The wind chill temperature is how cold people and animals feel when outside. 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. Therefore, the wind makes it FEEL much colder. If the temperature is 0°F and the wind is blowing at 15 mph, the wind chill is -19°F. At this wind chill temperature, exposed skin can freeze in 30 minutes.
The NWS Wind Chill Temperature uses advances in science, technology, and computer modeling to provide an accurate, understandable, and useful formula for calculating the dangers from winter winds and freezing temperatures. The index does the following:
Calculates wind speed at an average height of 5 feet, the typical height of an adult human face, based on readings from the national standard height of 33 feet, which is the typical height of an anemometer
Is based on a human face model
Incorporates heat transfer theory based on heat loss from the body to its surroundings, during cold and breezy/windy days
Lowers the calm wind threshold to 3 mph
Uses a consistent standard for skin tissue resistance
Assumes no impact from the sun, i.e., clear night sky.
The wind chill temperature is calculated using the following formula:
Wind chill (ºF) = 35.74 + 0.6215T - 35.75(V^0.16) + 0.4275T(V^0.16)
Where: T = Air Temperature (F)
V = Wind Speed (mph)
^ = raised to a power (exponential)
Wind chill temperature is only defined for temperatures at or below 50°F and wind speeds above 3 mph. Bright sunshine may increase the wind chill temperature by 10°F to 18°F.
Criteria for issuing official NWS Extreme Cold Warnings and Cold Weather Advisories are set locally. For example, the Rochester, NY area, Extreme Cold Warnings are issued when the apparent temperature with or without wind is expected to fall at or below -25°F. Cold Weather Advisories are issued when the air temperature or wind chill temperature is expected to fall between -15°F and -24°F. Cold is dangerous with or without wind.
Yes, wind chill applies only to people and animals. The only effect wind chill has on inanimate objects, such as car radiators and water pipes, is to cool the object more quickly to the current air temperature. The inanimate object will NOT cool below the actual air temperature. For example, if the temperature outside is -5°F and the wind chill temperature is -31°F, then your car's radiator will not drop lower than -5°F.
The air temperature has to be BELOW freezing for frostbite to develop on exposed skin. Wind chill can NOT bring the temperature to below freezing for humans and animals when the thermometer says it is above freezing, so you will not get frostbite; however, you might get hypothermia from exposure to cold. In summary, you can only get frostbite if the actual air temperature, not the wind chill temperature, near your skin is below freezing.
When NWS tested the new Wind Chill Temperature, NWS researchers applied the new index to 12 test subjects. The results of the tests showed that relative humidity was an insignificant weather parameter, affecting the outcome to less than 1 degree. To simplify the calculation, relative humidity was left out.
The tests that were done on wind chill were conducted on adult subjects. For legal and safety reasons, NWS did not ask for child volunteers. Use the existing chart as a starting point and be even more cautious with children, the elderly, and persons with compromised health.
Additional information on cold-related health safety from the Centers for Disease Control and Prevention (CDC) can be found here.
