How this instrument works
Wind chill temperature describes how cold the air actually feels against exposed skin, accounting for how wind accelerates heat loss from the body compared to perfectly still air at the same thermometer reading. It isn't a different physical temperature — the air itself is still whatever the thermometer says — but a calculated equivalent: the still-air temperature that would produce the same rate of heat loss from skin as the actual combination of temperature and moving air.
This instrument uses the official formula the U.S. National Weather Service, Canada's Meteorological Service, and the UK Met Office jointly adopted in November 2001, replacing an older 1945 formula. The new formula was derived from a model of human facial skin cooling in moving air, validated with actual trials in a chilled wind tunnel, and it produces noticeably milder (less extreme) wind chill values than the older formula did for the same conditions.
The formula is only defined, and only meaningful, within specific bounds: air temperature at or below 50°F, and wind speed at or above 3 mph. Below 3 mph, wind's cooling effect on skin is considered negligible, and above 50°F, wind chill isn't a meaningful concept in the same way — the NWS's companion heat index formula covers the analogous 'how hot it feels' calculation for warm, humid conditions instead.
- Enter Air temperature in degrees Fahrenheit — this formula is only valid at 50°F or below.
- Enter Wind speed in miles per hour, measured at standard 5-foot height — this formula requires at least 3 mph.
- Read Wind chill temperature — the still-air equivalent temperature reflecting how cold the moving air actually feels.
- Treat wind chill, not the raw air temperature, as your guide for frostbite and hypothermia risk when dressing for or working in cold, windy conditions.
Worked example — 20°F with a 15 mph wind
Enter 20 into Air temperature and 15 into Wind speed — a cold but ordinary winter day with a steady breeze. Wind chill temperature reads 6.22°F, computed as 35.74 + 0.6215x20 - 35.75x15^0.16 + 0.4275x20x15^0.16, where 15^0.16 ≈ 1.5423.
That means a 20°F day with a 15 mph wind cools exposed skin at roughly the same rate as a still-air temperature of 6.22°F would — a difference of nearly 14 degrees driven entirely by that moving air. The published NWS Wind Chill Chart, which rounds to whole degrees for quick field reference, lists the same 20°F/15 mph cell as 6°F, consistent with this more precise calculation.
Questions
Why does wind make cold air feel colder than the thermometer reading?
Exposed skin is normally warmed by a thin boundary layer of air it heats through contact. Wind continuously strips that warmed layer away and replaces it with fresh cold air, forcing the skin to keep losing heat to warm a new layer, over and over. The stronger the wind, the faster this replacement happens and the faster the skin loses heat — which is what the wind chill formula quantifies as an equivalent still-air temperature.
Why is this formula only valid at 3 mph and above?
Below roughly 3 mph, air movement is too slight to meaningfully accelerate heat loss beyond what still air already causes, so the formula's underlying wind-speed term becomes unreliable and the NWS simply doesn't define wind chill below that threshold — readings that light are effectively treated as calm air for this calculation.
Why is this formula only valid at 50°F and below?
Wind chill specifically models accelerated heat loss in cold conditions; above 50°F, that framework doesn't apply the same way, and wind generally isn't perceived as making warm air feel colder. The National Weather Service uses a separate formula, the heat index, to calculate how hot humid air feels in warm conditions instead.
How is the 2001 NWS wind chill formula different from the older one?
The formula in use before 2001 was based on a 1945 Antarctic expedition study using a plastic water-filled cylinder, not human skin, to estimate heat loss, and it tended to overstate how cold conditions actually felt. The 2001 revision, developed jointly by the US, Canada, and the UK, is based on an actual model of human facial skin cooling and produces milder wind chill values for the same temperature and wind inputs.
At what wind chill temperature does frostbite become a real risk?
The National Weather Service publishes frostbite-time guidance alongside its wind chill chart: at a wind chill of -19°F, for instance, exposed skin can freeze in as little as 30 minutes. The colder the wind chill drops beyond that, the faster frostbite can set in, which is why wind chill, not the plain air temperature, is the number to check before extended outdoor exposure in winter conditions.
Does wind chill affect car radiators, pipes, or other non-living objects?
No — wind chill is specifically a model of heat loss from human (and similarly warm-blooded) skin, and it has no meaning for inanimate objects. A car engine block or an outdoor pipe cools toward the actual air temperature regardless of wind chill; moving air can speed up how fast it gets there, but it will never cool an object below the true air temperature the way it changes how living skin perceives cold.