Why does thunder make sound?

During a thunderstorm, a brilliant flash of lightning is often followed by a deep rumble or a sharp crack: thunder. The two always come together, and for good reason. Thunder is not a separate event from lightning. Thunder is the sound that lightning makes. To understand the booming noise, we first have to understand what lightning does to the air around it.

Lightning is a powerful burst of electricity flashing through the sky. As it shoots along its path, it heats the narrow channel of air around it to an astonishing temperature, far hotter than the surface of the Sun, and it does so in a tiny fraction of a second. This sudden, extreme heating is the heart of the matter. It is what turns a flash of light into a sound that can be heard for many kilometers.

Air, like other gases, expands when it is heated. When air is heated gently, it expands gently. But lightning heats the air so violently and so quickly that the air has no time to expand calmly. Instead it expands explosively, bursting outward in an instant. This rapid expansion pushes hard against the surrounding air and creates a shock wave, a powerful pressure wave rushing outward from the lightning channel.

As this shock wave travels outward and spreads, it slows and becomes an ordinary sound wave, the sound we hear as thunder. Because a lightning bolt is long and jagged, sound reaches us from many points along it at slightly different times, which is why thunder can roll and rumble rather than make a single clean bang. One more familiar detail is explained by this: we always see the lightning flash before we hear the thunder, because light travels far faster than sound.

This article was written using information from Wikipedia.