What is the difference between the Old and New Wind Chill Formulas?

National Weather Service Windchill Chart
Weather channels and news organizations are still making reference to the "old" wind chill formulas. Wind Chill describes the rate of heat loss on the human body resulting from the combined effect of low temperature and wind. As wind increases the internal body temperature is driven down at a faster fast, quickly reducing the exterior and interior body temperatures.
 
The "old" wind chill formula was first instilled in the 1940s and based on the research from Antarctic explorers Siple and Passel. The Siple and Passell formula was implemented through a series of observations on freeze times of plastic cylinders filled with water and placed in differing temperatures and wind speeds. 

This old system did not take in consideration that the human body produces its own heat, unlike the plastic containers. In addition, weather stations measure wind speed, the basis for the WCT index, at a height of 33 feet where it is often blowing stronger. This lead to meteorologists forecasting wind chill readings much lower than actuality.

The "new" wind chill formula was developed in 2001 and according to the National Weather Service, the formula is more detailed, "accounting for wind effects at face-level. This should result in more accurate calculations of human body heat loss."

To see accurate readings for the Whiteface region of the Adirondack Park, check out the Atmospheric Sciences Research Center, part of the University of Albany, in Wilmington, NY. 

According to the National Weather Service: The NWS Windchill Temperature (WCT) index 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:
  • 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, typical height of an anemometer
  • Is based on a human face model
  • Incorporates heat transfer theory: 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.
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For those interested in attempting to calculate the old formula

Old Wind Chill = 0.081 * (3.71 * sqrt (V) + 5.81 - 0.25 * V) * (T - 91.4) + 91.4
Where T = temperature in degrees Fahrenheit and V = wind speed in MPH


New Wind Chill = 35.74 + 0.6125T - 35.75 (V0.16) + 0.4275T (V0.16)
Where T = temperature in degrees Fahrenheit and V = wind speed in MPH 



© Diane Chase is the author of the Adirondack Family Time™ guidebook series. Adirondack Family Time™guidebooks have easy, short Adirondack family hikes for ADK kids, parents, retired, seniors, dog-owners, Adirondack swimming holes, Lake Placid Olympic activities, Adirondack trivia, Adirondack horseback rides, Adirondack snowshoe family trails and more. Look for the Adirondack family guidebooks online or bookstores/museums/sporting good stores. Diane is currently working on the next Adirondack Family Activities™ guide.

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