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How It Works
Light from a lightning flash arrives almost instantly, but thunder travels at about 343 m/s (1,125 ft/s) at 20°C. Multiply the seconds between the flash and the thunder by the speed of sound to get the distance. The classic “5-second rule” says every 5 seconds equals roughly 1 mile.
Example Problem
You see lightning and hear thunder 8 seconds later. How far away is the storm?
- Speed of sound: 343 m/s (at 20°C)
- Distance: 343 × 8 = 2,744 m (about 1.7 miles)
Since that is under 10 km, the 30-30 rule says to seek shelter immediately.
Frequently Asked Questions
How many seconds per mile for lightning?
Roughly 5 seconds per mile (or 3 seconds per kilometer). If you count 10 seconds between the flash and thunder, the lightning struck about 2 miles away.
Does temperature change the speed of sound?
Yes. The speed of sound increases by about 0.6 m/s for each degree Celsius. At 0°C it is ~331 m/s; at 35°C it is ~352 m/s. For most purposes the default 343 m/s (20°C) is close enough.
What is the 30-30 rule for lightning safety?
If the flash-to-thunder time is 30 seconds or less (about 6 miles), seek shelter immediately. Wait at least 30 minutes after the last thunder before going back outside.
Related Calculators
- Cloud Base Calculator — estimate cloud ceiling from temperature and dewpoint.
- Relative Humidity Calculator — compute humidity from vapor pressure or dewpoint.
- Sound Wave Calculator — calculate speed of sound, frequency, and wavelength.
- Heat Index Calculator — check heat conditions during thunderstorm season.
- Speed Unit Converter — convert the speed of sound between m/s, mph, and km/h.
- Length Unit Converter — convert lightning distances between miles, kilometers, and feet.