New Wind Chill Temperature Index The NWS is planning to implement a replacement Wind Chill Temperature (WCT) index for the 2001/2002 winter season. The reason for the change is to improve upon the current WCT Index used by the NWS and the Meteorological Services of Canada (MSC, the Canadian equivalent of the NWS), which is currently based on the 1945 Siple and Passel Index. For over a year, there has been discussion within the NWS and Meteorological Services of Canada (MSC), about updating the WCT. During the Fall of 2000, the Office of the Federal Coordinator for Meteorological Services and Supporting Research (OFCM) formed a special group consisting of several Federal agencies, MSC, the academic research community (Indiana University-Purdue University in Indianapolis (IUPUI), University of Delaware, and University of Missouri), and the International Society of Biometeorology to evaluate the existing wind chill formula and make necessary changes to improve upon it. The group is called the Joint Action Group for temperature Indices (JAG/TI) and is chaired by the NWS. The goal of JAG/TI is to internationally upgrade and standardize the index for temperature extremes (e.g., Wind Chill Index).
After the October 2000 and February 2001
meetings, the JAG/TI reached agreement on a new wind chill formula, discussed a
process for scientific verification of the new formula, and developed plans for
implementation of the new formula. The new WCT index was presented at the JAG/TI
meeting in Toronto, Canada on August 2, 2001. The JAG/TI formula will make use
of advances in science, technology, and computer modeling to provide a more
accurate, understandable, and useful formula for calculating the dangers from
winter winds and freezing temperatures. In addition, clinical trials have been
conducted and the results of those trials have been used to verify and improve
the accuracy of the new formula. Standardization of the WCT Index among the
meteorological community is important, so that an accurate and consistent
measure is provided and public safety is ensured. Our goal is to implement the
new wind chill formula in Canada and the United States during the same time
frame in order to have a consistent WCT Index for North America.
Specifically, the new WCT index will: use wind speed calculated at the average
height (5 feet) of the human body's face instead of 33 feet (the standard
anemometer height); be based on a human face model; incorporate modern heat
transfer theory (heat loss from the body to its surroundings, during cold and
breezy/windy days); lower the calm wind threshold to 3 mph; use a consistent
standard for skin tissue resistance; and assume the worst case scenario for
solar radiation (clear night sky).
For example, assuming an air temperature of 5 degrees and a wind of 30 mph...
Old WCT = -41 New WCT = -18
To reflect the new formula, National Weather Service Forecast Offices have adjusted the threshold values of Wind Chill Temperatures that trigger Wind Chill Warnings and Wind Chill Advisories.
- When the wind chill temperature is forecast to be minus 25 degrees or lower, a wind chill warning will be issued.
- When the wind chill temperature is forecast to be between minus 10 to minus 24 degrees, a wind chill advisory will be issued.
In 2002, adjustments for solar radiation (i.e., the impact of sun) for a variety of sky conditions (sunny, partly sunny and cloudy) will be added to the calculation model.
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Here's the formula:
Wind Chill (�F) = 35.74 + 0.6215T - 35.75(V0.16)
+ 0.4275T(V0.16)
Where V = the wind speed value in mph and
T = the temperature in �F
Note: Frostbite occurs in 15 minutes or less at wind chill values of -18 or lower