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Wind Chill
Terms and Definitions
National
Weather Service plans to Implement a New Wind Chill Temperature Index
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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 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.
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The
JAG/TI formula will make use of advances in science, technology,
and computer modeling to provide a more accurate, understandable,
and useful 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.
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Specifically,
the new WCT index will:
- use calculated
wind speed at an average height of five feet (typical height of an adult
human face) based on readings from the national standard height of 33
feet (typical height of an anemometer);
- 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
no impact from the sun (i.e. clear night sky).
The new formula
will be incorporated into the latest software build installed on the NWS
Advanced Weather Interactive Prediction System (AWIPS) by November 2001.
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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