How this works
Wind chill is the temperature your skin actually experiences when wind strips away the warm boundary layer your body generates. The 2001 NWS / Environment Canada formula superseded older versions because field tests on volunteers (with thermistors taped to faces) showed the previous formula overestimated cooling at high winds. The current formula is calibrated against actual skin-cooling rates in the −20 to +5 °C range with wind 5–60 km/h.
The practical reading is frostbite time. At −15 °C with 30 km/h wind, the wind-chill is −25 °C and exposed skin can frostbite in roughly 30 minutes. At −30 °C with the same wind, wind-chill is around −45 °C and frostbite hits in under 10 minutes — which is why polar safety guidelines require covering all skin and limiting outside time. Children, the elderly, and people with peripheral circulation issues frostbite faster than the chart suggests.
A few caveats: wind chill applies to skin, not water — pipes don't freeze faster in wind, they just lose heat to the same thermal sink (ambient air). Cars don't suffer wind chill in the engineering sense — engine and battery temperatures are dictated by air temp, not wind chill. And wind chill stops mattering below ~5 km/h wind because there's no boundary-layer disruption.
The formula
T = air temperature, V = wind speed measured at standard 10 m height. Wind speed at face level (~1.5 m) is typically 50–70% of the 10 m value, so the formula slightly underestimates real face cooling — but is close enough for safety guidance.
Example calculation
- Air temperature −10 °C, wind 25 km/h.
- WC = 13.12 + 0.6215×(−10) − 11.37×(25^0.16) + 0.3965×(−10)×(25^0.16) ≈ −18 °C. Frostbite risk: moderate.
Frequently asked questions
Why does wind chill matter at the same temperature?
Your skin warms a thin boundary layer of air to roughly body temperature, which then insulates you. Wind strips that layer away faster than it can be replenished, so your skin loses heat to fresh, cold air at a higher rate. The body temperature itself doesn't drop instantly, but exposed skin surfaces can fall below freezing far faster than the air-temperature reading would suggest — hence the "feels like" framing.
Why no chill below 5 km/h?
At very light wind, the boundary layer around your skin is barely disturbed — even your normal walking speed (4–5 km/h) creates more disruption than the ambient wind. Heat-transfer rate is dominated by ambient air temperature, not wind. The 2001 formula is undefined below ~5 km/h because the cooling rate becomes nonlinear and indistinguishable from still-air convection.
Does humidity affect wind chill?
Negligibly in the cold, dry air ranges where wind chill matters. Humidity drops below 30% in deep cold and the boundary-layer dynamics are dominated by sensible (not latent) heat transfer. Humidity matters far more on the hot side via the Heat Index, where evaporative cooling from sweat is the body's main heat-loss path. Wet skin in cold weather is a different problem — it accelerates heat loss directly via conduction and evaporation, but isn't modelled in the wind-chill formula.