Frostbite risk is about minutes, not degrees alone

At wind-chill near −28 °C, frostbite can start in under 30 minutes on exposed skin; near −40 °C, in under 10.

Why it works

The NWS frostbite-time charts combine air temperature and wind speed. The calculator flags those bands, but clothing, wetness and exertion still dominate real risk.

Do it exactly

Estimate with the rule, then check it against the calculator that models it properly.

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More in Environmental

• Lightning distanceCount the seconds from flash to thunder, divide by 3 for kilometres (by 5 for miles).
• Air cools as you climbTemperature drops about 6.5 °C for every 1,000 m of altitude.
• Pressure halves every ~5.5 kmAtmospheric pressure falls by half roughly every 5,500 m you climb.
• Ten metres of water = one atmosphereEvery 10 m of seawater adds about 1 atmosphere of pressure.

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