In a cloud, fast moving air separates water droplets, causing electrical charges (like rubbing a pen on your sleeve). The electrical charges build up and causes huge electric shocks as they try to reach the ground. A huge current of electricity goes down, then back up, maybe four or five times in the flash of an eye. This makes a loud bang, which we call thunder. Light travels a lot faster than sound, so we see the flash before we hear the bang. The further away the lightning is, the longer the gap until we hear the rumble of thunder.

When lightning is close, we hear a sharp bang. When it is far away, we hear a rumble. Lightning is incredibly hot - it heats the air around it to a temperature hotter than the surface of the sun! The photo below is a time-lapse photo showing many different flashes of lightning hitting the ground over a period of time.

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