Lightning makes the flash and the boom at the same moment. The flash reaches your eyes almost instantly, but the sound has to jog through the air — about 1 kilometre every 3 seconds. Count the gap, divide by three, and you know how far away the storm is.
What's actually happening
A lightning bolt heats its channel of air to around 30,000 °C in a few millionths of a second — five times hotter than the sun's surface. The air has no time to expand politely; it detonates outward as a shockwave that relaxes into sound. So the flash and the bang are born together, identical twins leaving the same address at the same moment.
They just don't travel the same way. Light crosses a few kilometres in about a hundred-thousandth of a second — for human purposes, instantly. Sound plods along at 343 metres per second, a million times slower. The silence between the flash and the boom is simply the sound still being in transit, and it converts directly into distance: three seconds of waiting per kilometre of storm. Counting "one-thousand, two-thousand…" after a flash isn't folklore; it's a speed-of-sound calculation you run in your head.
And why a long rumble instead of one clean bang? Because the bolt isn't a point — it's a crooked channel several kilometres long, often running sideways through the cloud. Sound from the nearest section reaches you first; sound from the far, high end might arrive ten seconds later, bouncing off hills and clouds on the way. You're not hearing many noises. You're hearing one noise, smeared out by geometry.
- 1Next thunderstorm, sit by a window and wait for a flash. Start counting seconds the moment you see it ("one-one-thousand…").
- 2Stop at the first rumble and divide your count by three: that's the distance in kilometres.
- 3Track three or four flashes in a row. You'll know — with numbers — whether the storm is approaching, passing, or leaving, which is more than most weather apps can tell you.