Count the seconds between seeing a lightning flash and hearing the thunder, and estimate how far away the storm is.
Light from a lightning flash reaches your eyes almost instantly, but the sound of thunder travels much slower — roughly 343 metres per second in typical air conditions, or approximately 1 mile every 5 seconds, or 1 kilometre every 3 seconds.
Counting the seconds between seeing the flash and hearing the thunder, then converting that count using the speed of sound, gives a rough estimate of how far away the strike actually occurred — a technique often taught as a quick, no-equipment way to gauge storm distance.
This is a rough estimate rather than a precise measurement — actual sound speed varies slightly with air temperature and humidity, and human reaction time counting the delay introduces its own small error. Treat the result as a useful general indicator of how close a storm is, not an exact distance reading.
Light travels at roughly 300,000 km per second, while sound travels far slower at about 343 metres per second in typical air — light effectively arrives instantly for storm-distance purposes, while sound takes several seconds to cover even a short distance.
It's a reasonably good rough estimate, though not perfectly precise — the speed of sound varies slightly with temperature and humidity, and manually timing the delay introduces some human error. Treat it as a useful general indicator of distance, not an exact measurement.
Roughly 5 seconds of delay per mile, or 3 seconds per kilometre — this calculator applies the same underlying speed-of-sound conversion, just more precisely.
It only estimates distance based on the time delay — it's not a safety or weather warning system. If you can hear thunder at all, standard lightning safety guidance is to seek shelter, regardless of the estimated distance.
