When you see a flash of lightning, have you ever wondered how far away it is? There is a way to roughly calculate the distance using nothing but your eyes, ears and some basic arithmetic.
Begin counting as soon as you see the flash of lightning. Use a stop watch or begin counting in your head, "one, one-thousand," "two, one-thousand," "three, one-thousand."
Listen for the thunder. As soon as you hear it, stop counting.
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Divide the number of seconds you counted by five. Your answer will be the approximate number of miles between you and the the lightning. For example, if you counted to 15, the lightning is about 3 miles away.