Lightning 30-30 Rule Countdown
NWS 30-30 rule: seconds between flash and thunder -> distance in miles and seek-shelter advisory. Sound speed ~1125 ft/s.
Formula and source
distance_mi = flash_to_bang_seconds / 5 (sound at sea level ~ 1125 ft/s). 30-30 rule: under 30 s flash-to-bang (~6 mi) -> seek shelter.
NOAA / NWS lightning safety; 30-30 rule is a public guideline.
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