Wind chill is an estimate of how cold it feels on exposed skin when wind and cold air act together. The National Weather Service (NWS) wind chill index is based on the rate of heat loss from exposed skin and is given as a temperature in °F.
How it is calculated
The current NWS index took effect on 11/01/01. With air temperature T in °F and wind speed V in mph, wind chill = 35.74 + 0.6215T – 35.75(V^0.16) + 0.4275T(V^0.16).
The formula adjusts wind from the standard 33-foot measuring height to about 5 feet, the typical height of an adult face. It is only defined for temperatures at or below 50°F and winds above 3 mph.
What the numbers mean for skin
As wind increases it pulls heat from the body, lowering skin temperature and eventually internal body temperature. At 0°F with a 15 mph wind, the wind chill is -19°F, and the NWS says exposed skin can freeze in 30 minutes at that value.
The NWS wind chill chart shades combinations where frostbite can occur in 30, 10 or 5 minutes.
What it does not apply to
Objects cannot cool below the actual air temperature, however hard the wind blows. If the air is -5°F and the wind chill is -31°F, a car radiator still will not drop below -5°F.
The index also assumes no sunshine. The NWS notes that bright sun may raise the wind chill temperature by 10 to 18°F.
How the Weather Service warns on cold
On 10/01/24, the NWS renamed its cold products. The Wind Chill Warning became the Extreme Cold Warning, the Wind Chill Watch became the Extreme Cold Watch, and the Wind Chill Advisory became the Cold Weather Advisory.
These products can be issued for either a low wind chill or a low air temperature, and thresholds vary by office. As an example, the Baltimore/Washington office issues an Extreme Cold Warning east of the Blue Ridge when the wind chill or temperature is expected to reach -10 degrees or lower.