National Weather Service launches new wind chill formula

You may not be as cold as you think you are this winter.

A new formula for calculating wind chill was launched this week by the National Weather Service. It provides a more accurate, understandable and useful formula for evaluating potential danger from the combined effects of wind and cold, said David Blanchard of the National Weather Service at Paducah, Ky.

The weather service adopted the new formula to replace a wind chill index previously used by the weather service and Meteorological Service of Canada based on the 1945 Siple and Passel Index.

The new wind index uses calculated wind speed at an average height of 5 feet, considered the typical height of an adult human face. It previously was based on observations at 30 feet.

The new formula makes use of advances in science, technology and computer modeling to provide a more accurate, understandable and useful method 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.

Included in the index system are means of incorporating modern heat transfer theory, heat loss from the body to its surroundings during cold and windy days.

The new system produces warmer wind chill temperatures than the formula used in the past, said Blanchard and Kevin Smith, also of the weather service at Paducah.

For example, the old formula produced a wind chill temperature of minus 31 for a thermometer reading of zero with a wind speed of 15 mph. The new formula produces a wind chill factor of minus 19 for the same conditions.

The new system comes a year after the Office of the Federal Coordinator for Meteorological Services and Supporting Research formed a group of several federal agencies, universities and the International Society of Biometeorology to work with the National Oceanic and Atmospheric Administration to evaluate and improve the wind chill formula. The group, called the Joint Action Group for Temperature Indices, was chaired by the weather service. Its goal was to internationally upgrade and standardize the index for temperature extremes.

The goal is to implement the new formula in Canada and the United States at the same time to obtain a consistent wind chill index for North America.

"We usually start announcing wind chill temperatures when the thermometer hits about 33 degrees." said Blanchard.

The Meteorological Service of Canada started using the new system Oct. 1.

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