What Is a Wind Chill Factor? Definition, Formula & Safety

There’s a reason your cheeks sting and your fingers feel stiff on a windy winter day — the wind chill factor is at work. The National Weather Service calculates it using a formula that accounts for both air temperature and wind speed, and understanding this number can help you decide how to dress and when to head indoors.

Wind chill formula (US): 35.74 + 0.6215*T – 35.75*V^0.16 + 0.4275*T*V^0.16 ·
Freezing risk (exposed skin, -27°F): 30 minutes ·
Lowest recorded wind chill (US): -108°F (-78°C) ·
Speed threshold for warming effect: > 100°F air temperature

Four key facts that define the wind chill factor at a glance.

What does wind chill factor mean?

Difference between wind chill and actual temperature

The wind chill factor is the perceived temperature on exposed skin due to wind and cold. It is based on the rate of heat loss from the body via forced convection, as defined by the National Weather Service. The key distinction: wind chill does not change the actual air temperature. It only describes how cold it feels to a living, warm-blooded person.

In Canada, the Government of Canada defines wind chill similarly: how cold it feels when temperature is below zero and wind is present. Both agencies stress that the index is a heat-loss index, not a temperature measurement.

Why wind chill feels colder

When the wind blows, it strips away the thin layer of warm air that naturally surrounds your body. The faster the wind, the faster that heat is carried away. This process, known as forced convection, is what makes a 20°F day with 20 mph wind feel like 6°F on exposed skin, per the NWS wind chill chart.

• Wind removes the warm air layer around your body.
• Faster wind = faster heat loss = colder sensation.
• Wind chill values are only meaningful when air temperature is at or below 50°F and wind speed is above 3 mph (National Weather Service Boulder).

The implication: wind chill is a measure of risk, not just discomfort. The NWS uses it to warn of frostbite danger.

What is the wind chill factor of 20 mph?

Wind chill chart for common wind speeds

Wind chill values change non-linearly with wind speed. At 20°F with 20 mph wind, the wind chill is approximately 6°F. At 30°F with 20 mph wind, the wind chill is approximately 17°F, according to the NWS wind chill chart.

For a wind speed of 10 mph at 20°F, the wind chill is about 9°F. At 20 mph, it drops to 6°F — a three-degree difference that matters for frostbite risk. The Government of Canada publishes a similar metric chart for Celsius users.

• At 20°F, 10 mph wind → wind chill 9°F.
• At 20°F, 20 mph wind → wind chill 6°F.
• At 30°F, 20 mph wind → wind chill 17°F.

The pattern: the colder the base temperature, the more dramatic the wind chill drop. A 20°F day with 20 mph wind crosses the frostbite threshold for 30 minutes of exposure.

How to use the wind chill calculator

You can calculate wind chill yourself using the NWS formula: WCT = 35.74 + 0.6215*T – 35.75*(V^0.16) + 0.4275*T*(V^0.16), where T is temperature in °F and V is wind speed in mph. The National Weather Service provides the official calculator online.

In Canada, the formula uses Celsius and km/h: WC = 13.12 + 0.6215*T – 11.37*(V^0.16) + 0.3965*T*(V^0.16) (Government of Canada). Both formulas produce the same relative risk levels, just in different units.

What this means: always check the wind chill before heading out in cold weather, especially if you plan to be outdoors for more than 15 minutes.

What wind chill is too cold?

Frostbite risk by wind chill temperature

The National Weather Service provides clear thresholds for frostbite risk:

• Wind chill below -27°F (-33°C): frostbite possible in 30 minutes.
• Wind chill below -40°F (-40°C): frostbite possible in 10 minutes.
• Wind chill below -60°F (-51°C): frostbite possible in 5 minutes.

However, the NWS notes that frostbite can occur only if the actual air temperature near the skin is below freezing, not merely because the wind chill is low. The wind chill accelerates cooling, but the skin must drop below 32°F for ice crystals to form.

A Canadian wind chill chart from McGill University classifies -60 as a danger level where frostbite is possible in under 2 minutes.

Wind chill safety guidelines from the National Weather Service

The NWS recommends dressing in layers and covering all exposed skin when wind chill is dangerous. The agency’s safety page also warns that a wind chill near -25°F means frostbite is possible within 15 minutes. For extreme cold, limit outdoor exposure and stay dry.

• Wear a hat, gloves, and scarf to cover face and ears.
• Choose insulated, wind-resistant outer layers.
• Stay dry — wet clothing increases heat loss dramatically.

Does wind chill actually lower the temperature?

Wind chill and inanimate objects

No. Wind chill does not lower the actual air temperature. It only affects living, warm-blooded organisms with exposed skin. Inanimate objects like pipes, cars, and buildings do not experience wind chill, as confirmed by the National Weather Service.

This is a common misconception. A pipe can still freeze if the actual temperature is below 32°F, regardless of wind. The wind chill index is irrelevant to non-living things. The Government of Canada makes the same point: wind chill is an index for conditions below freezing, not a direct air temperature measurement.

• Pipes freeze based on actual air temperature, not wind chill.
• Cars don’t feel wind chill, but their batteries can fail in extreme cold.
• Home heating systems are unaffected by wind chill (though wind can increase heat loss through walls).

The catch: wind can increase heat loss from a house via convection, but that’s a building physics issue, not wind chill. The wind chill index is strictly about human exposure.

Wind chill and home heating systems

Wind does not affect the thermostat’s reading of indoor temperature. However, strong wind can accelerate heat loss through windows and poorly insulated walls, causing your furnace to run more often. This is a matter of building envelope efficiency, not wind chill. The National Weather Service clarifies that the wind chill index is not designed for inanimate objects or structures.

The trade-off: if you’re worried about frozen pipes, focus on the actual temperature forecast, not the wind chill. Insulate pipes and let faucets drip when temperatures drop below 20°F.

Does wind chill affect elderly people more?

Why elderly have higher hypothermia risk

Older adults lose body heat faster due to reduced circulation and a lower metabolic rate, according to the National Institute on Aging. Their bodies are less efficient at generating and retaining heat, making them more vulnerable to the effects of wind chill even at moderate levels.

Additionally, many seniors take medications that affect blood flow or temperature regulation. The National Weather Service includes age as a risk factor in its extreme cold warnings.

• Reduced circulation means less warm blood reaches the skin.
• Lower metabolic rate produces less internal heat.
• Medications can impair the body’s ability to regulate temperature.

Precaution measures for seniors in cold wind

Elderly individuals should limit outdoor exposure when wind chill is extreme, dress in warm layers, and keep their home heated to at least 68°F. The National Institute on Aging recommends checking on older neighbors during cold snaps.

• Check local wind chill forecasts before going out.
• Wear a warm hat, scarf, and gloves — even for short trips.
• Stay indoors if wind chill is below 0°F.

The pattern: the same wind chill that causes discomfort for a healthy adult can be dangerous for an older person. The safe threshold is effectively lower.

Confirmed facts vs. what’s unclear

Confirmed facts

• Wind chill index is calculated using a specific formula adopted by NOAA (National Weather Service).
• Wind chill does not affect inanimate objects (National Weather Service).
• Wind chill below -27°F can cause frostbite in 30 minutes (National Weather Service).
• The coldest recorded wind chill in the US was -108°F (National Weather Service).

What’s unclear

• The exact physiological response varies by individual (age, health, clothing).
• Wind chill tolerance limits for outdoor activities are not fixed for all people.
• The role of humidity in wind chill perception is not captured by the standard formula.
• Specific wind chill thresholds for different age groups are not precisely defined.
• The effect of wind chill on pets is not officially defined.

Expert perspectives on wind chill

Wind chill is based on the rate of heat loss from exposed skin caused by wind and cold. As the wind increases, it draws heat from the body, driving down skin temperature and eventually the internal body temperature.

— National Weather Service (NWS wind chill page)

Wind chill temperature is a measure of how cold people feel when exposed to wind. The wind chill temperature is not the same as the actual air temperature — it’s a representation of how the wind makes the air feel colder.

— NOAA (NOAA Technical Attachment report)

Wind chill factor takes into account wind speeds and humidity to assess how the human body actually feels temperature. It is a measure of the cooling effect of wind on the human body.

— Met Office (UK weather service) (Met Office)

For anyone heading outdoors in cold weather, the choice is clear: check the wind chill before you go, and cover all exposed skin. Ignoring it could mean frostbite in minutes. For older adults, the implication is even sharper: treat any wind chill below 0°F as a signal to stay indoors, or risk hypothermia.

Frequently asked questions