Estimate lightning distance from thunder delay.
Light reaches you effectively instantly while thunder crawls, so the gap between the two measures distance. Sound travels at about 343 m/s in 20 C air, and that speed rises roughly 0.6 m/s for every degree warmer, so each second of delay is close to a third of a kilometre, or about a fifth of a mile. Timing two consecutive strikes turns the same arithmetic into a closing speed: if the delay is shrinking, the cell is coming towards you, and the rate tells you how much time you have. The widely taught 30-30 rule follows from this — shelter when the delay drops to 30 seconds, because strikes can jump roughly 10 km, and wait 30 minutes after the final thunder before going back out.
Flash-to-bang distance
Speed of sound = 331.3 + 0.606 x temperature in C; distance = speed of sound x delay / 1000; closing speed = (previous distance - current distance) x 60 / minutes between strikes
Lightning is lethal and this is an estimate, not a safety system. No place outdoors is safe during a thunderstorm: move inside a substantial building or a fully enclosed metal-topped vehicle, and follow the guidance of local weather authorities rather than a calculated distance.
Close enough. At 20 C, sound covers a mile in about 4.7 seconds and a kilometre in about 2.9, so dividing by five for miles or three for kilometres is a good field approximation — this calculator just applies the temperature correction as well.
Because lightning does not only strike below the cloud it came from. Bolts from the anvil of a thunderstorm can travel 10 km or more horizontally and hit under a clear sky, which is why the 30-second threshold exists rather than waiting for rain.
Thunder is normally audible to about 15 km, though temperature inversions can bend the sound upward and silence it sooner. Silent, visible lightning means the cell is distant but not necessarily harmless — keep watching for the delay to shorten.