About wind chill
Wind chill is how cold the air feels on exposed skin once wind is factored in. Moving air strips away the thin insulating layer of warmth your body builds at the skin surface, so a 20 F day with a stiff wind can feel colder than a still 5 F day. The number this calculator returns is the wind chill temperature: the equivalent still-air temperature that would chill bare skin at the same rate. It does not change the actual air temperature, only how quickly you lose heat.
The formula used here is the official US National Weather Service (NWS) and Environment Canada wind chill index, adopted in 2001 to replace the older 1945 Siple-Passel model. It is calibrated for a human face about 5 feet off the ground and is considered valid only when the temperature is at or below 50 F and the wind is above 3 mph. Outside that range the index is not meaningful, which is why the tool flags those cases.
How it works
The NWS index is an empirical equation fitted to heat-loss measurements. Temperature is in Fahrenheit and wind speed in miles per hour:
WC = 35.74 + 0.6215 x T - 35.75 x V^0.16 + 0.4275 x T x V^0.16 WC = wind chill temperature (F) T = air temperature (F) V = wind speed (mph) Valid for T <= 50 F and V > 3 mph
- The V^0.16 term captures that wind has a big effect at first but diminishing returns: going from 5 to 15 mph chills far more than 35 to 45 mph.
- Wind chill is always at or below the air temperature within the valid range, never above it.
- It assumes bare skin; clothing, sun, and humidity are not in the model, so it is a worst-case for exposed face and hands.
Worked example
It is 20 F with a 15 mph wind. How cold does it feel, and how fast is frostbite a risk?
- Compute V^0.16: 15^0.16 is about 1.534.
- Plug in: WC = 35.74 + 0.6215 x 20 - 35.75 x 1.534 + 0.4275 x 20 x 1.534.
- Term by term: 35.74 + 12.43 - 54.84 + 13.12.
- Sum: about 6 F wind chill.
- Frostbite risk: at a wind chill around 6 F the danger is low for brief exposure, but at -18 F or colder frostbite can occur within 30 minutes.
Wind chill reference (F)
Wind chill temperature for a given air temperature and wind speed. The NWS marks frostbite danger zones at roughly -18 F (30 min), -32 F (10 min), and -48 F (5 min).
| Air temp | 5 mph | 15 mph | 30 mph |
|---|---|---|---|
| 40 F | 36 | 32 | 28 |
| 20 F | 13 | 6 | 1 |
| 0 F | -11 | -19 | -26 |
| -20 F | -34 | -45 | -53 |
Wind chill, heat index, and feels-like
Wind chill is one of two "apparent temperature" measures, and it is easy to confuse them. Wind chill applies only in the cold, when wind accelerates heat loss from skin. Its warm-weather counterpart is the heat index, which combines temperature and humidity to describe how hot it feels, because high humidity slows the evaporation of sweat and traps body heat. The generic "feels-like" temperature in weather apps simply switches between the two: it reports wind chill when it is cold and windy, and heat index when it is hot and humid.
The drivers differ in each direction. Cold-weather discomfort is dominated by wind speed, so a calm cold day feels milder than a breezy one at the same temperature. Hot-weather discomfort is dominated by moisture, so a humid 90 F day feels far hotter than a dry one. That is why a single feels-like number needs to know the season to pick the right model, and why this calculator focuses strictly on the cold-side wind chill equation.
Common pitfalls
- Using it above 50 F or below 3 mph wind. The formula is undefined there; a light breeze on a mild day produces no meaningful wind chill.
- Confusing wind chill with actual temperature. Your car, pipes, and thermometer respond to the real air temperature; wind chill only affects exposed living skin.
- Mixing units. This NWS formula needs Fahrenheit and mph. The metric (Environment Canada) version uses Celsius and km/h with different constants.
- Assuming clothing is included. The index models bare skin, so well-covered skin loses heat more slowly than the number implies.
- Ignoring frostbite time, not just the number. Two different temperature and wind pairs can give the same wind chill but different frostbite onset; the NWS chart shades exposure time directly.
Related tools
Frequently asked questions
What is wind chill?
Wind chill is how cold the air feels on exposed skin once wind is taken into account. Wind strips away the thin warm layer your skin builds, speeding up heat loss, so a windy 20 F day feels colder than a calm 5 F day. The wind chill temperature is the still-air temperature that would chill bare skin at the same rate.
What is the wind chill formula?
The US National Weather Service formula is WC = 35.74 + 0.6215 x T - 35.75 x V^0.16 + 0.4275 x T x V^0.16, where T is air temperature in Fahrenheit and V is wind speed in miles per hour. It was adopted in 2001 and is valid only when the temperature is at or below 50 F and the wind is above 3 mph.
Why does more wind not keep making it much colder?
Because the formula raises wind speed to the power 0.16, which has strongly diminishing returns. Going from 5 to 15 mph drops the wind chill noticeably, but going from 30 to 45 mph changes it only a few degrees. Most of the chilling effect comes from the first moderate breeze.
When does wind chill cause frostbite?
The NWS marks danger zones by wind chill temperature: frostbite on exposed skin in about 30 minutes at -18 F, about 10 minutes at -32 F, and about 5 minutes at -48 F or colder. Below those thresholds, cover all exposed skin and limit time outdoors.
Is wind chill the same as the actual temperature?
No. Wind chill only describes how fast exposed skin loses heat; it is not the real air temperature. Your thermometer, car, and water pipes respond to the actual temperature, which never drops below freezing just because of wind. Wind chill matters for human comfort and frostbite risk, not for objects.