Wind Chill
Calculate wind chill from temperature and wind speed.
About the Wind Chill
Wind chill is how cold the air feels on exposed skin once wind strips away the thin warm layer your body maintains. It uses the NOAA formula for Fahrenheit and mph.
How to use it
- Enter the air temperature in Fahrenheit.
- Enter the wind speed in mph.
- Read the apparent temperature.
The formula
Wind chill describes how cold the air feels on exposed skin, not how cold it is. The current North American formula dates from 2001.
WC = 35.74 + 0.6215T − 35.75V0.16 + 0.4275T × V0.16
T is air temperature in Fahrenheit and V is wind speed in miles per hour. The 0.16 exponent is the reason the first few miles per hour matter so much more than the last: the curve is steep at low wind speeds and flattens as wind rises, so going from 5 to 15 mph costs far more than going from 25 to 35.
The formula is defined for temperatures at or below 50°F and winds above 3 mph. Outside that range it returns numbers that are not meaningful, which is why forecasts simply stop quoting wind chill on mild days.
Worked examples
| Air temperature | Wind speed | Feels like | Difference |
|---|---|---|---|
| 40°F | 5 mph | 36.5°F | 3.5° |
| 30°F | 15 mph | 19.0°F | 11.0° |
| 10°F | 25 mph | −10.7°F | 20.7° |
| −10°F | 30 mph | −39.4°F | 29.4° |
The final column is the part worth internalising: the colder it already is, the more wind takes. At 40°F a light breeze costs three degrees; at −10°F a strong wind costs nearly thirty. This is why frostbite guidance is issued on wind chill rather than temperature — at −39°F exposed skin can be damaged in around ten minutes.
Common mistakes
- Thinking wind chill cools objects below air temperature. It cannot. Wind removes heat faster, so a warm body cools quicker, but a parked car or a water pipe will never drop below the actual air temperature no matter how hard the wind blows.
- Applying it above 50°F. The formula is not defined there. On mild days the relevant measure is the heat index, which works in the opposite direction.
- Assuming it applies through clothing. Wind chill is calculated for exposed skin, specifically the face. Properly insulated and windproofed, the effect on your body is very much smaller.
- Comparing figures from different countries. Canada and the United States use the same 2001 formula but in different units, and older sources use the 1945 version, which produced considerably more dramatic numbers.
Terms explained
- Wind chill
- The apparent temperature on exposed skin, combining air temperature and wind speed.
- Convective heat loss
- Heat carried away by moving air. The mechanism wind chill describes.
- Boundary layer
- The thin film of air warmed by your skin. Wind strips it away, which is why moving air feels colder.
- Frostbite
- Freezing of skin and underlying tissue. Risk times are published against wind chill rather than air temperature.
- Heat index
- The warm-weather counterpart, combining temperature and humidity to describe how hot it feels.
- Apparent temperature
- The general term for what conditions feel like, of which wind chill and heat index are two cases.
Common questions
- What exactly is wind chill?
- How cold the air feels on exposed skin, given the wind. Moving air strips away the thin warm layer your body maintains, so heat is lost faster than the thermometer alone suggests.
- Can wind chill freeze my pipes faster?
- It can make them reach air temperature sooner, but never colder than the air. If the air is above freezing, wind alone will not freeze anything.
- Why was the formula changed in 2001?
- The 1945 version was based on measuring how quickly water froze in plastic cylinders, and it substantially overstated the effect. The current version was calibrated using human volunteers with facial sensors.
- At what point does frostbite become a risk?
- Guidance generally begins around −18°F wind chill, with exposed skin at risk within about thirty minutes. Near −40°F that falls to roughly ten minutes.
- Does wind chill affect animals?
- Any warm body loses heat faster in wind, so the principle applies. The specific numbers are calibrated for human facial skin and do not transfer directly to a well-furred animal.
- Why does the wind speed use a fractional exponent?
- Because heat loss does not rise linearly with wind. The 0.16 power captures the steep gain at low speeds and the flattening at high ones — the first 10 mph matters far more than the next 10.
- Is there a wind chill above 50°F?
- The formula is not defined there, and forecasters stop quoting it. Above that, humidity matters more than wind and the heat index takes over.
- How much does clothing change this?
- Enormously. Wind chill assumes exposed skin, so a windproof outer layer removes most of the effect. The figure is a guide to what needs covering, not a prediction of how you will feel when dressed for it.