Lightning is a massive electrical discharge that produces a bright flash of light followed by a loud, rumbling sound known as thunder. Because these two phenomena occur simultaneously during a storm, the time delay between them provides a simple method for calculating the distance of the strike. This technique, often called the “flash-to-bang” method, relies on a fundamental difference in how light and sound travel through the air. Understanding the separation in their speeds allows anyone to assess the threat posed by an approaching storm.
The Physics of Light and Sound
The calculation relies on the enormous speed disparity between light and sound waves. Light travels at approximately 186,000 miles per second, meaning the flash reaches an observer almost instantaneously. Sound moves much slower, traveling through the atmosphere at roughly 1,100 feet per second.
This speed difference creates the noticeable delay between the flash and the thunder. The measured time delay is exclusively the time it takes for the sound wave to travel from the strike location to the observer. Although the speed of sound varies slightly depending on air temperature, a consistent approximation is used for practical calculation.
Executing the Count to Determine Distance
Calculating the distance begins the moment the lightning flash is seen, serving as the precise starting point for the count until the thunder is heard. Without a stopwatch, an effective way to measure this time is to use the traditional method of saying “one thousand one, one thousand two,” and so on, with each phrase approximating one second.
The count stops the instant the thunder reaches the ear, and the total number of seconds is recorded. Once the time interval is known, a simple division provides the distance to the strike.
To find the distance in miles, divide the number of seconds counted by five (e.g., a 15-second delay indicates the lightning is three miles away). To find the distance in kilometers, divide the number of seconds by three (e.g., a 15-second count places the strike at five kilometers away).
When Calculated Distance Dictates Action
The primary purpose of calculating lightning distance is to assess personal safety, as the presence of thunder means a person is within striking range of the storm. Meteorologists and safety organizations recommend following the “30/30 Rule” as a guideline for when to seek shelter.
The first “30” refers to the flash-to-bang time: if the time counted is 30 seconds or less, the storm is dangerous, and safety should be sought immediately. A 30-second count correlates to approximately six miles, which is considered the minimum safe distance from a thunderstorm.
The second “30” dictates the waiting period before resuming outdoor activities. Once shelter is reached, one must wait a minimum of 30 minutes after the last sound of thunder is heard before it is safe to leave. This waiting period ensures that the trailing edge of the storm has moved far enough away to prevent strikes from any lingering electrical activity.