The 3 second rule for estimating the distance of a thunderstorm is based on the speed of sound. When you see a lightning bolt, you can count the seconds until you hear the thunder. For every 3 seconds that pass between the flash and the sound of thunder, the storm is roughly 1 kilometer (or about 0.6 miles) away.
To understand why this rule works, it’s important to remember that light travels much faster than sound. While light enables you to see the lightning almost instantly, sound travels at approximately 343 meters per second (about 1,125 feet per second) in air. This means that there’s a delay between when you see the flash and when you hear the thunder, which can help provide a rough distance estimate.
On the other hand, the 5 second rule operates on a similar principle but provides a slightly different guideline. Some might suggest using a 5 second interval to estimate the distance. This method is often associated with a greater safety margin since it translates to a storm being about 1.5 kilometers (or 1 mile) away for every 5 seconds counted.
In summary, the 3 second rule is a quick and effective way to gauge how far away a storm is, while the 5 second rule is a more conservative approach that indicates greater distance. Both methods utilize the relationship between the speed of sound and the observed delay after lightning strikes. Knowing how to judge the distance of thunderstorms using these methods can help ensure safety during severe weather.