Science
What is Thunder?
Thunder is the loud sound made by lightning. When a lightning bolt superheats the air around it in an instant, the air expands explosively and then snaps back, sending out a shock wave we hear as a crack or rumble.
See it, don’t just read it.
Watch a 2-minute lesson with voice + animation that explains thunder.
Key things to understand
- 1Thunder is caused by lightning — they're two parts of the same event.
- 2Lightning heats the air to ~30,000°C in a fraction of a second, making it expand faster than the speed of sound.
- 3That rapid expansion creates a shock wave, and the shock wave is the sound of thunder.
- 4A nearby strike sounds like a sharp crack; a distant one rumbles because the sound spreads out and echoes.
- 5Counting the seconds between the flash and the thunder tells you roughly how far away the lightning was (about 3 seconds per kilometre).
Frequently asked questions
- What causes thunder?
- Lightning. The bolt heats the air so fast that it explodes outward as a shock wave, and that shock wave is the thunder you hear.
- Why does thunder sometimes rumble and sometimes crack?
- A close strike makes a sharp crack. A distant one rumbles because the sound comes from along the whole bolt and echoes off clouds and hills, arriving stretched out over time.
- Can you have thunder without lightning?
- No — thunder is the sound of lightning, so every clap of thunder is made by a lightning bolt, even if you didn't see the flash.