How Lightning Distance Works
Lightning produces a visible flash that travels at the speed of light (essentially instantaneous) and a sound wave (thunder) that travels much slower at about 343 m/s in air at 20 degrees C. By measuring the time between seeing the flash and hearing the thunder, you can estimate the distance to the lightning strike. The common rule of thumb is "5 seconds per mile" or about 3 seconds per kilometer.
The speed of sound varies with temperature, humidity, and altitude. Warmer air transmits sound faster. This calculator accounts for temperature to provide a more accurate distance estimate than the simple rule of thumb.
The Formula
Where t is the time delay in seconds and T is the air temperature in degrees Celsius. At 20 C, sound travels at about 343 m/s.
Lightning Safety
| Time (seconds) | Distance (km) | Safety Level |
|---|---|---|
| < 10 | < 3.4 | Immediate danger - seek shelter |
| 10 - 30 | 3.4 - 10.3 | Danger - move indoors |
| 30 - 60 | 10.3 - 20.6 | Be alert |
| > 60 | > 20.6 | Relatively safe (for now) |
Frequently Asked Questions
Why is the 5-second rule only approximate?
The "5 seconds per mile" rule assumes a sound speed of about 322 m/s, which corresponds to cold air around 0 degrees C. At typical summer thunderstorm temperatures (25-35 C), sound travels faster, making the actual distance slightly greater than the simple rule predicts. Temperature corrections improve accuracy by 5-10%.
How far away can you hear thunder?
Thunder is typically audible up to about 15-25 km (10-15 miles). Beyond this, the sound wave is absorbed and refracted by the atmosphere. Temperature inversions and terrain can extend or reduce this range. Lightning without audible thunder is called "heat lightning."
What is the 30-30 rule?
The 30-30 rule states: seek shelter when the flash-to-bang time is less than 30 seconds (lightning within ~10 km), and remain sheltered until 30 minutes after the last thunder is heard. This provides a margin of safety since lightning can strike up to 15 km from the storm center.