Wind Chill Factor: How It's Calculated and What It Means
Wind chill factor (or wind chill temperature) is a measure of how cold it actually feels on exposed human skin when wind is taken into account. It is calculated using the National Weather Service formula that combines air temperature and wind speed. Wind chill only applies to people and animals, not to objects, and only when the air temperature is at or below 50 degrees Fahrenheit with wind speeds above 3 mph.
Quick Answer
- 1. Formula: Wind Chill = 35.74 + 0.6215T - 35.75(V0.16) + 0.4275T(V0.16), where T = air temp (F), V = wind speed (mph).
- 2. At 0 F with 15 mph wind, the wind chill is -19 F. Frostbite can occur in 30 minutes.
- 3. Wind chill values near -25 F mean frostbite is possible within 15 minutes (NWS).
- 4. The current NWS formula was developed in 2001 using human trial data from refrigerated wind tunnel experiments.
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Wind Chill Calculator - FreeThe NWS Wind Chill Formula Explained
The National Weather Service uses the following formula to calculate wind chill temperature in Fahrenheit:
Wind Chill (F) = 35.74 + 0.6215T - 35.75(V0.16) + 0.4275T(V0.16)
Where T is the air temperature in degrees Fahrenheit and V is the wind speed in miles per hour. This formula is only valid when the air temperature is at or below 50 degrees Fahrenheit and the wind speed is above 3 mph. Below those thresholds, wind chill is equal to the air temperature.
For example, if the air temperature is 10 degrees Fahrenheit and the wind speed is 20 mph, the calculation produces a wind chill of approximately minus 9 degrees Fahrenheit. This means exposed skin will lose heat at the same rate as if it were minus 9 degrees with no wind.
How the Formula Was Developed
The current NWS wind chill formula was adopted in 2001, replacing the older Siple-Passel index from 1945. The new formula was developed through clinical trials at the Defence and Civil Institute of Environmental Medicine in Toronto, Canada. Twelve volunteers walked on treadmills in a refrigerated wind tunnel at 1.4 meters per second (about 3.1 mph) while researchers placed thermal transducers on their faces to measure skin temperature changes at varying wind speeds and temperatures. The resulting formula directly measures heat loss from the human face, the most exposed body part during cold weather.
Wind Chill Chart: Temperature, Wind Speed, and Frostbite Risk
The NWS wind chill chart shows how quickly frostbite can develop at different combinations of temperature and wind speed. Frostbite times are for exposed facial skin:
| Air Temp (F) | 10 mph Wind | 20 mph Wind | 30 mph Wind | Frostbite Risk |
|---|---|---|---|---|
| 30 | 21 F | 17 F | 15 F | Low |
| 15 | 3 F | -2 F | -5 F | Low |
| 0 | -16 F | -22 F | -26 F | 30 min |
| -15 | -35 F | -42 F | -46 F | 10 min |
| -30 | -53 F | -61 F | -67 F | 5 min |
According to the National Weather Service, wind chill values near minus 25 degrees Fahrenheit mean that frostbite is possible within 15 minutes of exposure. At wind chill values of minus 40 degrees Fahrenheit or below, frostbite can occur on exposed skin in as little as 5 minutes. At extreme values below minus 60 degrees Fahrenheit, exposed skin can develop frostbite in under 2 minutes.
What Is Frostbite and How Does Wind Chill Cause It?
Frostbite occurs when skin and underlying tissues freeze, typically affecting extremities like fingers, toes, ears, and nose first. Wind accelerates frostbite by stripping away the thin layer of warm air that normally insulates exposed skin. The faster the wind, the more rapidly this protective layer is removed and replaced with cold air.
There are three stages of frostbite:
- Frostnip: The mildest form. Skin turns red, feels cold, and may tingle or feel numb. No permanent damage occurs if treated promptly by rewarming.
- Superficial frostbite: Skin turns white or pale, may feel warm (a dangerous sign), and blisters may form within 24 to 36 hours after rewarming.
- Deep frostbite: The most severe form. Affects all layers of skin and underlying tissue. Skin turns white, blue, or grayish-yellow. Joint and muscle function may be lost. Can result in permanent damage or amputation.
What Is Hypothermia?
Hypothermia occurs when your core body temperature drops below 95 degrees Fahrenheit (normal is 98.6 degrees). Unlike frostbite, which affects extremities, hypothermia is a systemic condition that can be fatal. Wind chill accelerates hypothermia by increasing the rate of overall body heat loss.
Prolonged exposure to temperatures as mild as 60 degrees Fahrenheit can trigger hypothermia, particularly in wet or windy conditions. Symptoms progress from shivering and confusion to slurred speech, drowsiness, and loss of consciousness. Hypothermia kills approximately 1,300 people per year in the United States, according to the CDC.
Wind Chill vs. Actual Temperature: What's the Difference?
A critical distinction: wind chill does not lower the actual temperature of objects. If the air temperature is 20 degrees Fahrenheit with a wind chill of minus 5, a thermometer will still read 20 degrees. Water will not freeze at 20 degrees regardless of wind chill. Pipes will not freeze at above-freezing temperatures because of wind.
Wind chill only describes how cold it feels on exposed human and animal skin. However, wind does cause objects to reach the ambient temperature faster. A warm car parked in 10-degree weather with strong winds will cool down to 10 degrees more quickly than in calm conditions, but it will not cool below 10 degrees.
NWS Wind Chill Advisories and Warnings
The National Weather Service issues two levels of wind chill alerts:
- Wind Chill Advisory: Issued when wind chill values are expected to reach minus 15 to minus 24 degrees Fahrenheit. At these levels, frostbite can occur on exposed skin within 30 minutes. You should dress in layers, cover exposed skin, and limit time outdoors.
- Wind Chill Warning: Issued when wind chill values are expected to reach minus 25 degrees Fahrenheit or lower. Life-threatening conditions exist. Frostbite can occur within 15 minutes on exposed skin. Avoid outdoor activities if possible.
How to Stay Safe in Extreme Wind Chill
Dress in Layers
Wear three layers: a moisture-wicking base layer (synthetic or merino wool), an insulating middle layer (fleece or down), and a windproof outer shell. The outer layer is critical because it blocks wind from penetrating and stripping away body heat. Cotton should be avoided as a base layer because it absorbs moisture and loses insulating properties when wet.
Cover All Exposed Skin
Frostbite most commonly affects the face, ears, fingers, and toes. Wear a balaclava or neck gaiter to cover your face, insulated gloves or mittens (mittens are warmer because fingers share heat), and warm, moisture-wicking socks with insulated boots. Exposed skin at minus 25 degrees wind chill can develop frostbite in 15 minutes.
Know the Warning Signs
Numbness, tingling, and skin discoloration (redness followed by whiteness) are early signs of frostbite. Shivering is the body's first defense against hypothermia. If shivering stops despite continued cold exposure, this is a medical emergency indicating the body is losing the battle against heat loss.
The Bottom Line
Wind chill factor tells you how cold it actually feels on exposed skin by accounting for wind's ability to strip away body heat. The NWS formula combines air temperature and wind speed to produce a value measured in degrees. At wind chill values of minus 25 degrees Fahrenheit, frostbite can occur within 15 minutes. At minus 40 degrees, it can happen in 5 minutes. Dress in windproof layers, cover all exposed skin, and heed NWS advisories and warnings during extreme cold events.
Check wind chill conditions instantly with our free wind chill calculator.
Frequently Asked Questions
What is wind chill factor and how does it work?
Wind chill factor describes how cold it feels on exposed skin when wind speed is factored in with the actual air temperature. Wind increases the rate at which your body loses heat by removing the thin layer of warm air that normally insulates your skin. The faster the wind blows, the more quickly heat is carried away, making the air feel colder than the thermometer reads. Wind chill is measured in degrees (Fahrenheit or Celsius) and calculated using the NWS Wind Chill Temperature Index formula, which was developed using human trial data in refrigerated wind tunnels.
Can wind chill actually freeze water or pipes?
No. Wind chill only affects living organisms and describes how cold it feels on exposed skin. Objects like water pipes, car engines, and outdoor equipment cannot cool below the actual air temperature regardless of wind speed. If the air temperature is 20 degrees Fahrenheit, a pipe will not freeze from wind chill alone. However, wind does cause objects to reach the ambient air temperature faster. So if the actual temperature is already below freezing, wind will cause exposed pipes to freeze sooner than in still air, but not at a lower temperature.
At what wind chill temperature should you stay indoors?
The National Weather Service issues a Wind Chill Advisory when wind chill values reach minus 15 to minus 24 degrees Fahrenheit, meaning frostbite can occur within 30 minutes. A Wind Chill Warning is issued when values reach minus 25 degrees Fahrenheit or lower, where frostbite can occur within 15 minutes or less. At wind chill values below minus 40 degrees Fahrenheit, frostbite can occur on exposed skin in as little as 5 minutes. If a Wind Chill Warning is in effect, limit outdoor exposure as much as possible and cover all exposed skin.
Does wind chill affect cars and engines?
Wind chill does not make your car engine colder than the actual air temperature. If it is 0 degrees Fahrenheit outside with a wind chill of minus 30, your engine and coolant will stabilize at 0 degrees, not minus 30. However, wind can cause your engine to reach that 0-degree temperature faster by stripping away residual heat more quickly. This matters when a warm car is parked in extreme cold and wind. Block heaters help by maintaining engine warmth regardless of wind. For electric vehicles, cold temperatures (not wind chill) reduce battery range by 20 to 40 percent.
How accurate is the NWS wind chill formula?
The current NWS wind chill formula, adopted in 2001, is based on controlled human trial data and is considered significantly more accurate than the older Siple-Passel formula it replaced. Volunteers walked on treadmills in refrigerated wind tunnels while researchers measured facial skin temperature at various wind speeds and temperatures. The formula assumes a 5-foot person walking at 3.1 mph with wind measured at the standard weather station height of 33 feet. For most people in most conditions, the formula provides a reliable estimate. However, individual factors like body composition, clothing, and sun exposure can affect perceived cold.
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