Lightning 30-30 Rule Countdown

NWS 30-30 rule: seconds between flash and thunder -> distance in miles and seek-shelter advisory.

Run the calculator

Example

You enter

Flash to bang (s) 15

You get

Distance miles 3
Distance (km) 4.828
Seek shelter Yes

Checked against published worked examples from: NOAA / NWS.

Details, formula, and sources

ScopeSound speed ~1125 ft/s.

Formula or tabledistance_mi = flash_to_bang_seconds / 5 (sound at sea level ~ 1125 ft/s). 30-30 rule: 30 s or less flash-to-bang (~6 mi or closer) -> seek shelter.

Source and editionNOAA / NWS lightning safety; 30-30 rule is a public guideline.

Free accessweather.gov/safety/lightning.

What governsGeometry is not forecasting. Avalanche advisory and incident commander govern.

API field namesflash_to_bang_s, distance_miles, distance_km, seek_shelter

Sound speed ~1125 ft/s at sea level; 5 s ~ 1 mistandard atmospheric model
30-30 rule threshold 30 s flash-to-bang (~6 mi); resume after 30 min of last detected strikeNWS public guideline