National Weather Service United States Department of Commerce

Learning Lesson: Determining distance to a Thunderstorm


Thunder is a result of the rapid expansion of super heated air caused by the extremely high temperature of lightning. As the 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 modifies 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 the thumb rule of 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

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


  • Instruct the students about thunder and why it occurs. Ensure they know sound travels about one mile every five seconds (three 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 and off.
  • After you count five seconds, 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 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 thunder sounds (distant rumbles) that, by themselves, are an indication of distance.


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 the second hand of watches or use stop watches.

For advanced students, during the next thunderstorm, have the class record 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.

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. (DO NOT have the students contact one another during a thunderstorm unless it is by cell or cordless phone. Some people have died while using the phone when lightning struck a nearby telephone pole.)

Building a Weather-Ready Nation

Lightning kills an average of 49 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 kills more people each year than tornadoes and hurricanes combined.
  • 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; 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 lightning is seen or thunder is heard, 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. Listen to forecasts and warnings through 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 automobile (not a convertible). Don't wait for rain to begin before you act. Once indoors, do not use corded telephones unless it is an emergency as the phone line is the leading cause of indoor lightning injuries in the United States. 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.