Wind Chill Index

Wind chill coefficient: the NWS 2001 formula

The wind chill coefficient tells you how much moving air sharpens the feeling of cold. Wind keeps stripping away the thin warm layer that forms against your skin, and that convective heat loss is what makes a windy day feel colder than the thermometer says. The current standard, adopted by the US National Weather Service and Environment Canada in 2001, is WC = 13.12 + 0.6215·T − 11.37·V^0.16 + 0.3965·T·V^0.16, with T in °C and V in km/h, valid for V ≥ 4.8 km/h. Take T = 0 °C and V = 30 km/h and you land at WC ≈ −10 °C. This equation replaced the 1945 Siple-Passel formula, which had been built around a can of water freezing in Antarctica and tended to overstate the cooling. Below WC −27 °C, frostbite can set in on exposed skin within roughly 30 minutes.

Applications

You will see it behind INMET cold-front forecasts for southern and southeastern Brazil, in safety planning for outdoor sports and trail running, in occupational protection under the NR-29 cold exposure rules, in snow tourism advisories, and in the risk warnings sent to elderly and vulnerable people when a cold wave hits.

FAQ

Why V ≥ 4.8 km/h? Once the wind drops below that, convection barely matters and what you feel is just the real air temperature, so the equation stops fitting.

Does humidity enter the formula? No. NWS wind chill leaves humidity out entirely. When the cold is damp, reach for the Steadman apparent temperature instead.

Does wind chill freeze objects faster? It speeds up how quickly they reach the air temperature, but it never drives them below the actual T.