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Computer graphic showing method to approximate distance
 to a lightning strike.

Air is a gas, and a very important property of any gas is the speed of sound through the gas. The speed of "sound" is actually the speed of transmission of a small disturbance through the gas. Sound is a sensation created in the human brain in response to sensory inputs from the inner ear. The speed of sound through the atmosphere is a constant that depends on the daily temperature. On a standard day at sea level static conditions, the speed of sound is about 760 mph, or 1100 feet/second. We can use this knowledge to approximately determine how far away a lightning strike has occurred.

When lightning strikes, a bright flash of light is generated. Light travels at a constant 186,000 miles/second, which means that we see the flash immediately as it happens. The intense heat of the lightning generates a sound called thunder which is transmitted through the air at the speed of sound. Because the speed of sound is so much slower than the speed of light, we always see the flash before we hear the thunder.

On the figure, we show the sound waves that are generated by a lightning strike. The waves move at about 1100 feet/second. How far has the wave moved from the lightning strike after 2 seconds? We can use a rate equation to solve this problem:

d = s * t

distance d equals speed s times time t. In 2 seconds, at 1100 feet/second, the wave has moved 2200 feet. How far does it travel in 5 seconds? 5500 feet which is just a little over a mile (1 mile = 5280 feet). So, since the flash reaches our eyes instantly, if we count the number of seconds between the flash and when we hear the thunder, we can approximate the distance to the lightning strike:

d = 1100 * t (distance in feet)

Or, if we divide by 5280 feet/mile:

d ~= t / 5 (distance in miles)

Try it out during the next thunderstorm! But take shelter if the time is less than a couple of seconds!


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Editor: Nancy Hall
NASA Official: Nancy Hall
Last Updated: May 05 2015

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