The NWS 2001 wind-chill formula
In English units, with air temperature T in °F and wind V in mph,
WCT = 35.74 + 0.6215 T − 35.75 V0.16 + 0.4275 T V0.16.
The SI form, with T in °C and V in km/h, is
WCT = 13.12 + 0.6215 T − 11.37 V0.16 + 0.3965 T V0.16.
The two expressions are official equivalents of the same fit. They replaced the 1945 Siple–Passel index in November 2001 after a joint U.S.–Canadian review, and they assume a walking face in clear weather, not a wet-bulb or a solar-load correction. The NWS publishes the index only for T ≤ 50 °F and V ≥ 3 mph; this page follows that domain rather than extrapolating. A 0 °F air temperature in a 10 mph wind gives a wind chill of about −16 °F, matching the NWS chart.
How pilots usually use it
On a preflight or a ramp walk, wind chill is a personal-exposure number, not an aircraft-performance correction. Airframe icing, density altitude, and engine inlet temperature still come from the true OAT. For a heat-stress counterpart on a hot day, use the heat-index calculator. Current observations belong on a METAR — see the METAR decoder.
Sources
- NOAA / National Weather Service, “Wind Chill Temperature Index” (2001 formula, English and metric, official thresholds).
- Osczevski and Bluestein, “The New Wind Chill Equivalent Temperature Chart,” Bulletin of the American Meteorological Society, 2005.
- Environment and Climate Change Canada, wind-chill program (joint U.S.–Canadian index).
Frequently asked questions
Why does a light wind leave the wind chill equal to the air temperature?
The official NWS index is defined only for winds of 3 mph or more at the 10-meter anemometer height. Below that, the service reports the air temperature, and so does this page.
Does wind chill change takeoff performance or density altitude?
No. Performance and density altitude use the actual outside air temperature, pressure, and humidity. Wind chill is a skin-heat-loss index for people.
Is this the same as a frostbite timer?
No. The NWS publishes separate frostbite-time guidance that depends on the wind-chill value. This page reports only the index.