At sea level the speed of sound in air is about 340 m/s. The speed of light is much much higher than that (about 300,000 kilometers per second!). So you will first see ? the light of the lightning and then you will hear ?️ the sound that arrives at lower speed.
Because of that you can count the seconds elapsing and then multiply by 340 to get the distance between you and the storm in meters.
Of course this is a simple approximation, since the speed of sound can change with the air conditions.
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