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What is thunder?

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Quick answer

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.

Key things to understand

  1. Thunder is caused by lightning — they're two parts of the same event.
  2. Lightning heats the air to ~30,000°C in a fraction of a second, making it expand faster than the speed of sound.
  3. That rapid expansion creates a shock wave, and the shock wave is the sound of thunder.
  4. A nearby strike sounds like a sharp crack; a distant one rumbles because the sound spreads out and echoes.
  5. Counting the seconds between the flash and the thunder tells you roughly how far away the lightning was (about 3 seconds per kilometre).

People also ask

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.