Thunder is produced when lightning heats up the surrounding air very rapidly, causing it to expand faster than the speed of sound. This produces a ‘sonic boom’ like the one caused by a supersonic aircraft. If you count the seconds between seeing a flash of lightning and hearing thunder and divide this number by 3, it will tell you how many kilometres away from the lightning you are. Sound travels at about 1220 km/h, while light from the lighting strike travels so fast that you see it almost instantaneously.
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