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The speed of sound is 344 m/s (1238 km/h) or 1128 ft/s (769 mph) . . .
. . . where the medium of transmission is air and the temperature around the 20ºC mark. Hence, for rough calculations, 3 seconds between flash and boom is one kilometre to the source. 4.5 seconds gives you about a mile. These are very rough but usually enough for us poor sods who have trouble judging time in such small packets to guesstimate the distances.
Of course there are many variables. My rule of thumb is that if you hear the 'tearing' sound often associated with lightning then it's too bloomin' close.
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