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New Formula Will Provide More Accurate Warnings for North America

NOAA image of winter storm aftermath.November 1, 2001 — NOAA's National Weather Service today implemented a new method to compute wind chill temperature to provide better winter weather warnings throughout the nation. For the first time, the index will include specific wind chill threshold values showing frostbite danger at given periods of time.

"We're proud that the new index reflects the best science, technology and computer modeling technology," said retired General Jack Kelly, director of NOAA's National Weather Service. "For the first time, a consistent standard of wind chill will be used by both Canada and the United States."

The new wind chill formula is the product of a year-long effort by scientists and wind chill experts from the academic community, and the U.S. and Canadian governments. The Office of the Federal Coordinator for Meteorological Services and Supporting Research created this Joint Action Group for Temperature Indices, which 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).

In October 2000, scientists began evaluating the previous wind chill formula and developed plans for a standardized index among the meteorological community for North America. This new wind chill formula was developed after extensive analysis of the factors that impact wind chill, using the latest advances in science, technology and computer modeling to provide a more accurate, understandable and useful model for calculating the dangers from winter winds and freezing temperatures. The resulting formula was then tested using human volunteers at the wind tunnel and climatic chamber of the Defense and Civil Institute of Environmental Medicine in Toronto, Canada.
Specifically, the new wind chill 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 has been incorporated into the latest software installed on the NWS Advanced Weather Interactive Prediction System (AWIPS) effective Nov. 1, 2001. In 2002, adjustments for solar radiation (the impact of the sun) and for a variety of sky conditions (sunny, partly sunny and cloudy) may be added to the calculation model.

"The new wind chill index provides us with specific warning of time-to-frostbite at given levels of wind chill," Kelly said. "Since it is the responsibility of the National Weather Service to help protect lives, we believe this will be an important service to the American people during winters to come."

The previous index has been used by the National Weather Service since 1973, and was based on science produced by Antarctic explorers in 1945.

The new wind chill index is available online.

NOAA's National Weather Service is the primary source of weather data, forecasts and warnings for the United States and its territories. NWS operates the most advanced weather and flood warning and forecast system in the world, helping to protect lives and property and enhance the national economy.

Relevant Web Sites
NOAA's New Wind Chill Chart

NOAA's Winter Weather Awareness

NOAA's Office of the Federal Coordinator for Meteorology

NOAA's Meteorological Calculator

Meteorological Tables

NOAA's Weather Page

NOAA's Storm Watch

Media Contacts:
Susan Weaver or Curtis Carey, NOAA's National Weather Service, (301) 713-0622