Thunder is the loud acoustic event that always follows a lightning flash. It is a direct physical consequence of the massive electrical discharge that creates lightning. The sound is generated as the extreme energy of a lightning bolt momentarily alters the air around it. The sound we hear is the atmosphere reacting to the sudden, violent passage of electrical current.
The Mechanism: Extreme Heat and Shockwaves
The origin of thunder lies in the instantaneous and intense heating of the air along the lightning channel. When a lightning stroke occurs, it creates a narrow column of superheated gas, often called plasma. The temperature within this channel can instantly soar to around 50,000 degrees Fahrenheit, which is five times hotter than the surface of the sun.
This rapid energy transfer occurs in mere microseconds, leaving the surrounding air no time to move away gradually. Because air expands when heated, this instantaneous temperature spike forces the air to expand explosively outward. The air expands so rapidly that it travels faster than the speed of sound, generating a powerful shockwave.
This initial shockwave is essentially a sonic boom created along the entire length of the lightning path. The pressure wave propagates outward through the atmosphere, eventually decaying into the sound wave we perceive as thunder. Every point along the lightning’s path contributes to this auditory event, meaning the sound is not generated from a single point.
The Acoustics of Thunder
The familiar sound of thunder changes depending on the listener’s distance from the lightning strike. When a strike occurs very close by, the listener hears a sharp, loud “crack” or “bang.” This sharp sound is the immediate arrival of the initial shockwave before it has had a chance to dissipate significantly.
More commonly, thunder is heard as a prolonged, low-frequency “rumble.” This drawn-out sound results from sound waves originating from different sections of the long, winding lightning channel arriving at staggered times. The sound from the nearest point reaches the ear first, followed by sound from increasingly distant points.
Sound waves also reflect and scatter off objects like terrain, hills, and atmospheric layers. This reflection stretches the duration of the sound, transforming the initial sharp shockwave into the familiar, continuous rumble. The atmosphere absorbs higher-frequency components more quickly, which is why distant thunder sounds lower and more muted.
Estimating the Storm’s Proximity
The difference between the speed of light and the speed of sound allows for a simple method to estimate a storm’s distance, known as the “flash-to-bang” rule. Since light travels nearly instantaneously, seeing the lightning flash and starting a timer are considered simultaneous events. Sound travels much slower, approximately one mile every five seconds.
To calculate the storm’s distance, count the number of seconds between seeing the flash and hearing the thunder. Dividing that count by five provides the approximate distance to the lightning strike in miles. For example, a 10-second delay indicates the strike was about two miles away.