A thunderstorm often begins with a brilliant flash of light, followed by a delayed, rumbling sound. This common experience showcases two distinct atmospheric phenomena: lightning and thunder. Lightning is a powerful electrical discharge in the atmosphere, often between clouds or between clouds and the ground. Thunder is the sound wave generated by the rapid expansion of air superheated by a lightning bolt.
The Speed Difference
The noticeable delay between seeing a lightning flash and hearing its thunderous roar stems from a fundamental difference in how light and sound travel. Light moves at an incredibly fast pace, approximately 186,282 miles per second (300,000 kilometers per second). This speed means that the light from a lightning strike reaches our eyes almost instantaneously, regardless of how far away the strike occurs.
Sound, by contrast, travels at a much slower rate. The speed of sound in air varies depending on factors like temperature, but at typical atmospheric conditions, it moves at about 343 meters per second, or 767 miles per hour. This significant difference in speeds explains why light reaches us almost immediately, while sound takes a measurable amount of time to cover the same distance.
Calculating the Distance
The disparity in the speeds of light and sound provides a practical way to estimate how far away a lightning strike has occurred. This method, often called the “flash-to-bang” technique, involves simply counting the seconds between seeing the lightning and hearing the thunder.
To determine the approximate distance in miles, divide the number of seconds you counted by five. For example, if you count 10 seconds between the flash and the bang, the lightning strike is roughly 2 miles away. If you prefer to calculate the distance in kilometers, divide the counted seconds by three. Counting the seconds helps in understanding whether a storm is approaching or moving away; if the time between the flash and bang decreases, the storm is getting closer.
When Thunder Means Danger
Understanding the time delay between lightning and thunder is not just a scientific curiosity; it is a crucial element of lightning safety. A widely recognized guideline for staying safe during thunderstorms is the “30/30 rule.” This rule states that if the time between seeing a lightning flash and hearing the subsequent thunder is 30 seconds or less, the lightning is close enough to pose a significant danger. In such a scenario, it is important to immediately seek safe shelter, such as a sturdy building or an enclosed metal vehicle.
After the storm has passed, the second part of the 30/30 rule advises waiting at least 30 minutes after the last sound of thunder before resuming outdoor activities. This waiting period is important because lightning can strike even at a considerable distance from the main storm, sometimes as far as 10 miles away. If you can hear thunder, you are within striking distance, even if the sky overhead appears clear.