How far away is lightning? Count the seconds between flash and thunder

01

What is the 5-second rule for lightning?[1]

The 5-second rule says every 5 seconds between the lightning flash and the thunder equals about 1 mile of distance. Divide your count by 5 for miles, or by 3 for kilometers. A 15-second gap means the strike hit roughly 3 miles or 5 kilometers away.

The rule works because light and sound travel at wildly different speeds. Light covers 10 km in about 33 microseconds, which is effectively instant. Sound covers the same distance in roughly 29 seconds. So the gap you count is almost entirely the time thunder spends traveling to your ears.

At 20 °C in dry air, sound moves at 343 meters per second. That is 1 kilometer every 2.9 seconds, or 1 mile every 4.7 seconds, which is where the familiar 3 and 5 divisors come from. They are rounded for mental arithmetic, not for accuracy.

02

How accurate is counting seconds?[2]

Counting is accurate to roughly ±20% under normal conditions. The divide-by-5 rule assumes 343 m/s, but real sound speed ranges from about 325 m/s on a freezing day to 351 m/s in hot, humid air, a yearly spread of roughly 8%. Human reaction time adds a further half-second or so.

Three things push the true answer away from the rule of thumb. Temperature is the largest: sound speed rises about 0.6 m/s for every degree Celsius, so air at 30 °C carries thunder roughly 6% faster than air at 0 °C.

Humidity matters more than most people expect. Water vapor is lighter than the nitrogen and oxygen it displaces, so humid air actually carries sound faster than dry air. At 20 °C and 100% humidity, sound travels 344.6 m/s against 343.4 m/s when dry.

Wind is the third factor, and the one the classic rule ignores completely. Sound is carried by the air it moves through, so a 10 m/s tailwind delivers thunder about 0.85 seconds early over 10 km, while the same headwind delays it by a similar margin.

03

How do you convert seconds to distance?

Multiply the seconds you counted by 343 to get meters, or use the shortcuts: divide by 3 for kilometers and by 5 for miles. A 30-second gap puts the storm about 10 km or 6 miles away, which is the outer edge of where thunder is normally audible at all.

3 seconds

about 1 km / 0.6 miles. Dangerously close, shelter immediately

5 seconds

about 1.7 km / 1 mile. Still well inside strike range

15 seconds

about 5 km / 3 miles. The storm is nearby and can reach you

30 seconds

about 10 km / 6 miles. The 30-30 safety threshold

45+ seconds

about 15 km / 9 miles, near the limit of audible thunder

04

Why can't you hear distant thunder?[1]

Thunder normally becomes inaudible beyond about 16 km (10 miles). Air usually cools with height, so sound travels slower higher up, bending the wavefront upward and away from the ground. Past that distance the sound passes overhead instead of reaching you, an effect called an acoustic shadow zone.

This is why you can watch a distant storm flash silently on the horizon. The lightning is real and the thunder exists, but refraction has lifted the sound above your head.

A temperature inversion reverses the effect. When a warm layer sits above cooler surface air, sound bends back down toward the ground and thunder can carry 25 km or more. Inversions are common on calm nights, which is why distant storms sometimes sound unusually close after dark.