For more than 25 years, weather forecasters have used the wind chill index to tell people how cold it really feels. Schools close, employees go home early and families huddle before the fireplace watching the Weather Channel and waiting for the ice to bring down the power lines.

Well, it's not that bad. In fact, wind chill generally overestimates the effect of wind by at least 10 degrees, and the error worsens as the wind increases. Scientists have pointed this out for some time, and this year a task force led by the National Weather Service has told two of the best-known heretics to change the formula.

"Even though we all pretty much know it's not correct, people said, 'Don't bother with it, you're erring on the side of safety,' " said mechanical engineer Maurice Bluestein, of Indiana University-Purdue University in Indianapolis. "But that's not true."

Indeed, counsels Bluestein, wind chill gives a false sense of security. If ambient temperature is above 32 degrees Fahrenheit, you can't get frostbite, no matter how bad the wind chill. But if the temperature is below 32 degrees, even without any wind chill, you can freeze to death.

This summer Bluestein and Randall Osczevski, an environmental physicist with Canada's Department of National Defence, will pool their skills to redraw the wind chill index under the auspices of the Weather Service-led Joint Action Group for Temperature Indices. The idea is to have the new table in place before next winter.

"What we're trying to do is devise a new revised formula in Canada and the United States to be implemented at the same time," said the Weather Service's Mark Tew, chairman of the Action Group. "It will take advantage of new science, technology and computer modeling."

The change is long overdue, according to Bluestein, Osczevski and others. Wind chill shows all the signs of an imperfectly realized government innovation that has endured largely through inertia -- there was no public clamor for change.

The wind chill index attempts to measure the rate of heat loss by the human body as wind blows across it at different temperatures and speeds. In the United States the index is a relative measure of discomfort expressed in degrees Fahrenheit -- what the wind makes the temperature feel like.

The index was developed in the 1940s by geographer Paul A. Siple and geologist Charles F. Passel during an Antarctic expedition. The two researchers hung plastic cylinders of water in the open air and measured the rate at which the water froze as temperature and wind speed changed. The National Weather Service adopted the wind chill index in 1973.

"It's flawed," Bluestein said, explaining that the experiments ignored the insulating effect of the plastic, assumed a skin temperature of 90 degrees (much too high) and measured the wind 33 feet off the ground (where its speed is much faster than at the level of the human face).

"Also, they measured different conditions and plotted the data all over the graph," Bluestein continued. "There was no consistent pattern for what they did. The chart is off anywhere from 10 to 15 degrees, and it gets worse at higher speeds."

Over the years, researchers in many countries have tried to improve the index based on their own needs.

"In the United States, we basically look at the extremes, because we're interested in warnings," said Richard Schwerdt, deputy chief of Meteorological Services in the Weather Service's Central Region. "But in many countries the extremes are not as great as here, so in a country like Germany they are looking at overall 'thermal comfort.' "

Interest in finding a world standard last year prompted the International Society of Biometeorology to form the "Commission 6" to develop what Schwerdt, a member of the panel, called "a continuous spectrum" that would replace both wind chill and the heat index with a single table that would describe comfort levels at all temperatures and weather conditions.

AccuWeather, the Pennsylvania-based commercial forecaster, has already done this, and has been posting its RealFeel Temperature for about a year.

"The biggest problem is that the wind chill index only takes into account two factors," said Michael Steinberg, AccuWeather's senior vice president. RealFeel, developed over several years, blends several climatic factors in addition to temperature and wind speed, he said.

These include humidity, atmospheric pressure, the amount and intensity of precipitation, and the amount and intensity of sunshine. "When you stand outside in the sun, even in winter, you notice a difference," Steinberg said. "And even on a day where it's not sunny, there may be some heat reaching you."

Still, old habits die hard. In Canada, meteorologists for years have calculated wind chill as the rate of heat loss in watts per meter squared, but working forecasters routinely translate the readings into degrees Celsius because that's what people understand.

And while Steinberg said that the use of RealFeel "is growing rapidly," he acknowledged that preferences for RealFeel over traditional wind chill vary. (The Post, an AccuWeather client, uses wind chill.)

So while AccuWeather has opened "indirect contacts" with the Commission 6 looking for a single index solution, Steinberg said, the Weather Service-led Action Group is focusing, at least for the moment, on wind chill, and has its hopes riding on Osczevski and Bluestein.

In the mid-1990s, Osczevski began developing a new wind chill model using an artificial human head he had constructed to measure the insulation of combat helmets. "I came to the conclusion that wind chill has to be related to facial cooling," Osczevski said, "but the [traditional measurement] was wrong proportionately."

Bluestein had reached the same conclusion by modeling the whole head as a cylinder. Osczevski then demonstrated that if researchers used only the upwind side of the face, the two models produced similar results.

The Action Group plan is for Bluestein and Osczevski to work together in a wind tunnel with a compromise device -- the windward side of a cylinder, and put it at face height, five feet off the ground.

The pair will test the resulting index by measuring skin temperature and heat loss in the faces of human volunteers.

Bluestein acknowledged that the sun can have a significant effect on wind chill, and he predicted that he and Osczevski will "put sunshine in the mix," but they had no immediate plans to create an index to cover all climate conditions.

"The first thing is to fix the current wind chill temperature," Bluestein said. "I'm just particularly interested in cold."