Real-time thunderstorm detector based on the Blitzortung community network. Each yellow flash is a strike detected in the last seconds.
A lightning stroke is a kilometres-long antenna that emits a radio pulse at very low frequencies — what specialists call a sferic. That pulse travels thousands of kilometres following the curvature of the Earth, and there lies the trick: you do not need to see the storm to detect it. Blitzortung is a volunteer project whose home-built stations keep their clocks synchronised by GPS. When several receive the same pulse, the difference of milliseconds between arrival times is enough to triangulate the strike point, GPS in reverse. Something like half a dozen stations are needed to fix a discharge precisely.
Two practical consequences follow. The first is that accuracy depends on how many stations are nearby: over mainland Europe, where density is good, the typical error is a few kilometres; at the edge of coverage or out over the ocean it can be tens. The second is that the count is a floor, not a total: the network sees cloud-to-ground strokes well, those being the powerful ones, and misses much of the in-cloud discharge, which is far weaker at those frequencies. An active storm always produces more discharges than the map shows.
No map needed for that, just counting. Light arrives instantly and sound takes about three seconds per kilometre, so if nine seconds pass between flash and thunder, the strike landed some three kilometres away. From which comes the safety rule the weather services recommend, known as 30/30:
If fewer than 30 seconds pass between flash and thunder, the storm is within ten kilometres and can already reach you: get inside a building or a hard-topped car. And wait 30 minutes after the last thunder before going back out, because the closing phase of a storm keeps discharging when it seems to have passed, and the strikes of that phase land further from the core.
What does not protect you: a tree, an open porch, an umbrella, a summit, a fence line, water. What does: a building with electrical wiring and plumbing, or a closed metal vehicle, which works as a Faraday cage.
Because it is a volunteer network, not official infrastructure. Where few stations are online, discharges in that sector go unlocated even while they are happening, and if the data link drops the map goes blank for a few minutes. A map with no flashes means «the network is not detecting anything right now», not «there are no storms». For the official picture, radar and AEMET’s warnings always take precedence.
