How it is calculated
- Temperature with altitude: in the standard atmosphere the air cools by 0.65 °C per 100 m. In reality it is between about 0.5 °C (moist air) and almost 1 °C (dry air) – and in winter inversions it can even be warmer above the fog than in the valley.
- Wind chill (the formula used by Canadian and US weather services since 2001):
13.12 + 0.6215·T − 11.37·v0.16 + 0.3965·T·v0.16, with T in °C and wind v in km/h. It is defined for temperatures up to 10 °C and wind above 4.8 km/h. It tells how cold it feels on exposed skin – the air temperature itself does not change. - Air pressure from the barometric formula of the standard atmosphere, and the boiling point of water from the Clausius–Clapeyron equation – why pasta takes longer at a mountain hut.
Frostbite on exposed skin becomes possible within 10–30 minutes when the wind chill drops below about −27 °C. Plan the tour with the Hiking Time Calculator and the sunrise and sunset times.
Mountains are not a calculator. Weather, conditions, fitness and experience change everything. Use these numbers for planning, and turn back in time.