We see lightning before we hear thunder because light travels much faster than sound, so the flash reaches our eyes almost instantly while the sound waves take noticeably longer to reach our ears.

Lightning and thunder: same event, different speeds

Lightning and thunder actually happen at almost the same moment in the storm cloud. The lightning superheats the air, which then rapidly expands and creates the sound we call thunder.

But:

  • Light from the lightning travels at about 300,000,000 meters per second.
  • Sound (thunder) travels at about 340 meters per second through air.

Because light is so incredibly fast, the time it takes to reach you is essentially zero over the distances of a typical thunderstorm, while sound lags behind.

What this looks like in real life

Imagine you are 1 kilometer away from a lightning strike:

  • The flash reaches your eyes almost instantly (in a few microseconds, far too fast to notice).
  • The thunder reaches your ears about 3 seconds later, because 1,000 meters á 340 m/s ≈ 3 seconds.

That delay is why you often count “one-one-thousand, two-one-thousand…” between the flash and the boom. The longer the gap, the farther away the storm is.

A handy storm-distance trick

People use this speed difference as a rough “storm distance calculator”:

  1. Start counting as soon as you see the lightning.
  2. Stop when you hear the thunder.
  3. Divide the number of seconds by 3 to estimate the distance in kilometers.

For example, if you hear thunder 9 seconds after the flash, the storm is roughly 3 kilometers away (9 ÷ 3 ≈ 3). If you see lightning and hear thunder almost at the same time, the storm is very close and you should seek shelter immediately.

Same idea in other situations

The lightning–thunder effect is just a dramatic version of a common pattern:

  • Fireworks: you see the burst, then hear the bang a bit later when you’re far away.
  • Air shows: a fast jet can pass overhead before you hear its roar, because its sound lags behind the visual.

In all of these, light and sound are created together, but you notice them at different times because light outruns sound by an enormous margin.

TL;DR: Lightning and thunder happen together, but light travels so much faster than sound that your eyes get the message first and your ears catch up a few seconds later.

Information gathered from public forums or data available on the internet and portrayed here.