About Where's That Lightning

Why this exists

There are several good live lightning maps. All of them show you where lightning struck; none of them tell you when you will hear it. The familiar advice, count the seconds and divide by five, assumes a fixed speed of sound and ignores humidity, pressure and wind entirely.

That approximation is off by up to 8% across a normal year's weather. This site does the real calculation instead, and draws the expanding thunder wavefront on the map so you can watch it reach your position.

How the calculation works

• Speed of sound

  The Cramer (1993) equation from the Journal of the Acoustical Society of America, using temperature, humidity, pressure and CO₂. It is accurate to roughly 300 parts per million between 0 and 30 °C.

• Live conditions

  Weather is fetched for both your position and the strike location, then averaged along the path. Sound crosses both air masses, and they are frequently several degrees apart.

• Wind advection

  Sound is carried by the air it travels through. The component of wind along the strike-to-listener path is added to the propagation speed, worth about 0.85 seconds over 10 km in a 10 m/s wind.

• Path geometry

  Thunder arrives from the nearest point of the lightning channel, and sound speed changes with altitude, so the speed is averaged along the slant path rather than taken at ground level.

What this site is honest about

The largest error is not in the physics. It is in the strike position. Community detection networks locate strikes to within a few kilometers on average, and one published study of Blitzortung.org data in Japan found a mean error around 5.6 km. At the speed of sound that is roughly 16 seconds of arrival-time uncertainty.

That is far larger than the fraction of a second the precise formula buys back. Rather than hide it behind a confident-looking number, the map draws that uncertainty as a visible band around every wavefront, and the strike panel shows the ± window alongside the countdown.

Data sources

• Blitzortung.org supplies strike locations, from a volunteer network of ground stations that triangulate the radio pulse each discharge emits. Used non-commercially with attribution.
• Open-Meteo supplies temperature, humidity, pressure, wind, precipitation and air quality.
• OpenStreetMap contributors via OpenFreeMap supply the base map tiles.