How do I tell how far away a thunderstorm is?
The speed of sound in air is considerably less than the speed of light. It is therefore possible to calculate the distance of a storm, provided you can observe the associated lightning, and the thunderstorm is close enough for the sound (of the thunder generated) to reach you (*).
A 'thunder-clap', or the noise our ears hear (ranging from a sharp crack for nearby, short path-length lightning strikes to a long, low rumble for a distant, long path-length discharge), is caused by the rapid expansion (due to heating) of air in the lightning channel that the stroke passes through. ( If you are frightened of thunder, it is useful to remember that by the time that noise has reached you, the really dangerous lightning strike has already occurred. )
Very roughly, the speed of sound in the lower atmosphere is 330 metres / second (or 1 mile / 5 seconds). If you observe the 'flash' of the lightning stroke, then begin counting (or timing) in seconds until the sound of the thunder just reaches you, using the relationships above will give a rough guide to the distance of the storm. Thus a time difference of 3 seconds will give you three-fifths of a statute mile, or about a kilometre; 6 seconds=> 2 km etc.
((*) thunder, under average atmospheric conditions, should be audible up to 8, possibly 10 km away. Strong winds & low-level temperature variations may though alter these values considerably, with values quoted up to 20 km in some cases; lightning at night can be seen up to 100 km away, depending upon your observing location, obstructions etc.)
[ Also see this FAQ entry ]