Wind Chill Calculator
The US National Weather Service (2001 update) wind chill formula, valid for air temperatures at or below 10 °C and winds above 4.8 km/h.
Result
- Frostbite time (exposed skin)
- Low risk
How the calculation works
The standard North American wind chill index (Osczevski & Bluestein, 2001):
T is air temperature in °C and V⁰·⁰¹⁵ the wind speed in km/h raised to the power 0.16. The formula is valid for T ≤ 10 °C and V > 4.8 km/h; the calculator returns the feels-like temperature and the experimental frostbite-time band for exposed skin.
What the index ignores: sunshine. It is defined for overcast conditions - bright sun can make exposed skin feel roughly 5-10 °C warmer than the number says, which is why the chart sometimes feels pessimistic. It also applies to living skin only; water bottles and tent walls have no wind chill.
Field use: estimate the feels-like number before committing to exposed ridgelines, and feed the air temperature (not the wind chill) into the warmth calculator - sleep systems are sheltered, so wind chill does not transfer. The altitude temperature calculator gives you that air temperature in the first place.
Planning estimate only - not safety advice.
Frequently asked questions
What is wind chill, physically?
Moving air strips the warm boundary layer off exposed skin, so your skin loses heat to air that is effectively colder than the thermometer reads. The index expresses that as the equivalent calm-air temperature. It changes how cold you feel and how fast frostbite can occur - it does not make objects colder than the air.
Why is there a lower wind speed limit of 4.8 km/h?
The NWS formula is only defined above that threshold; below it, the boundary layer stays mostly intact and "calm air" applies. The calculator tells you when the inputs are outside the valid range instead of extrapolating nonsense.
Does wind chill matter under clothing or in a tent?
Almost not at all - which is the entire point of layering and shelter. The index applies to exposed skin. Under a shell, inside a sleeping bag or behind tent walls, the air is near-still and the air temperature is the number that matters. The <a href="/warmth-calculator/">warmth calculator</a> discounts wind the same way.
Does sunshine affect wind chill?
The index is deliberately computed for the shade: the NWS formulation excludes solar radiation. On a bright day the same air can feel roughly 5-10 °C warmer than the index, so treat the number as the conservative case. And it applies to exposed skin only - gear, water and tent walls have no wind chill, which is why feeding air temperature (not wind chill) into sheltered sleep-system planning is the correct move.
How should I read the frostbite times?
They are for exposed facial skin under the NWS experimental curves: under 30 minutes below roughly −27 °C feels-like, under 10 minutes below about −50. They assume dry skin, no wind protection and no rewarming - treat them as order-of-magnitude warnings, not guarantees.