Wind chill explained: what the number really means
Wind chill is the temperature it feels like on exposed skin when wind and cold combine. Wind strips away the thin layer of warm air around your skin, so skin cools faster than it would in still air. The U.S. National Weather Service index, in use since November 2001, models heat loss from a face at walking pace and estimates how quickly frostbite can occur.
What is wind chill?
The National Weather Service defines wind chill temperature as the temperature it feels like outside, based on the rate of heat loss from exposed skin caused by wind and cold. It is a way to express faster cooling in familiar temperature units.
The physics is convection. Skin warms a thin boundary layer of air that acts as a little insulation. Wind blows that layer away and replaces it with cold air, so skin keeps losing heat faster. Mayo Clinic describes the same mechanism: wind removes body heat by carrying away the thin layer of warm air at the skin surface.
How is wind chill calculated?
The current U.S. index took effect on November 1, 2001. It uses air temperature in degrees Fahrenheit (T) and wind speed in miles per hour (V):
Wind chill (°F) = 35.74 + 0.6215T minus 35.75(V to the power 0.16) plus 0.4275T(V to the power 0.16).
The model is built on heat transfer theory applied to a human face. Wind speed is adjusted to an average height of 5 feet, typical of an adult face, and the model was developed with volunteers walking at 3 mph on a treadmill. It assumes no sunshine, as on a clear night. The index is defined only for temperatures at or below 50°F and wind speeds above 3 mph.
What do typical wind chill values look like?
Values from the NWS wind chill chart show how quickly moderate wind lowers the felt temperature.
| Air temperature | Wind speed | Wind chill (NWS chart) |
|---|---|---|
| 0°F | 15 mph | -19°F |
| 5°F | 20 mph | -15°F |
| -10°F | 20 mph | -35°F |
How does wind chill relate to frostbite?
The NWS chart shades zones where frostbite can develop on exposed skin in 30 minutes, 10 minutes or 5 minutes. The NWS notes that at a wind chill of about -19°F, exposed skin can freeze in 30 minutes. Colder wind chills shorten that window.
Frostbite is an injury caused by freezing of body tissue. Early signs include loss of feeling and a white or pale appearance, and it needs prompt medical attention. Hypothermia, a core temperature below 95°F, can also develop in cold, windy conditions, especially when clothing is wet. Anyone with symptoms of either should talk to a clinician or seek emergency care.
What wind chill does not tell you
It does not cool objects below air temperature. Car radiators and water pipes cannot get colder than the actual air, so wind chill does not make them freeze at a higher temperature.
It ignores sunshine. The index assumes a clear night, so bright sun can make conditions feel milder than the number suggests.
It applies to exposed skin, mainly the face. Well-covered skin under windproof layers does not experience the full wind chill.
It assumes walking pace. Riding a bike, snowmobile or motorcycle adds your own speed to the wind.
It does not account for wet skin or clothing, which increases heat loss further.
How should you use wind chill when planning time outside?
Treat wind chill as a clock for exposed skin. The lower the number, the shorter the time before uncovered cheeks, ears or fingers are at risk. Check it alongside the actual air temperature, because the air temperature still governs what happens to water, vehicles and equipment.
Plan for the highest wind you will meet, not the average. Ridgelines, open lakes, rooftops and loading docks can be much windier than a sheltered street, and your own speed adds to the wind when you ride or ski.
For outdoor work, wind chill is one input to warm-up breaks and task rotation. Build in shelter, dry spare gloves and a buddy system so early signs of cold injury are noticed.
How should you dress for wind chill?
Because wind chill is about exposed skin and convective loss, the priorities are covering skin and blocking wind. NWS guidance recommends layers of loose-fitting, lightweight, warm clothing, a hat, mittens over gloves and outer garments that are tightly woven, water repellent and hooded. Cover your mouth and stay dry.
A windproof outer shell protects the warm air trapped in the layers beneath it. Without it, wind can push through insulation and flush out that air, a problem that grows when you are moving.
Face and hands deserve special care because they are the most exposed. A neck gaiter or balaclava, goggles in strong wind and mittens rather than thin gloves make a large difference at low wind chills.
Key takeaways
Wind chill is the felt temperature on exposed skin when wind speeds up heat loss.
The U.S. index dates to November 2001 and models a face at 5 feet height, walking at 3 mph, with no sun.
At 0°F with a 15 mph wind, the wind chill is -19°F, where exposed skin can freeze in about 30 minutes.
Wind chill does not cool cars or pipes below the air temperature.
Covering skin and blocking wind with a windproof outer layer addresses the risk directly.
Frequently asked questions
Does wind chill affect cars or pipes?
No. Wind chill describes heat loss from warm skin. Inanimate objects like car radiators and water pipes cannot cool below the actual air temperature, although wind can help them reach it faster.
At what wind chill does frostbite occur?
The NWS chart shows frostbite possible within 30 minutes at a wind chill of about -19°F, and within 10 or 5 minutes at colder values. Wet skin, poor circulation and tight clothing raise the risk.
Is wind chill the same as the feels-like temperature?
In cold weather, the feels-like temperature is usually the wind chill. In hot weather, forecasters use the heat index instead, which combines temperature and humidity.
Why does the wind chill feel less severe on sunny days?
The index assumes a clear night with no sunshine. Direct sun adds radiant heat to skin and clothing, so real conditions can feel milder than the calculated number.
Can wind chill cause hypothermia?
Wind increases the rate of heat loss, which raises the risk of hypothermia in poorly protected or wet people. Hypothermia is defined by core temperature falling below 95°F, not by the wind chill number itself.
Related reading
Sources
Wind Chill Chart and Frostbite Times, National Weather Service (NOAA)
Wind Chill Temperature Index, National Weather Service (NOAA)
Wind Chill Chart, National Weather Service (NOAA)
Hypothermia: Symptoms and causes, Mayo Clinic
Frostbite: Symptoms and causes, Mayo Clinic
About HEATJAC. HEATJAC is a thermal architecture company founded by an anesthesiologist. We design patented garment systems that capture, conduct and broaden warmth across the body, and we publish this knowledge hub because the science of staying warm should be public. HEATJAC products are not medical devices and are not intended to diagnose, treat, cure or prevent any disease.