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 
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 

The new formula will be incorporated into the latest software 
build installed on the NWS Advanced Weather Interactive 
Prediction System (AWIPS) in early November 2001. 

Local National Weather Service Offices will need to adjust 
the threshold values of Wind Chill Temperatures that trigger 
Wind Chill Advisories and Wind Chill Warnings....to reflect 
the new formula. 

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) 

Note: V   is the wind speed value in mph 

Note:  T  is the temperature in °F 

Note: Frostbite occurs in 15 minutes or less at wind 
         chill values of -18 or lower 

As history the old wind chill chart and formula has been retained here.