Counting the seconds
Lightning and thunder happen at the same moment, but they reach you at very different speeds. The flash travels at the speed of light and arrives almost instantly; the sound of thunder travels at about 343 meters per second (1,125 feet per second) at room temperature. Counting the seconds between the two therefore tells you how far away the strike was.
- Miles = seconds ÷ 5 (the sound covers about a mile every 4.7 seconds)
- Kilometers = seconds ÷ 3
- Exact distance = seconds × speed of sound, where the speed of sound is 331.3 × √(1 + T ÷ 273.15) m/s with T in °C
Worked example: 40 seconds at 68 °F
- 68 °F = 20 °C, so sound travels at 343.2 m/s.
- Distance: 40 × 343.2 = 13,729 m, which is 8.5 miles (13.7 km).
- The quick rule gives 40 ÷ 5 = 8 miles: close enough to decide what to do.
Distance by seconds counted
| Seconds | Distance | Quick rule |
|---|---|---|
| 3 | 0.6 mi (1.0 km) | 0.6 mi |
| 5 | 1.1 mi (1.7 km) | 1 mi |
| 10 | 2.1 mi (3.4 km) | 2 mi |
| 15 | 3.2 mi (5.1 km) | 3 mi |
| 30 | 6.4 mi (10.3 km) | 6 mi |
| 60 | 12.8 mi (20.6 km) | 12 mi |
The 30-30 rule and “When thunder roars, go indoors”
Lightning can strike 10 miles or more from the storm that produces it, sometimes out of a clear sky. That is why the National Weather Service’s advice is simple: if you can hear thunder, you are within striking distance. The older 30-30 rule says to seek shelter if the count is 30 seconds or less (about 6 miles), and to wait 30 minutes after the last thunder before going back outside.
Where to shelter
- Safe: a fully enclosed building with wiring and plumbing, or a hard-topped car or truck with the windows closed.
- Not safe: open shelters, picnic pavilions, tents, golf carts, under a tree, or near a fence, a pole or water.
- Indoors, stay off corded phones, away from windows, and avoid showers and baths during the storm.
If you are caught outside
There is no completely safe place outdoors. Get off ridges and summits, away from tall isolated objects and water, and keep moving toward shelter. Groups should spread out so that a single strike cannot injure everyone. A person struck by lightning carries no electrical charge: call 911 and start CPR right away if they are not breathing.
Why the count is approximate
Temperature changes the speed of sound by about 0.6 m/s per degree Celsius, and wind and terrain bend sound waves. Thunder is also a long rumble because different parts of the lightning channel are at different distances: count to the first sound you hear. Beyond 10 to 15 miles, thunder is usually inaudible.