The intense heat from a lightning strike causes the surrounding air to expand violently, creating a powerful shockwave heard as thunder. Rolling thunder is a prolonged, deep rumbling sound. This distinct sound profile is not caused by a unique type of electrical discharge, but rather by how the sound waves travel and interact with the environment. To understand the risk, it is necessary to separate the physics of the sound from the true danger posed by the electrical source.
The Physics Behind Rolling Thunder
The sound of thunder originates from the rapid heating of air along the lightning channel, which can reach temperatures up to 50,000 degrees Fahrenheit. This extreme heat causes the air to expand supersonically, creating a shockwave perceived as a sharp clap or crack when the lightning is close. The lightning discharge is not a single point source, but a long, often tortuous path that can span several miles.
The “rolling” effect occurs because sound waves generated along this extended channel reach the listener at different times. Sound travels approximately one mile every five seconds, meaning the sound from the nearest segment arrives first. Sound from progressively farther segments arrives moments later, stretching the instantaneous explosion into a prolonged rumble.
Echoes and reverberations further contribute to the drawn-out sound. Sound waves reflect off terrain features like hills, buildings, and layers of air with differing temperatures. These multiple reflections, combined with the time delay, blur the initial sharp sound into a continuous, low-frequency rumble.
Assessing the Real Danger
The true danger lies solely in the lightning that produces the sound. A rolling rumble generally indicates that the lightning strike occurred farther away from the observer. When a strike is very close, the sound arrives almost instantly as a sharp, loud crack because the shockwave has not been stretched out by distance.
A distant strike sounds like a low rumble rather than a crack because of sound dissipation over distance. As the sound wave travels, it loses energy. The high-frequency components (the sharp crack) are absorbed more quickly by the air than the lower-frequency components, leaving the residual, low-frequency sound heard as a sustained rumble.
The “Flash-to-Bang” technique estimates the distance to a strike. By counting the seconds between seeing the flash and hearing the first sound of thunder, and then dividing by five, one determines the approximate distance in miles. A longer count indicates a greater distance and a lower risk of a direct strike. However, if any thunder is audible, the observer is within striking distance and must seek safe shelter immediately.
Safety Guidelines During Thunderstorms
Since the sound of rolling thunder is merely an indicator of distance, the primary focus during a thunderstorm must shift to protective action against the lightning itself. The widely recommended “30/30 Rule” provides a clear guideline for when to seek shelter and when it is safe to resume outdoor activities. If the time between seeing the lightning flash and hearing the thunder is 30 seconds or less, the storm is close enough to pose a danger, and safe shelter must be sought immediately.
The second part of the rule dictates that individuals should wait 30 minutes after the last sound of thunder is heard before returning outdoors. This waiting period ensures that the main body of the storm has passed. Lightning can strike well away from the center of the storm cloud, traveling up to 10 miles or more from the main storm even when the sky appears clear overhead.
Seeking Safe Shelter
A sturdy, substantial building that is fully enclosed, such as a house or business, offers the best protection from lightning. If a building is unavailable, a hard-topped, fully enclosed vehicle with the windows rolled up provides the next safest option. Open structures like porches, carports, sheds, or picnic shelters offer virtually no protection and should be avoided.
Indoor Precautions
Once indoors, certain precautions are necessary because lightning can still travel through a home’s utility systems. Individuals should avoid contact with plumbing, which includes washing dishes, showering, or bathing, as water and metal pipes can conduct electricity. It is also advised to stay away from corded phones, electrical appliances, and outlets, as the electrical current from a strike can travel through wires and cords. Staying on the lowest floor in an interior room, away from windows and concrete walls containing metal rebar, further reduces the risk.
Outdoor Safety
Outdoor activities carry specific risks, particularly in open areas or near tall objects. Golfers, fishermen, and those in open fields should immediately drop metal objects like clubs or fishing rods and seek appropriate shelter. If no safe shelter is available, individuals should crouch low to the ground with their feet together, minimizing contact with the earth, which can carry ground current from a nearby strike. Never lie flat on the ground, as this increases the body’s exposure to potentially dangerous ground current.