Flash-to-Bang Distance to a Lightning Strike

Light takes about eleven microseconds to cover three kilometres and sound takes nearly nine seconds, so for any purpose you have standing in a field, the flash is instantaneous and the thunder is not. Count the gap, multiply by the speed of sound, and you have the distance to the part of the channel nearest you. That is the whole of the physics, and it is genuinely reliable — which is exactly why the page has to be careful about what it does not tell you.

Start with the thing this page cannot do, because everything else is arithmetic. A distance is not a safety margin. Lightning reaches ground more than ten kilometres from the storm that produced it — the phenomenon has a name, the bolt from the blue — striking ahead of the visible cloud, behind it, and out of a patch of sky that looks perfectly clear overhead. A measured eight kilometres tells you where that flash went. It says nothing whatsoever about where the next one goes. The published guidance belongs to the meteorological services and is quoted here rather than invented: NOAA's National Weather Service and Environment and Climate Change Canada both publish the 30-30 rule — if the gap between flash and thunder is under thirty seconds, be inside a substantial building or a hard-topped vehicle already, and wait thirty minutes after the last thunder before going back out. NOAA's shorter public version of the first half is "When Thunder Roars, Go Indoors".

And silence proves nothing at all. Sound in the lower atmosphere is refracted upward by the ordinary decrease of temperature with height and by wind shear, so beyond roughly twenty-five kilometres — often far less, and less again in wind, in hills, or in traffic noise — the thunder from a completely normal flash simply passes over your head and never arrives. A flash you can see but cannot hear is not a distant flash. It is a flash whose sound went somewhere else.

Now the folk rules, which is why the temperature is an input here. "Five seconds per mile" and "three seconds per kilometre" are both rounded from one calculation done at one temperature. At 20 °C sound covers a mile in 4.69 s and a kilometre in 2.91 s, so both rules round up, and rounding up reports the storm as closer than it is — about 6 % closer, which is the conservative direction and is why nobody ever minds. What the rules cannot do is follow the air. The speed of sound in dry air is very nearly linear in temperature, $v = 331.3 + 0.606\,T_C$, running from about 319 m/s at −20 °C to about 353 m/s at +35 °C. That is a 10 % spread, and an identical five-count means 1.067 mi on a summer afternoon and 0.992 mi in a −20 °C snow squall.

Two practical cautions on the count itself. Thunder from a distant flash arrives as a long roll, and the distance corresponds to the first sound — timing to the loudest part or to the end overstates the range, sometimes badly. And in a busy storm it is very easy to time one flash against another flash's thunder, which produces a confident number about nothing. If the counts from successive flashes are wildly inconsistent, that is usually what has happened.

Humidity, incidentally, barely matters: moist air is slightly less dense than dry air at the same pressure, so sound travels marginally faster in it, but the effect is a fraction of a per cent and is swamped by the temperature term. Wind matters more, since it adds vectorially to the propagation speed along the path, but for a storm at a few kilometres it is still small compared with the uncertainty in your count.