What causes thunder?

A lightning stroke heats the narrow channel of air it passes through to somewhere around 50,000 degrees Fahrenheit, several times hotter than the sun's surface, in a few millionths of a second. Air cannot expand fast enough to accommodate that, so it is driven outward violently, creating a shock wave that decays into a sound wave as it travels. That is thunder. There is no separate cause: every lightning stroke produces thunder, and every clap of thunder was produced by lightning, even when the flash was hidden by cloud or occurred beyond the horizon.

The character of the sound comes from the geometry of the channel. A lightning bolt is miles long and jagged, so sound from different parts of it arrives at your ear at different times, spread across seconds. Nearby strikes produce a sharp crack from the closest portion followed by rumbling from the rest. Distant strikes produce only a low rumble, because higher frequencies attenuate faster over distance and are refracted away by temperature layers in the atmosphere. Thunder is rarely audible beyond about ten miles, which is why heat lightning is not a separate phenomenon but simply lightning too far away for its thunder to reach you.

The practical use of thunder is ranging. Light arrives essentially instantly while sound travels roughly one mile every five seconds, so counting the seconds between flash and thunder and dividing by five gives the distance in miles. The safety rule built on this is that if you can hear thunder at all, you are within striking distance, since lightning can travel horizontally well away from the parent storm. The National Weather Service guidance is to move indoors when thunder is first heard and to wait thirty minutes after the last thunder before going back out, which is when most lightning fatalities occur, as people return to activity too early.