Why does thunder always come after lightning? Have you ever counted the number of seconds between lightning and thunder to see how far away a storm was? If you have, then you were doing a linear-relationship math problem using time and the speed of sound!

#### Why It Matters

Seeing a lightning bolt in the sky and hearing a boom of thunder are two results of the same storm event. This event is a huge electrical discharge between charged regions in the clouds and on the ground. So why do we see lightning before we hear it? It’s because light travels a whole lot faster than sound. In just one second, light travels 186,282 miles, while sound travels only about \begin{align*}\frac{1}{5}\end{align*}^{th} of a mile. If we assume that the extraordinary speed of light allows us to see lighting at *almost* the instant it occurs, then we can use the linear relationship between distance and time to estimate how far away a storm is… and to determine if we might want to run for cover!

Imagine that you saw a flash of lightning and then counted *one-mississippi, two-mississippi, three-mississippi… *BOOM! How far away did the lightning strike? Well, if the sound of thunder travels at an approximate rate of 0.2 miles per second, and it took 3 seconds to get to you, then it must have traveled a journey of about \begin{align*}0.2\times3=0.6\end{align*} miles. Notice the linear relationship between time and distance: 5 seconds is 1 mile, 10 seconds is 2 miles, and so on.

See for yourself: http://www.lightningsafety.noaa.gov/distance.htm

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Explore More

Learn more about lightning, thunder, and the speed of sound with the following link. Then consider the questions below.

http://www.srh.noaa.gov/jetstream/lightning/thunder.htm#distance

- Is the speed of sound always the same?
- What kind of relationship do atmospheric temperature and the speed of sound have?