Overview
Thunder is a result of the rapid expansion of super heated air caused by the extremely high temperature of lightning. As a lightning bolt passes through the air, the air expands faster than the speed of sound, generating a "sonic boom".
Since the sonic boom is created along the path of the lightning bolt, in effect, millions of sonic booms are created, which we hear as a rumble.
Thunder from a nearby lightning strike will have a very sharp crack or loud bang, whereas thunder from a distant strike will have a continuous rumble. The primary reason for this is that the sound shock wave alters as it passes through the atmosphere.
Sound travels roughly 750 mph (1,200 km/h), or approximately one mile every 5 seconds (one kilometer every 3 seconds). The speed actually varies greatly with the temperature, but 5 seconds per mile (3 seconds per kilometer) is a good approximation.
Through a series of examples, the student will be able to determine the distance to a lightning strike.
| TOTAL TIME | 10 minutes |
|---|---|
| SUPPLIES | Flashlight. Optional: thunder sound files (see below); camera flash. |
| PRINTED/AV MATERIAL | None |
| TEACHER PREPARATION |
None unless you would like to use the sound files. You can download the following thunder sounds to a computer or smartphone. The sounds are in mp3 format. Very Sharp Thunder (161k), Sharp Thunder (201k), Close Rumble (237k), Far Rumble (246k) |
| SAFETY FOCUS | Lightning safety |
Procedure
- Instruct the students about thunder and why it occurs. Ensure they know sound travels about one mile every five seconds (one kilometers every three seconds). Instruct the student that they can approximate seconds by counting "One-Mississippi", "Two-Mississippi", "Three-Mississippi", etc.
- Have the student look at the end of the flashlight and instruct them to begin counting once they see it light up.
- Rapidly turn the flashlight on, then off.
- Count five seconds, then either say "BOOM" or play one of the sharp thunder sounds.
- Have the students divide the time from the first light to hearing the sound by 5 seconds to determine the distance in miles from the lightning bolt.
- Repeat the procedure but wait ten seconds between flashing the light and playing the sound.
- Repeat the procedure again, but wait 15 seconds between flashing the light and playing the sound.
- Repeat the procedure several more times, but vary the time from flash to sound (two seconds, 14 seconds, etc.). Remember, the longer the time between flash and sound, the farther away the lightning is, so use the appropriate thunder sounds (distant rumbles) that are an indication of distance.
Discussion
Each time you do the procedure, there will be some variability in the student's results due to inconsistent counting of the seconds. However, you will quickly be able to understand the student's grasp of the concept by inquiring how many seconds they counted. For more accurate results, have the student use stop watches or the second hand of a clock.
For advanced students, during the next thunderstorm, have them record from home the local time (in hours, minutes, and seconds) and direction of up to 20 cloud-to-ground lightning strikes and the time thunder was heard. Then have the student compare their results with each other. Inform students that cell phones and computers can only be safely used during a thunderstorm as long as they are not plugged in.
On a map of your local area, plot the student's homes and, by triangulation, determine the location of the strikes based upon the time and direction of occurrence at each dwelling.
Building a Weather-Ready Nation
Lightning kills between 20 and 30 people in the United States each year, and hundreds more are severely injured. Many of these tragedies can be avoided. Finishing the game, getting a tan, or completing a work shift are not worth death or crippling injury.
- All thunderstorms produce lightning and are dangerous.
- Lightning can strike more than 25 miles (40 km) away from any rainfall. Many deaths from lightning occur ahead of the storm because people wait until the last minute before seeking shelter.
- Lightning can strike well beyond the audible range of thunder. If you hear thunder, the thunderstorm is close enough that lightning could strike your location at any moment.
- Lightning injuries can lead to permanent disabilities or death. On average, 20% of strike victims die and 70% of survivors suffer serious long term effects.
- Look for dark cloud bases and increasing wind. Every flash of lightning is dangerous, even the first. Head to safety before that first flash. If you hear thunder, head to safety!
- NO PLACE outdoors is safe during a lightning storm. If you hear thunder or see lightning, or if dark clouds are gathering overhead, quickly move indoors or into a hard-topped vehicle and remain there until 30 minutes after the final clap of thunder. Check forecasts and warnings through weather apps, NOAA Weather Radio, or your local TV and radio stations. If lightning is forecast, plan an alternate activity or know where you can take cover quickly.
What to do!
The best thing you can do is stop your outdoor activity and move indoors or get in a hardtop vehicle (not a convertible). Don't wait for rain to begin before you act. Once indoors, stay away from electrical appliances and corded phones (cell phones are safe to use, as are laptops that are not plugged in). Lightning can travel long distances in both phone and electrical wires, particularly in rural areas.
There are many more lightning safety rules to live weatherwise. You can find them at the NWS Lightning Safety website.