Skip to story
ELI5 Yahaaa

Air exploding

Why Does Thunder Rumble?

Lightning heats air so fast that it explodes outward, and the bang reaches you slowly.

1 The bolt heatsair to 30,000°C2 Air explodesoutward3 You hear thebang, secondslater
Light arrives instantly. Sound takes its time.

  1. Step 1 of 5

    Lightning heats air almost instantly.

    Lightning channelabout 30,000°CSun's surface5,500°CHot summer day30°C

    In under a thousandth of a second the bolt's channel reaches about 30,000°C — five times hotter than the Sun's surface.

  2. Step 2 of 5

    Hot air expands violently.

    Superheatedchannel A shockwavepunches outward

    Heated that fast, the air cannot get out of the way politely. It blasts outward faster than sound, which is what an explosion is.

  3. Step 3 of 5

    That shockwave becomes the sound.

    Right next to it A single sharp crackFar away A long rumble

    Within a few metres the shock slows to an ordinary sound wave — but an extremely loud one. Close by it is a crack, not a rumble.

  4. Step 4 of 5

    The rumble comes from the bolt's length.

    1 A 5 km longbolt2 Near end'ssound arrivesfirst3 Far end arrivesseconds later —a rumble

    A bolt can be several kilometres long. Sound from the near end reaches you first and the far end much later, smearing one bang into a roll.

  5. Step 5 of 5

    Counting tells you the distance.

    Light Arrives instantlySound 1 km every 3 seconds

    Light arrives instantly; sound travels about a kilometre every three seconds. Count from flash to bang and divide by three for kilometres.

    Echoes off clouds and hills stretch the rumble further, which is why mountain thunder goes on so long.

The short version

Thunder is air exploding away from a lightning channel, and the rumble is the sound of a long bolt arriving in stages.

Try it yourself

Next storm, count seconds between flash and bang. Divide by three for kilometres — and watch the number shrink as it nears.

Topics

Next tiny story