Wind chill factor gets new formula

By Thom Patterson
CNN

(CNN) -- Change is in the wind. The U.S. National Weather Service is about to revise how it measures the wind chill factor for the first time in 56 years.

The wind chill index measures how wind speed affects outdoor temperatures that are felt by the human body. The index is designed to give people an idea of how cold-weather air temperatures actually feel when factoring in the speed of the wind.

The old system -- used by the United States and Canada since 1945 -- measured wind speed at 33 feet above ground. But the new formula accounts for wind speed at what the weather service calls "face level."

Face level is officially defined as "about 5 feet above ground," the average height of the human face, according to the weather service.

The new system also factors in heat transfer theory, which accounts for heat loss from the body to its surroundings.

The public may have trouble with the new system at first because it makes temperatures appear warmer than they did under the old index.

Under the old system, an air temperature of 20 degrees Fahrenheit with a 15-mph wind speed would result in a wind chill of 5 degrees below zero, according to the weather service.

Under the same conditions, the new index would show an 11-degree increase in the wind chill factor: 6 degrees above zero.

"We're trying to ... generate a public education campaign, and we urge our users out there to take this seriously even though the numbers have been warmed up," said Mark Tew, who heads the weather service project as chairman of the Joint Action Group on Temperature Indices.

Tew said he hopes to implement the new wind chill factor in early November.

Forecasters are expected to use the new formula as they did the old one: to issue severe cold warnings for people who plan to be outside in dangerous conditions, Tew said.

Weather service Director Jack Kelly said practical considerations prompted creation of the new formula. "This information will help people make sound decisions about how to dress for the weather," Kelly said in a statement.

"Exposure to cold, biting air for long periods of time is dangerous," Kelly said. "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."

Despite its practicality, one forecaster fears that the public may not immediately embrace the new wind chill formula.

"It sounds like they're factoring in things that actually affect the human body, important factors that are more realistic," said CNN meteorologist Jill Brown. "But everybody hates change, and some people are not going to like it."

Brown also said that people in colder climates will have to adjust themselves mentally to the newly calibrated temperatures.

"You'll have to kind of recalculate your feeling of how cold it actually is out there," she said.

In addition, the new formula might have a temporary chilling effect on weather broadcasters. Brown said it may become tiresome for weathercasters to explain the wind chill change repeatedly all winter long.

"I think for everyone it's going to be a little bit confusing -- not that it's a bad thing," Brown said. "It's probably an improvement, but it's going to take a while to get used to a new standard."