Light arrives effectively instantly, sound travels about 343 metres per second. Count the seconds between the flash and the thunder and divide by three for the distance in kilometres. Three seconds is roughly one kilometre, nine seconds three kilometres.
If you hear the thunder within ten seconds the storm is inside three kilometres, and the next strike could just as easily land where you stand. Lightning regularly jumps several kilometres sideways from the cloud, so the distance to the last strike tells you little about where the next one goes.
Thunder rarely carries beyond about fifteen kilometres. Past that, the sound waves bend upwards through warmer air aloft and pass over your head. The map can therefore be full of dots on a quiet evening without you hearing a thing, and what you are seeing is usually a storm fifty to a hundred kilometres away.
The reverse also holds: if you see the flash but hear nothing after half a minute, it is beyond earshot and poses no danger where you are.
Thunderclouds are built by strong updraughts, and updraughts need the ground to warm first. Swedish thunder therefore peaks in July and usually arrives in the afternoon and evening, once the day's heat has accumulated. A storm rolling in at night is generally a different animal: it travels with a front rather than growing in place, and can sweep across whole regions.
Frontal storms move fast and pass. Heat storms often stand nearly still and can dump a great deal of rain on a small area, which is what floods basements in one neighbourhood while the next village stays dry.
Lightning data comes from the Finnish Meteorological Institute, which publishes the Nordic sensor network's observations as open data under CC BY 4.0. SMHI's own lightning service is a paid product and is not used here, which is why this map is free. The longer treatment is in the guide to thunder and lightning, and every source behind Nollan is listed under about Nollan.
Written for Nollan.se. Knowledge base · About Nollan · Nollan.se