For the first time in 56 years, the US National Weather Service is about to revise how it measures the wind chill factor.
The wind chill index measures how wind speed affects outdoor temperatures as felt by the human body.
The old system - used by the United States and Canada since 1945 - measures wind speed at 33 feet above ground. 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 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 F. with a 15 m.p.h. wind speed would result in a wind chill of 5 degrees below zero.
Under the same conditions, the new index would show an 11 degree increase in the wind chill factor to 6 degrees above zero.
"We urge our users out there to take this seriously, even though the numbers have been warmed up," says Mark Tew, who heads the Weather Service project as chairman of the Joint Action Group on Temperature Indices.
Mr. Tew 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," Mr. Kelly said in a statement.
"Exposure to cold, biting air for long periods of time is dangerous," Kelly added. "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."