Thunder is the sound of air expanding and contracting after lightning heats it rapidly. The flash reaches your eyes almost immediately, while the sound takes about five seconds to travel one mile. Count the gap between the flash and the boom, then divide by five for a rough distance in miles. A 15-second gap is about three miles. If thunder is audible, the National Weather Service says the storm is close enough to be dangerous, so the calculation belongs indoors.
Sources: National Weather Service, Understanding Lightning: Thunder; NOAA, Lightning Safety; NOAA, Lightning.
Visual material: Pexels photo 16587242.
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