Physics instructor grew up with a 3 second rule for lightning bolts to allow estimation of the distance of thunderstorms. Why is that? Explain the difference between the “3 second rule” and the “5 second rule”.
3 s measures the km to the lightning
5 s measures the miles
The "3 second rule" and the "5 second rule" are commonly used methods to estimate the distance of a thunderstorm based on the time interval between seeing a flash of lightning and hearing the accompanying thunder. Let me explain why these rules exist and the difference between them.
Lightning and thunder are both outcomes of the same phenomenon, known as an electrical discharge occurring within a thunderstorm. Light travels incredibly fast, while sound travels relatively slowly. Therefore, the time difference between seeing the lightning and hearing thunder can be used to estimate the distance between the observer and the storm.
The "3 second rule" suggests that for every three seconds elapsed between seeing a lightning flash and hearing the thunder, the storm is approximately one kilometer (or about 0.62 miles) away. This estimation is based on the speed of sound, which travels at around 343 meters per second or approximately 1,125 feet per second in dry air at a temperature of around 20°C (68°F).
To understand the "3 second rule," you need to know the speed of sound and how it relates to the distance estimation. Sound travels at a uniform speed through the air, and by multiplying the speed of sound by the time interval, you can estimate the distance. In this case, since sound travels around 343 meters per second, every three seconds would correspond to approximately one kilometer.
On the other hand, the "5 second rule" is a less conservative estimation and is commonly used by people as a simpler guideline. It suggests that if you count five seconds from the moment you see the lightning until you hear thunder, the storm is roughly one mile away. The idea is similar to the "3 second rule," but the estimate is less accurate because it assumes a higher speed of sound. By considering the speed of sound as about 1,100 feet per second (which is a common approximation), the five-second interval corresponds to roughly one mile.
To summarize, the "3 second rule" estimates that for every three seconds between seeing the lightning and hearing the thunder, the storm is approximately one kilometer away, while the "5 second rule" suggests that a five-second interval corresponds to approximately one mile. Both rules provide rough estimations and assume certain average values for the speed of sound, but they can give you a general idea of how close a storm is to your location.