Why Does Thunder Sound Like Both a Sharp Crack and a Deep Rumble?
― The Sound Is Made in an Instant, but Takes Seconds to Reach Your Ears
Thunder is made at the very instant the lightning flashes, all along its path at once. Yet the sound goes on for seconds. Up close it splits the air with a sharp crack. From far away it growls low and deep. What makes the difference is not the lightning itself, but the distance the sound travels to reach you.
It is a summer evening, and the window outside flashes bright. You count: one, two, three… and before long a low "rumble…" reaches you. The sound just won't stop. It goes on for 5 seconds, then 10.
On another day, lightning struck very close by. This time it was different. A sharp "crack!" came almost together with the flash, as if the sky were being torn open.
It is the same thunder, so why does it sound so different? And lightning lasts only an instant, so why does the sound go on so long?
Just Two Reasons
A lightning path can be several kilometres long. Sound leaves the whole path at once, but each part is a different distance from your ear. Sound from the near parts arrives first and sound from the far parts arrives later, so the noise gets stretched out.
Air soaks up high notes more than low ones. In distant thunder, the high notes that give it sharpness are lost on the way. Only the low notes are left for your ear, and they sound like a rumble.
So what your ear hears is not the sound the lightning made. It is that sound after the journey has stretched it out and worn it down. Let's look at each reason in turn.
How Can One Instant of Sound Last Several Seconds?
When lightning flashes, the air along its path is heated to a very high temperature in an instant. The heated air swells suddenly and shoves the surrounding air aside. That shove sets off a vibration, and the vibration is thunder.
The key point is that the sound comes from the whole path almost at the same moment. A lightning bolt stretches upward for several kilometres, from the ground into the cloud. The part near the ground is close to you, and the part inside the cloud is far from you.
Sound travels at about 340 metres per second. That is very slow compared with light, so a difference in distance turns directly into a difference in arrival time. Look at Figure 1. The sound from near the ground arrives first, and the sound from the top arrives last. In between, sound from the points along the path keeps arriving one after another.
If you bring the lightning closer in Figure 1, the gap between the two lines widens. For a strike nearby, the distance to the top, almost overhead, is very different from the distance to the base. For a distant strike, the gap becomes small.
Real lightning, though, is not just one vertical line. Inside the cloud it branches sideways for several kilometres, and those branches make sound too. Sound bouncing off hills and buildings also arrives late and joins in. That is why even distant thunder often rumbles for a long time.
Why Does Distant Thunder Become a Low Rumble?
The sound of thunder is a mixture of high and low notes. Heard up close, the high notes give it its sharpness, and it splits the air with a "crack" or a "snap."
But air slowly soaks up sound and turns it into heat. What is more, the higher the note, the more strongly it is absorbed. Over a journey of several kilometres, the high notes fade first and only the low notes remain. Look at Figure 2. The close thunder on the left is a sharp, spiky wave. The distant thunder on the right has had its corners worn off into a gentle wave.
There is one more factor, on the side of the ear. Human ears are poor at sensing very low notes. Thunder is thought to contain notes so low that we cannot hear them at all. This is believed to be why distant thunder can be quiet and yet make the windows rattle.
Light arrives almost instantly, and sound travels about 340 metres per second. Count the seconds from the flash until you hear the sound, then divide by 3. That gives the rough distance in km. Nine seconds means about 3 km. Flashes too far away to hear are thought to be ones where the sound fades or bends up and away into the sky before it reaches you.
So What Should You Do?
If you can hear thunder, the lightning is within the range sound can travel, which means within about ten-odd kilometres. The Japan Meteorological Agency also says that once thunder is audible, a strike is imminent, and urges people to get to a safe place at once. Do not think, "It's only a distant rumble, so I'm fine."
- If you hear a rumble, go inside a building or a car right awayDo not stay in the open or in a wide field.
- Keep away from tall trees and utility polesDo not shelter from the rain right under a tree. Stay at least 4 m away from the trunk and branches.
- Stay inside for about 30 minutes after the last soundEven when the sound moves away, do not relax right away.
Call 119 (Japan's emergency number) at once. A person who has been struck does not carry any electricity, so touching them will not shock you. If they are not breathing, follow the dispatcher's instructions and give chest compressions, and use an AED if one is nearby. If the thunderstorm is still going on, rescuers should also make sure they are in a safe place themselves.
Summary
Thunder is made in the instant the lightning flashes, along the whole path at once. It keeps sounding for seconds because each part of the path is a different distance from your ear, so the sound arrives at different times. Distant thunder sounds low because the air absorbs the high notes, which fade first on the way.
The length of the rumble is the length of the lightning.
The lowness of the sound is how far away it is.
How counting the seconds between flash and sound gives the distance is covered in "How much of a household's electricity could one lightning bolt supply?", and why distant sounds carry well at night is covered in "Why can we hear distant sounds better at night?". For how to protect yourself from lightning, see also "Why shouldn't you shelter under a tree during a thunderstorm?"
- On a day with thunder, stay inside a building without fail, and count the seconds from the flash until the sound starts.
- Then count the seconds from when the sound starts until it stops, and make a note.
- Record a few strikes, and compare how sharp or low the sound is between thunder with a short wait (close) and thunder with a long wait (far).
Make your observations indoors, away from windows, and do not go outside. You should notice that close thunder starts with a sharp sound, while distant thunder is only a low growl.
For Those Who Want More ― Terms, Formulas and Links to TextbooksEach section is labelled with the level it belongs to, from junior high science to university courses
- Junior highCovered in junior high school science
- High schoolCovered in high school "Basic Physics" and "Physics"
- High school+Advanced high school material, or textbook sidebar content
- UniversityUniversity-level specialist subjects (acoustics, atmospheric physics) not taught in high school
- ResearchTopics that are not settled even at university level and that researchers are studying now
Junior highTerms: This Phenomenon Has Names
- Thunder (raimei): the sound of lightning. It is made when the air along the lightning path is suddenly heated, swells, and shoves the surrounding air aside.
- Speed of sound: how fast sound travels through air. At 15 °C it is about 340 metres per second, and it gets a little faster as the temperature rises.
- Sound absorption: air gradually turning the energy of sound into heat, which weakens it. High notes are absorbed more strongly.
Junior highHigh schoolChecking with a Formula: How Many Seconds Does the Rumble Last?
Use the same conditions as Figure 1: lightning strikes 3 km away, and its path extends 4 km straight up. We calculate the difference in arrival time between sound from the nearest point, the base, and the farthest point, the top.
| In symbols | T ＝ ( √( d² ＋ h² ) − d ) ÷ v |
| In words | Rumble duration ＝ (distance to the top − distance to the base) ÷ speed of sound |
| Where it comes from | Sound travels at a steady speed, so "time taken ＝ distance ÷ speed." Sound leaves the whole path at once, so the difference in arrival time depends only on the difference in distance. The distance to the top comes from the Pythagorean theorem. |
| Symbol | Meaning and unit |
| T | Time the sound lasts (seconds) |
| d | Horizontal distance to the strike point (the base) (m) |
| h | Vertical height of the path (m) |
| v | Speed of sound (m/s) |
| Distance to the strike d | 3 km (3000 m) |
| Height of the path h (up to the charged region in the cloud) | Assumed to be 4 km (in reality said to be a few km) |
| Speed of sound v at 15 °C | About 340 m per second |
| Speed of sound from temperature (0.6 × temperature) | 0.6 × 15 ＝ 9 |
| Add to the speed at 0 °C | 331.5 ＋ 9 ＝ 340.5 |
| Distance squared | 3 × 3 ＝ 9 |
| Height squared | 4 × 4 ＝ 16 |
| Add the two | 9 ＋ 16 ＝ 25 |
| Distance to the top | Square root of 25 is 5 km (5000 m) |
| Until the first sound (base) arrives | 3000 ÷ 340 ≒ 8.8 s |
| Until the last sound (top) arrives | 5000 ÷ 340 ≒ 14.7 s |
| Rumble duration T | 14.7 − 8.8 ＝ 5.9 s |
Although lightning lasts an instant, the 4 km of vertical path alone stretches the sound to about 6 seconds. For lightning that branches sideways more than 10 km inside the cloud, the calculation suggests the rumble can last tens of seconds.
High schoolHigh school+The Speed of Sound and How Sound Bends
High schoolThe speed of sound in air is roughly "331.5 ＋ 0.6 × temperature (°C)" metres per second. The speed of sound depends on the air temperature, and sound travels faster in warmer air.
High school+In the daytime the air is warmer near the ground, so sound bends upward and escapes into the sky. This is called refraction of sound. Thunder usually becomes hard to hear beyond about ten-odd kilometres. This is thought to be because, besides absorption, refraction creates a "sound shadow" where sound does not reach the ground.
UniversityFrom Shock Wave to Sound, and Absorption by the Atmosphere
The lightning path is thought to be heated to about 30,000 °C within a few microseconds, and its expansion first spreads as a shock wave. The shock wave weakens over a few metres and turns into an ordinary sound wave. Its later travel is affected by classical absorption, due to the viscosity and heat conduction of air, and by absorption from the vibrational relaxation of oxygen and nitrogen molecules. The absorption coefficient grows roughly in proportion to the square of the frequency, so the farther the thunder, the more only its low components remain. Some research works backward from the distribution of thunder arrival times to find the shape of the path.
📖 Derivations and further reading: Thunder (Wikipedia, in Japanese) ／ Speed of sound (Wikipedia, in Japanese)
ResearchWhat Is Still Not Well Understood
- Where the inaudible low notes come from Whether the very low notes in thunder come only from heat-driven expansion, or whether electrical forces inside the cloud also play a part, is said to be still under debate.
- Drawing the lightning's shape from sound There are attempts to record thunder with several microphones and reconstruct the 3D shape of the path. How accurately the unseen branches inside the cloud can be drawn is still being researched.
- The sound of winter lightning Winter lightning on the Sea of Japan coast is said to differ in nature from summer lightning. What differences there are in how it sounds has not yet been studied in detail.
So the content of this article is also "an explanation of what is known so far." The broad picture of the length and lowness of the rumble is solid, but some questions about the finer details of the sound's character remain unanswered.
Links to Textbooks (by Level)
| Level | Subject and unit | Where it appears in this article |
|---|---|---|
| Junior high | Year 7 science, "Properties of sound" | Finding distance from the seconds between flash and sound |
| High school | Basic Physics, "Sound waves" | Speed of sound and temperature, calculating the time difference |
| High school+ | Physics, "Refraction of waves" | The "sound shadow" where thunder cannot be heard |
| University | Acoustics, atmospheric physics | Shock waves, classical absorption and vibrational relaxation |
| Research | Atmospheric electricity | How low notes arise, reconstructing the path from sound |
| ― | Links to daily life | Why you should take shelter as soon as you hear thunder |
- Japan Meteorological Agency (気象庁), "Lightning observation and statistics" (雷の観測と統計)
- Wikipedia (ウィキペディア), "Thunder" (雷鳴)
- Rakov, V. A. & Uman, M. A. "Lightning: Physics and Effects", Cambridge University Press, 2003
- Few, A. A. "Power spectrum of thunder", Journal of Geophysical Research, 1969
- National Astronomical Observatory of Japan (国立天文台), ed., Rika Nenpyo (理科年表, Chronological Scientific Tables), Maruzen Publishing (丸善出版) (value for the speed of sound)
※This article is a general-audience science explainer. The numbers given are rough guides to help you understand the mechanism. During a thunderstorm, follow information from the Japan Meteorological Agency and instructions from the fire department, local authorities and others.