Distance to Lightning

Lightning is an occasional visitor to almost every school district in the nation. Most students are aware that if you divide the time between the flash and the thunder by five, you get the number of miles away the flash is. The idea can be shown in most schools if the playground is large enough.

For this exercise, all you need is something to make noise and can be seen by students a few hundred feet away, a stopwatch and a large tape measure. (Pacing will do in a pinch) I've used two boards which are clapped together as a noisemaker; however, a hammer and board will do as well.

Have the students measure the distance across the playground and have a student hit the boards together on one side while the other students record the time between when they see the hit and the time the sound arrives. Since light travels almost instantaneously but sound takes around one second to travel 1000 feet, counting the time between lightning flash and thunder tells you how many thousand feet the closest point of the bolt hit. (Since a mile is around 5000 feet, the dividing by 5 does work..)

The Shortest Length of a Lightning Bolt

On a day with lightning and thunder, it is interesting to get a measure of the length of the lightning bolt assuming that it is one linear electrical discharge. Branching occurs frequently but the sounds from two branches are jumbled. The shortest length of a linear bolt can be estimated by starting as before, noting the closest distance to the bolt and continuing to count until the last thunder roll has been heard. The number of seconds yields the distance away of the other end of the lightning bolt. Of course, if the thunder grades into silence after 25 or so seconds, it may be too far away to hear. The difference between the times for the first and last gives the shortest length of the lightning bolt. Most bolts zig-zag considerably in their path so this method does not give the actual length, only the minimum length of the bolt.

Another observation which is interesting is to determine how far you can hear thunder. Continue to determine the distance to lightning bolts as a storm moves away. At some point, you will still see lightning but be unable to hear the thunder. The furthest you can hear thunder is the limit to this technique, probably about 5 miles.

Some computer sound cards also provide wave analysis programs which are installed when the sound card is installed. You can record with the sound cards as well. Record a few thunder claps and look at the waves with the analysis program.