When observing a thunderstorm, it is common to notice a delay between the bright flash of lightning and the subsequent rumble of thunder. This phenomenon is a consequence of physics and a practical tool. Understanding this time difference can help estimate a thunderstorm’s proximity.
The Science Behind the Delay
The gap between seeing lightning and hearing thunder occurs due to a difference in how light and sound travel through the atmosphere. Light travels at an extremely high speed, approximately 299,792,458 meters per second (186,282 miles per second). During a storm, light from a lightning flash reaches our eyes almost instantly.
Sound travels much slower. The speed of sound in air is about 343 meters per second (1,125 feet per second) at 20 degrees Celsius (68 degrees Fahrenheit). Its speed varies slightly with temperature and humidity but is consistently far less than light. Thunder, created by the rapid heating and expansion of air along the lightning channel, takes time to reach an observer.
Estimating Distance to a Strike
The difference in light and sound travel speeds allows for a simple method to estimate lightning strike distance, known as the “flash-to-bang” method. To use this technique, count the seconds immediately after seeing a lightning flash until you hear the thunder.
Each five-second interval indicates the strike is approximately one mile away. For example, if you count 10 seconds between the flash and the bang, the lightning was about two miles distant. Alternatively, for every three seconds counted, the strike is roughly one kilometer away. This calculation provides a reliable estimate, though terrain or atmospheric conditions can subtly influence sound travel.
When Lightning Strikes Close
Knowing lightning strike distance is important for safety. The “30/30 rule” helps determine when to seek shelter and when to resume outdoor activities. If the time between seeing a lightning flash and hearing thunder is 30 seconds or less, the thunderstorm is close enough to pose a hazard, and immediate shelter is necessary.
Safe shelters include substantial buildings with plumbing or electrical wiring, which can help conduct electrical current to the ground. Fully enclosed, hard-topped metal vehicles also offer protection. Avoid open fields, isolated tall trees, and bodies of water during a storm. After the last sound of thunder, wait at least 30 minutes before returning to outdoor activities, as lightning can strike even when the main storm appears to have passed.