Why do we see lightning before we hear thunder?

Lightning and thunder happen at exactly the same moment. Lightning is a giant electric spark inside a storm cloud. The spark heats the air so fast that the air explodes outwards — that explosion is the sound of thunder.

So why do we see the flash first? Because light and sound travel at very different speeds. Light is incredibly fast — about 300,000 kilometres every second. Sound is slow — about 340 metres every second.

The flash reaches your eyes almost instantly. The sound takes time to arrive. If you count the seconds between the flash and the thunder and divide by three, you get the distance of the storm in kilometres.

Three seconds means the lightning was about one kilometre away. If the flash and the bang come together, the storm is right above you — go inside.