Why do we see lightning before we hear thunder?

Why do we see lightning before we hear thunder?

Lightning and thunder are produced at the same instant, but light travels at about 300,000 km/s while sound travels at only about 343 m/s. The flash reaches you almost immediately; the thunder takes seconds to cover the same distance.

That gap is a free rangefinder. Every $3$ seconds between flash and thunder means the strike was about $1$ km away.

How do you find the distance to a lightning strike?

Start counting when you see the flash and stop when you hear the thunder. Because light takes a negligible time to arrive, the delay $t$ is simply the travel time of sound, so distance is speed times time:

Example: you count $6$ seconds. Then $d = 343 \times 6 \approx 2058$ m, a little over $2$ km. Light took only about $7$ microseconds to cover that distance, so ignoring it is safe.

What causes thunder?

A lightning bolt heats the air along its path to about $30{,}000\,°\text{C}$, several times hotter than the surface of the Sun, within microseconds. The air expands explosively, launching a shock wave that becomes the sound we call thunder.

Why does thunder rumble instead of bang?

A lightning channel is several kilometres long. Sound from the nearest part reaches you first and sound from the farthest part arrives later, so one instant of lightning is stretched into a rolling rumble. Reflections from hills and clouds add to the effect. Close strikes, where the whole channel is at a similar distance, produce a sharp crack.

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