Everyone knows you count the seconds between the flash and the bang. Fewer people know why it works, how accurate it is, or which of the safety rules they grew up with are actually true.
Light travels fast enough that for anything you can see, it arrives instantly. Sound does not: it moves at roughly 343 metres per second in ordinary summer air.
So the gap between seeing the flash and hearing the thunder is simply how long the sound took to reach you. Count the seconds and divide by three, and you have the distance in kilometres:
Beyond about 15 kilometres you usually stop hearing thunder at all, even though the flash may still be visible. A storm you can see but not hear is often called heat lightning, though there is nothing special about it: it is an ordinary storm too far away for the sound to carry.
A lightning channel heats the air around it to somewhere near 30,000 degrees, several times hotter than the surface of the Sun, in a small fraction of a second. Air that hot expands explosively, and the shockwave from that expansion is the thunder.
That also explains the two kinds of sound. A nearby strike gives a single sharp crack, because the whole channel is roughly the same distance from you. A distant one gives a long rolling rumble, because the channel is kilometres long and the sound from its far end arrives noticeably later than from its near end. Rolling thunder is not an echo, it is the length of the bolt.
A thunderstorm needs air rising hard and fast. The usual engine is the ground: sunshine warms the surface, the surface warms the air just above it, and that warm air becomes buoyant and climbs.
As it rises it cools, moisture condenses, and if the climb is vigorous enough the cloud builds into a towering anvil-shaped mass full of ice crystals and water droplets in violent motion. Collisions between them separate electrical charge, negative towards the bottom of the cloud and positive towards the top. When the difference grows large enough, the air breaks down and the charge equalises in a single enormous spark.
Sweden gets the necessary surface heating mainly from May to August, with July the peak. It is also why thunder favours the afternoon and evening: the ground needs most of the day to build up the heat.
The 30/30 rule is the one worth memorising. If the gap between flash and thunder is under 30 seconds, the storm is within about 10 kilometres and you should be indoors. After the last thunder, wait a full 30 minutes before going back out.
The second half is the part people skip. Strikes can reach several kilometres ahead of the rain, from a part of the cloud that looks harmless, and the back edge of a storm is not safe just because the downpour has stopped.
Where to go, in order of preference:
What to avoid: being the tallest thing around, sheltering under a lone tree, open water, and open fields. If you are caught with no shelter at all, move away from anything prominent and keep your feet close together, since a strike nearby spreads current through the ground.
Rubber tyres do not protect you. A car is safe because its metal shell carries the current around the outside, an effect that works the same whether the car is on tyres or on blocks. Rubber-soled boots offer nothing against a discharge that has just crossed several kilometres of air.
Lightning strikes the same place constantly. Tall buildings and masts are hit repeatedly, which is precisely why lightning conductors are worth installing. The saying is comforting and wrong.
Nollan.se has a live lightning map showing recent strikes, which is the quickest way to tell whether a storm is heading towards you or away.
Count the seconds between the flash and the thunder and divide by three to get the distance in kilometres. Sound travels roughly 343 metres per second, so three seconds is about one kilometre. Light arrives effectively instantly, so the delay is all sound.
Yes, provided it has a metal roof and you are not touching metal parts. The body of the car conducts the current around you and into the ground. It is nothing to do with the rubber tyres, which is a persistent myth. A convertible or a fibreglass shell gives no protection.
Thirty minutes after the last thunder you hear. Lightning can strike well ahead of the visible storm, and the tail end of a cell is still dangerous. Most people go out too early rather than too late.
Because thunderstorms need strong upward air movement, which comes from warm ground heating the air above it. Sweden gets very little of that between October and April. Winter thunder exists but is rare and usually brief.
Routinely. Tall structures are hit repeatedly, sometimes several times in one storm, because they offer the shortest path. The saying is comprehensively wrong.