What wind chill means
On a windy winter day, the air feels colder than the thermometer says. Your body warms a thin layer of air next to the skin; wind blows it away, so exposed skin loses heat faster. The wind chill is the temperature at which, in calm air, your skin would lose heat at the same rate.
The formula
The US National Weather Service and Environment Canada adopted the current wind chill formula in 2001. It is based on heat loss from the face, measured on volunteers in a cold wind tunnel:
- Wind chill (°F) = 35.74 + 0.6215T − 35.75V^0.16 + 0.4275TV^0.16
- T is the air temperature in °F and V the wind speed in mph.
The formula applies at 50 °F or below with wind of at least 3 mph. Outside that range, the calculator shows the air temperature.
Worked example
The thermometer reads 14 °F (−10 °C) and the wind blows at 15 mph.
- V^0.16 = 15^0.16 = 1.54.
- Wind chill: 35.74 + 0.6215 × 14 − 35.75 × 1.54 + 0.4275 × 14 × 1.54 = −1 °F (−19 °C).
- It feels about 15 °F colder than the air temperature.
Wind chill and frostbite
| Wind chill | Frostbite on exposed skin |
|---|---|
| Above −18 °F | Low risk for most people |
| −18 to −39 °F | Possible in 10 to 30 minutes |
| −40 to −53 °F | Possible in 5 to 10 minutes |
| −54 to −66 °F | Possible in 2 to 5 minutes |
| −67 °F and below | Possible in under 2 minutes |
The National Weather Service issues cold weather advisories and extreme cold warnings when wind chills reach dangerous levels, with thresholds that vary by region.
What wind chill does not do
Wind chill applies to people and animals, which produce their own heat. Objects cannot cool below the actual air temperature, so water pipes, car batteries and puddles freeze only if the air itself is at or below 32 °F. Wind does speed up how quickly they reach that temperature.
Staying safe in the cold
- Dress in layers: a wicking base layer, an insulating middle layer and a windproof outer shell.
- Cover exposed skin: hat, scarf or face mask, and mittens, which keep hands warmer than gloves.
- Stay dry: wet clothing loses most of its insulation.
- Watch for frostbite: numbness and white or grayish-yellow skin on the nose, ears, cheeks, fingers or toes.
- Watch for hypothermia: shivering, confusion, slurred speech and drowsiness need urgent medical care.