opposite direction) Also, note thunder (boom) from second main channel 2 miles away following initial thunder. This second bolt flashed simultaneously with the close one.
Cloud-to-ground flash (MP3, 118KB): April 11, 1999 - distance: about 1/2 mile away. (Click at the end of the clip is the camera shutter opening) Crackling branch thunder is followed by a crash from the main channel.
Cloud-to-ground flash (MP3, 79KB): June 29, 1998 - distance: less than 1 mile - NOTE: 'crackling' sound from branches before main channel thunder- (the faint click toward the end of this sound clip is my camera shutter closing- the bright lightning flash (out of the frame to the left) that produced this thunder fully exposed the film, so I had to advance to the next frame)
Cloud-to-ground flash (MP3, 40KB): June 29, 1998 - distance: 1 mile - NOTE: 'crackling' sound from branches before main channel thunder
Cloud-to-ground flash (MP3, 81KB): June 29, 1998 - distance: 2 miles - (see the lightning bolt that produced this thunder)
Close Cloud-to-ground flash (MP3, 92KB): August 10, 1998 - thunder from distant cloud-to-ground flash followed by CG flash 1000-1500 feet away (Again, lightning behind camera)
CG about 1.5 miles away (MP3, 50KB): August 10, 1998
Cloud-to-ground flashes (MP3, 131KB): April 11, 1999: (pictured here) distance: less than 1 mile, 2 miles (clicking in photo is camera shutter opening and closing)
*CLOSE* Cloud-to-Ground flash (less than 100 feet away) (MP3, 69KB): July 30, 1999: The closest I've been to a lightning flash. (missed this one with the camera)
Cloud-to-ground strikes (RA, 450KB): May 15, 2001: Several crashes of thunder precede a sudden bang from a close strike near Wallback, WV. Clicking/clanking noise from tripod is audible, along with noise from interstate traffic.
Tower strike (MP3, 46KB): August 11, 1999: Thunder from a ground-to-cloud strike to a tower 1 mile away in Teays Valley, WV (pictured here).
Cloud-to-ground flash (MP3, 321KB): July 10, 2000: Thunder from a complex discharge involving anvil crawlers (heard first), followed by the connected cloud-to-ground channel (pictured here).
Cloud-to-ground flash (MP3, 282KB): July 10, 2000: Thunder from another complex discharge involving anvil crawlers (heard first), followed by the connected cloud-to-ground channel (pictured here).
Cloud-to-ground flash (MP3, 210KB): July 10, 2000: Thunder from a third complex discharge involving anvil crawlers (heard first), followed by the connected cloud-to-ground channel (pictured here).
Cloud-to-ground flash (MP3, 121KB): July 28, 2001: Thunder from a cloud-to-ground strike 1 mile away (view a video of the strike).
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