The rolling sound of thunder
EVERY thunder rumble has a different frequency. The noise we hear as thunder is created when lightning superheats the air it travels through. When air is heated it expands and, in a lightning flash, the expansion is so fast that a shock wave is created, causing the sound wave we hear as thunder. Essentially, the frequency of the sound wave depends on how fast the air expands, which in turn depends on the stability of the air and other complicated, thermodynamic factors.
But each roll of thunder sounds slightly different. You may have noticed that at the start of a roll of thunder you will hear a sharp clap and then the rest of the thunder rumble continuing after it. The sharp clap is at a much higher frequency. The reason we hear different types of thunder – the long roll or the short crack – is due to the path the lightning takes.
Imagine a flash that starts a kilometre above our heads and travels diagonally to hit the Earth a kilometre from our feet. Every point on that path is (very roughly) the same distance from us, so the sound from every point of the flash hits us at about the same time. This gives a loud “crack” kind of noise. Now, imagine a flash that starts about a kilometre up but ends close to our feet. The sound from the start of the flash’s path has to travel much further than the sound from the end and will, as a result, reach us much later. This creates the long, slow roll of thunder at a lower frequency.