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August 17, 2001 — As most of the nation copes with simmering summer heat, NOAA's National Weather Service this week was preparing for winter. Starting with the 2001-02 winter, forecasters will use a new Wind Chill Temperature Index, designed to calculate a more accurate reading of how the cold air feels on the human skin. (Click image for larger view.) [NOAA Photo: The Potomac River in Sandy Hook, Maryland.]

Since 1945, the United States and Canada have used an index, which relied on observed winds 33 feet above the ground, and focused on how fast the cold temperatures—combined with winds—made water freeze. The new index accounts for the wind effects at face level, and a better calculation for body heat loss. For example, under the old index system, an air temperature of 20 degrees, with a 15 mph wind, translated into a reading of five degrees below zero. The new index calculation would translate the same conditions to six degrees above zero.

Click image for larger view.

NOAA wind chill index chart comparing old and new formulas.

"Exposure to cold, biting air for long periods of time is dangerous," said retired General Jack Kelly, director NOAA's National Weather Service. "Our main goal was to use modern science in revising the index so that it's more accurate and makes the human impact more prominent."

The new index will be based on:

• Wind speed calculated at the average height of the human face, about five feet (the human face is most often exposed to the cold).
• An updated heat transfer theory, which factors in heat loss from the body to its surroundings during cold, windy days.
• A consistent standard for skin tissue resistence.
• Clear night sky conditions.
• A lowered calm wind threshold from four miles to three miles.

For the past year, the National Weather Service, acting on behalf of the U.S. Office of the Federal Coordinator for Meteorological Services and Supporting Research, has led a team of international scientists with the goal of creating an international standard wind chill index among the meteorological community. Last spring, the scientists conducted clinical trials and the results helped to verify and improve the accuracy of the new formula.

Kelly said, "This information will help people make sound decisions about how to dress for the weather."

Relevant Web Sites
New Wind Chill Index Chart — This is a PDF file. You'll need Adobe Acrobat Reader to view it.

KNOW YOUR WINTER WEATHER TERMS

U.S. THREAT ASSESSMENT

NATIONAL WEATHER SERVICE FORECAST OFFICES ACROSS THE USA

NOAA SATELLITE IMAGES — Real-time satellite images

NOAA's CLIMATE PREDICTION CENTER

Media Contact:
Susan Weaver, NOAA's National Weather Service, (301) 713-0622