Inside a mature thunderstorm, collisions between ice particles separate electric charge, until the charge jumps as lightning. The thunder that follows lets you measure how far away the flash was, using unit rates.
Key idea
Lightning heats a channel of air to around 30,000 °C in a fraction of a second. The air expands so violently that it makes a shock wave, which we hear as thunder. Light arrives almost instantly, but sound travels about 343 m/s, so each 3 seconds between flash and thunder is about 1 km (about 5 seconds per mile).
Why it works
In the cold upper part of the storm, small ice crystals and soft hail pellets (graupel) collide in the strong updraft. Each collision passes a little charge: the light crystals, carried up, end up mostly positive; the heavier graupel, lower down, mostly negative. Charge builds until the air between can no longer hold it apart, and a spark tens of kilometers long jumps between the cloud and the ground or within the cloud. Distance from the delay is a rate problem: light covers a kilometer in about 3 millionths of a second, so all of the delay is the sound's travel time.
Worked example: How far away?
You see a flash and count 12 seconds before the thunder. How far away was the lightning?
- Distance = speed × time = $343 \times 12 = 4116$ m.
- About 4.1 km (or with the shortcut, $12 \div 3 = 4$ km; in miles, $12 \div 5 \approx 2.4$ mi).
Answer: About 4 km
Worked example: Is it coming closer?
At 3:00 p.m. the delay is 15 s; at 3:10 p.m. it is 6 s. Estimate how fast the storm is approaching, using 3 s per km.
- At 3:00: $15 \div 3 = 5$ km away. At 3:10: $6 \div 3 = 2$ km away.
- It came 3 km closer in 10 minutes ($\tfrac{1}{6}$ hour).
- Speed: $3 \div \tfrac{1}{6} = 18$ km/h, toward you.
Answer: About 18 km/h
Common mistake
Believing lightning never strikes the same place twice. Lightning strikes tall, pointed and isolated objects again and again; the Empire State Building is struck more than 20 times in a typical year, according to the National Weather Service. Check: a place that was struck once is still tall and still exposed, so nothing protects it from the next strike.
Thunder can be heard about 15 km away or a little more, and lightning can strike that far from the storm. The National Weather Service's advice is short: "When thunder roars, go indoors."