How Cold Is It Really?

By CBSNews.com staff CBSNews.com staff

November 1, 2001 / 3:53 PM EST / CBS

It may not seem quite so cold this winter.

The National Weather Service introduced a new wind chill index Thursday that eases some of the extreme coldness estimates of the old formula.

The wind chill index is a formula that estimates how cold it feels based on the combination of temperature and wind. It is widely reported in winter to help people decide how warmly to dress.

"People may have gotten a little blase about wind chill but now what we have is a very specific threshold of time, wind and temperature that produces frostbite," said NOAA spokeswoman Susan Weaver. "This should give people a very specific and clear warning."

Under the new formula, a 10 mph wind makes a reading of 30 degrees Fahrenheit feel like 21 degrees and a reading of 15 degrees feel like 3 degrees. The old formula calculated that with a 10 mph wind 30 degrees felt like 16 and 15 degrees felt like minus 3.

A 20 mph wind under the new formula makes a 30 degree reading feel like 17 and 15 degrees fell like minus 2. The old formula showed a 20 mph wind making 30 degrees feel like 4 and 15 feel like minus 17.

Besides changing some estimated temperature readings, the new index adds a calculation of how long it will take exposed skin to develop frostbite.

"The new wind chill index provides us with specific warning of time-to-frostbite at given levels of wind chill," National Weather Service Director Jack 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."

For example, the chart shows frostbite occurring in 30 minutes at minus 10 degrees F and a 5 mph wind. At the same temperature a 30 mph wind brings on frostbite in 10 minutes.

In addition, Kelly noted that with the new formula, "For the first time, a consistent standard of wind chill will be used by both Canada and the United States."

The new formula was developed in a yearlong effort by scientists and wind chill experts from the academic community and the U.S. and Canadian governments, the Weather Service said.

It was tested, using human volunteers, at the wind tunnel and climatic chamber of the Defense and Civil Institute of Environmental Medicine in Toronto, Canada. The previous index, used by the National Weather Service since 1973, had been developed by Antarctic explorers in 1945.

The new index calculates the wind chill at five feet above the ground, the average height of a human face. It also incorporates modern heat transfer theory, in which heat is lost from the body to its surroundings, during cold and windy days.

Essentially, the wind chill calculates how rapidly the body will cool to the air temperature. It affects people and animals but not machines and regardless of the index the body will not cool below the air temperature.