How the distance is calculated

Weather satellites in geostationary orbit watch for the brief light lightning makes at the cloud tops: NOAA's GOES Geostationary Lightning Mapper over the Americas and EUMETSAT's Lightning Imager over Europe, Africa and the Middle East. They see total lightning — flashes inside clouds as well as those that reach the ground — and report each flash as one point near its centre. The full list, with licences and limits, is on the data sources page.

We take your point — your phone's location in the app, your browser's location on lightning near me, or the city centre on a city page — and measure the great-circle (haversine) distance to every flash from the last 60 minutes. The shortest one is the number you see.

The word “strike” in our headings is the everyday word. Strictly, the point is a flash centre seen from orbit: it may be a flash that never touched the ground, and when it did, the ground contact can be several kilometres from the point.

Both satellites place a flash within roughly 8–14 km (5–9 mi). A reading of “~6 mi” means the nearest flash was reported about 6 miles away, give or take that much. Treat any reading inside that band as lightning overhead. Your own location adds its own error: an approximate phone location can be off by a few kilometres more, and the app says so on screen when that happens.

New GOES data arrives about every 20 seconds, and a flash over the Americas usually reaches the app within about 2 minutes. EUMETSAT publishes its flashes in batches, so over Europe, Africa and the Middle East it can take up to about 10 minutes. The free tier adds a delay of 1–2 minutes; the subscription removes it. Every screen says how old its data is.

The trend compares where the nearest flashes sat over the last 10 minutes. If they keep getting closer to your point, the storm reads as approaching; if they keep getting farther, as moving away. The same calculation decides the “storm approaching” alert, so the screen and the notification always agree. With too few flashes to tell, the app says the trend is unknown.

Alerts are matched on our server, so the app never needs your location in the background. When a flash lands inside the radius you set around a saved place, the server sends a push. Because of the satellite's accuracy, the smallest radius you can set is 5 miles.

StrikeCast reports lightning that has already happened. It does not forecast the next strike and it is not a warning service. Follow official warnings from the National Weather Service or your national weather service, and go indoors when you hear thunder — sound reaches you even when no satellite has reported anything yet.