When a thunderstorm rolls in, a flash of lightning is followed by the rumble of thunder. Understanding this delay provides a practical method for estimating a storm’s distance, useful for safety. The perceptible gap between seeing lightning and hearing thunder is a direct consequence of physics.
The Rule of Thumb
The “flash-to-bang” rule is a simple method for estimating the distance to a lightning strike. This technique involves counting the seconds between observing the lightning flash and hearing the thunderclap. For every five seconds counted, the lightning strike is approximately one mile away. For instance, if you count 10 seconds, the lightning is about two miles distant. This straightforward calculation allows for a quick assessment of a thunderstorm’s proximity.
To apply this rule, begin counting immediately upon seeing the lightning until the thunder is heard. Once you have the total number of seconds, divide that number by five to determine the approximate distance in miles. For those using the metric system, dividing the number of seconds by three will give the distance in kilometers.
Why There’s a Delay
The perceptible delay between seeing lightning and hearing thunder arises from the vast difference in the speeds of light and sound. Light travels at an incredible speed, approximately 186,282 miles per second in a vacuum, reaching our eyes almost instantaneously.
Sound, however, travels much slower. Its speed in dry air at room temperature is roughly 767 miles per hour, meaning sound travels approximately one mile in about five seconds. The further away the lightning strike, the longer the delay before its thunder is heard.
What a Close Strike Means
A very short delay between lightning and thunder indicates that the storm is quite close, posing a greater risk. If the time between seeing the flash and hearing the thunder is 30 seconds or less, it signifies that the lightning is within approximately six miles. At this proximity, it is generally recommended to seek immediate shelter.
When thunder is heard, even if rain has not yet started, it means lightning is within striking distance, which can be up to 10 miles or more from the storm’s center. Safe shelter includes fully enclosed buildings with plumbing or electricity, or hard-topped, enclosed vehicles with windows closed. It is advisable to remain indoors for at least 30 minutes after the last clap of thunder to ensure the storm has passed.