When lightning is generated, the electric discharge produces a great quantity of heat that ionizes the surrounding air, transforming it into a plasma (gas consisting of only electrons and nuclei) at an extremely high temperature (10-15,000° C). The air all around expands violently, in a few millionths of a second, provoking the propagation of a compression wave though the air, which we hear as an acoustic wave that produces thunder. Thunder propagates at the speed of sound (350 m/s), while lightning travels at the speed of light (300,000 km/s), therefore the sound produced by the lightning is heard with a slight delay. We can enjoy calculating the distance of a storm cloud by measuring the time-interval from when we see the lightning to when we hear the thunder. With a rather approximate calculation, dividing the number of seconds that have passed by three, we obtain the distance in kilometres.

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