Re: How can I tell how far away lightning strikes when I hear the thunder?

Lightning travels at the speed of light...186,000 miles per second.  
Thunder, the sound generated by the lightning strike travels at the speed 
of sound, about 1/5 that of light.  So, when you see a strike, count the 
number of seconds until you hear thunder then divide by five.  The result 
is the approximate number of miles to the lightning strike.  

That said, lightning bolts aren't straight lines, they can be horizontal 
rather than vertical, and one end may be much further away than another.  
This means the sound from one part of the bolt may reach your ears at a 
different time than from another part, accounting for the different sounds 
thunder can make...anything from a sharp clap to a long low rumble.  

Admin note:

Michael Parker adds, "the statement 'the sound generated by the
lightning strike travels at the speed of sound, about 1/5 that of light'
is not correct.  The speed of sound is approximately 1/545,077 that of
light."

The 1/5 refers, instead, to an approximation of the speed of sound of 1/5
miles per second, or 1056 ft/sec - the actual speed of sound is closer to
1100 ft/sec, or 750 mph.