When a thunderstorm lights up the sky, the flash of lightning is followed by a deep, drawn-out sound known as thunder. This familiar acoustic event is the direct physical consequence of the lightning itself. Thunder is the noise produced when the massive electrical energy of a lightning bolt rapidly alters the air through which it passes.
The Explosive Cause of Thunder
The sharp, initial sound of thunder originates from the intense, near-instantaneous heating of the air surrounding the lightning channel. A lightning bolt carries a massive electrical current that heats the air along its path to temperatures approaching 50,000 degrees Fahrenheit. This superheating occurs in a fraction of a second within the narrow channel, which is often only an inch or two wide.
Because the heating is so rapid, the air has no time to move out of the way and instead undergoes explosive expansion. This sudden, violent expansion of the air creates a strong pressure wave that travels outward from the channel faster than the speed of sound. This phenomenon is known as a sonic shockwave, similar to the boom produced by a supersonic aircraft. As the shockwave travels away from the lightning channel, it dissipates and slows down, eventually decaying into the sound wave that we perceive as thunder.
The Geometric Reason for the Rumble
While the initial energy release generates a sharp shockwave, thunder is heard as a prolonged rumble due to the complex geometry of the lightning channel. A lightning bolt is not a single point of sound, but a long, often miles-long path of electrical discharge that is zig-zagged or branched. Sound waves are generated simultaneously along every point of this extended channel.
Sound waves from the nearest part of the lightning channel reach the observer first, creating the sharp crack. Sound waves from points farther along the path arrive progressively later. This difference in travel time, known as path length difference, stretches the single explosive event into a continuous, drawn-out sound.
The rumbling is further prolonged by echoes and atmospheric filtering. Sound waves reflect off objects like hills, mountains, and clouds, adding reverberations that stretch the duration of the noise. As sound travels over long distances, the atmosphere absorbs higher-frequency components more effectively than lower frequencies. This selective absorption causes the sound to lose its sharp crack and retain only the deeper, lower-frequency components, perceived as a hollow, rolling rumble.
Why Lightning Always Precedes Thunder
The observation that lightning is seen before thunder is heard is explained by the vast difference between the speeds of light and sound. Light travels at approximately 186,000 miles per second, meaning the flash of lightning reaches the observer virtually instantaneously.
In contrast, sound travels through the air at about 1,125 feet per second, or roughly one mile every five seconds. This significant disparity in speed creates a measurable time delay between seeing the light and hearing the sound. This phenomenon allows for the “flash-to-bang” method to estimate the distance to the lightning strike. By counting the seconds between the flash and the first sound of thunder, and then dividing by five, a person can approximate the distance to the storm in miles.