ELi5: Why does the time between lightning striking and when you hear the thunder correlate to how far the lightning landed?
In: Physics
It comes down to Distance = Speed x Time.
Sound and light do no travel at the same speed. The speed of sound is about 343 metre per second (m/s), while light travels at 3 * 10^8 m/s—it’s a big difference. If you are close enough to an object, you will see it and hear it at the same time. However, if the object is far away, you will see it first, then it will take some time for that sound to reach you.
If you see a flash, then it takes a full second to hear a noise, that means the flash was 343 metres away. If you see a flash of lighting, count to three, then hear the thunder that means that you are 3 x 343m away, which is 1029 metre, or about kilometre away. As a rule of thumb, every three seconds is kilometre, and every five seconds is a mile.
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