Nib asked, and Nib — askNib's tutor — drew the answer live at a whiteboard. This is the spoken transcript; enable JavaScript to watch it drawn.
what causes the sound of thunder
Thunder is not clouds bumping together. It is the sound of the lightning itself, and everything starts with the heat of the bolt.
In microseconds, a bolt heats a thin channel of air to about thirty thousand degrees, five times hotter than the Sun's surface.
Air that hot cannot stay put. It expands violently, faster than sound, shoving the cold air aside like a tiny explosion. That shock wave is the crack.
The flash reaches you at once, but sound crawls: one kilometer every three seconds. Count from flash to bang to find how far away it struck.
So why a rumble, not one bang? A bolt is kilometers long. Its nearest part reaches you first, its top last, smearing the sound out in time.
Sound also bounces off hills and cloud layers, arriving late from new directions. That is why a rumble can roll on for many seconds.
So thunder is lightning's shock wave: super-hot air bursting outward, heard late and stretched into a rumble. Count the seconds to find the storm.
Next storm, count the seconds: you are timing a sound that was born in air hotter than the Sun.