What Is Wind Chill? How the NWS Wind Chill Chart Works

Wind chill is the temperature it feels like to your body when cold air and wind combine. The thermometer may read 0°F, but if a 15 mph wind is blowing, your exposed skin loses heat as fast as it would at -19°F in calm air. That makes it a quick way to judge how dangerous it is to be outside.

As the first cold snaps of the season arrive, here is what wind chill means, how the National Weather Service calculates it, how to read the official wind chill chart, what it does and does not affect, and which cold weather alerts to watch for, based on NWS guidance.

What is wind chill?

The NWS defines the wind chill temperature as how cold people and animals feel when they are outside. It is based on the rate of heat loss from exposed skin caused by wind and cold together.

As the NWS puts it, as the wind increases it draws heat from the body, driving down skin temperature and eventually internal body temperature. The wind makes it feel much colder than the air alone.

How the NWS calculates wind chill

The current NWS Wind Chill Temperature index has been in effect since Nov. 1, 2001. According to the agency, the index:

• Calculates wind speed at an average height of 5 feet, the typical height of an adult human face, adjusted from the standard 33-foot height where wind instruments (anemometers) usually sit.
• Is based on a model of a human face.
• Uses heat transfer theory, based on heat lost from the body to its surroundings on cold and breezy days.
• Sets the calm wind threshold at 3 mph.
• Uses a consistent standard for how well skin tissue resists heat loss.
• Assumes no warming from the sun, as on a clear night.

The formula, in degrees Fahrenheit, is:

Wind chill (°F) = 35.74 + 0.6215T – 35.75(V^0.16) + 0.4275T(V^0.16)

Here T is the air temperature in degrees Fahrenheit and V is the wind speed in miles per hour. The NWS says wind chill is defined only for temperatures at or below 50°F and wind speeds above 3 mph.

Two factors are deliberately left out. Humidity was dropped because NWS testing on 12 adult subjects found relative humidity changed the result by less than 1 degree. Sunshine is not included either, but the NWS notes that bright sun can raise the wind chill temperature by 10°F to 18°F.

How to read the wind chill chart

The NWS publishes the formula as a wind chill chart so you do not have to do the math. Air temperature runs across the top, from 40°F down to -45°F, and wind speed runs down the side, from 5 mph to 60 mph. Find the column for the temperature and the row for the wind speed. The number where they meet is the wind chill.

Here are a few values taken directly from the official NWS chart:

Two patterns stand out. First, the first few miles per hour of wind make the biggest difference. At 0°F, going from 5 mph to 15 mph drops the wind chill from -11°F to -19°F, while going from 45 mph to 60 mph lowers it only from -37°F to -40°F.

The chart is also shaded to show frostbite times: zones where exposed skin can freeze in 30 minutes, 10 minutes or 5 minutes. The NWS uses the 0°F, 15 mph example to illustrate the first zone: at a -19°F wind chill, exposed skin can freeze in 30 minutes. Lower wind chills move into the faster zones.

What wind chill does and does not do

The NWS answers several common questions directly.

Does wind chill affect cars, pipes and other objects?

Not the way it affects people. Wind chill applies only to people and animals. The only effect wind has on objects such as car radiators and water pipes is to make them cool down to the air temperature faster. They do not get colder than the air. The NWS example: if the air is -5°F and the wind chill is -31°F, your car’s radiator will not drop below -5°F.

Can you get frostbite when it is above freezing?

No. The NWS says the air temperature has to be below freezing for frostbite to develop on exposed skin. If the thermometer reads above 32°F, wind chill cannot push your skin below freezing, even if the reported wind chill is lower. You can still develop hypothermia from exposure to cold.

Does the chart apply to children?

The wind chill tests were done on adult volunteers only. The NWS did not test children for legal and safety reasons, and it advises using the chart as a starting point while being even more cautious with children, older adults and people with health problems.

Frostbite and hypothermia: the real dangers

Wind chill matters because of what cold does to the body. The NWS describes the two main risks this way:

• Frostbite happens when body tissue freezes. The fingers, toes, ear lobes and tip of the nose are most at risk. Signs include loss of feeling and a white or pale look to the skin. The NWS says to get medical attention right away and to rewarm the area slowly with warm, not hot, water.
• Hypothermia happens when body temperature falls below 95°F. Warning signs include uncontrollable shivering, memory loss, disorientation, slurred speech, drowsiness and exhaustion. The NWS advises getting medical help immediately and, if help is delayed, warming the body core first rather than the arms and legs.

Cold is not only an outdoor problem. The NWS notes that about 20% of cold-related deaths occur in the home, and that children under 2 and adults over 65 are most susceptible to hypothermia. It recommends keeping the thermostat above 69°F, eating for warmth, staying hydrated and avoiding alcohol, which lowers body temperature.

Cold weather alerts: what changed in 2024

If you remember Wind Chill Warnings, those alerts no longer exist under that name. As part of its Hazard Simplification effort, the NWS consolidated its cold products on Oct. 1, 2024:

• Wind Chill Watches became Extreme Cold Watches.
• Wind Chill Warnings became Extreme Cold Warnings.
• Wind Chill Advisories became Cold Weather Advisories.
• Hard Freeze Watches and Warnings were folded into Freeze Watches and Freeze Warnings.

The agency said the change was meant to make clear that cold can be dangerous with or without wind. The new alerts are based on the apparent temperature, which is the air temperature when winds are calm or the wind chill when wind is present. Wind chill is not going away; it is now one input into a single set of cold alerts.

Here is what each NWS cold weather alert means:

• Extreme Cold Warning (take action): extremely dangerous cold air temperatures or wind chill values are expected or occurring. Avoid going outside if you can.
• Extreme Cold Watch (be prepared): dangerous cold is possible, but timing, location or occurrence is still uncertain.
• Cold Weather Advisory (be aware): dangerously cold conditions are expected that do not reach warning level.

Criteria are set locally by each NWS office. As one example, the NWS says that in the Rochester, New York, area an Extreme Cold Warning is issued when the apparent temperature is expected to fall to -25°F or lower, and a Cold Weather Advisory when it is expected to fall between -15°F and -24°F. You can follow active alerts and forecasts on the LiveNewsWorld U.S. Weather Live center.

How to stay safe when wind chills drop

The NWS says the best way to avoid frostbite and hypothermia is to stay warm and dry, indoors and out. When you need to go outside in low wind chills:

• Wear several layers of loose-fitting, lightweight, warm clothing; the air trapped between layers insulates you.
• Choose outer layers that are tightly woven, water repellent and hooded.
• Wear a hat, cover your mouth and pick mittens snug at the wrist, which the NWS says are better than gloves.
• Stay dry and out of the wind when possible, and remove layers before you start sweating.
• Under an Extreme Cold Watch, keep at least half a tank of gas in your car and update your winter survival kit; under a warning, make sure someone knows where you are going.

The NWS notes that dangerous cold can accompany or follow wintry precipitation, so watch for it after strong storms such as a bomb cyclone. For the seasonal picture, see our explainer on El Niño vs La Niña and U.S. winter.

The bottom line

Wind chill tells you how quickly cold, moving air will pull heat from your skin, not how cold the air itself is. The NWS calculates it with a formula built on a model of the human face, valid only at 50°F or below with winds above 3 mph, and publishes a chart that also marks how fast frostbite can set in. Use the wind chill to decide how to dress and how long to stay outside, use the actual temperature for pipes, plants and engines, and watch for Extreme Cold Watches, Extreme Cold Warnings and Cold Weather Advisories from your local NWS office.

Sources

• National Weather Service, Understanding Wind Chill (formula, index assumptions, thresholds, Rochester alert criteria)
• National Weather Service, Wind Chill Chart (PDF, effective Nov. 1, 2001) and Wind Chill Questions (chart values, frostbite times, frostbite and hypothermia guidance)
• National Weather Service, Understanding Cold Weather Alerts
• National Weather Service, “National Weather Service revises watch, warning and advisory products for cold” (news release, 2024)
• National Weather Service, Service Change Notice 23-44 Updated (March 26, 2024), consolidation of cold products effective on or about Oct. 1, 2024