Wind chill thawing.

Byline: MIKE STAHLBERG The Register-Guard

IT'S 10 DEGREES Fahrenheit on a crisp winter morning in the Oregon Cascades, so cold the snow squeaks under your cross-country skis. Suddenly, a stiff wind slaps you in the face. It picks up speed quickly, finally steadying at about 25 miles per hour.

And you're cold. Teeth-chattering cold. So cold it feels like it's 29 degrees below zero, even though the little plastic thermometer clipped to your parka still reads 10 degrees.

But that's what a 25-mile-per-hour wind is supposed to feel like on a 10-degree day, like 29 below zero.

At least that's what generations of Americans thought it was supposed to feel like before the government came up with some heart-warming news about frostbite and how wind and cold air affects people.

That cold wind you're skiing in, the government now says, actually feels like 11 degrees below zero, not 29.

That 18 extra degrees of perceived warmth comes to you courtesy of a new "Wind Chill Temperature Index" adopted last month by the National Weather Service. The Meteorological Services of Canada implemented the same change, meaning all of North America should feel 10 to 20 degrees warmer just about any time the wind blows.

The Ice Age of wind chill calculations ended Oct. 31. That's when the National Weather Service dropped a 50-year-old method of calculating the wind chill index. As a result, wind chill temperatures all across the continent soared the very next day.

"The latest advances in science and technology and computer modeling have resulted in a significantly improved wind chill index," said James E. Lee, chief of the Fire and Public Weather Services Branch of the National Weather Service in announcing the new formula.

"The new index will provide a more accurate, understandable and useful formula for calculating the potential danger from the combination of wind and cold temperatures."

Wind chill temperatures have been published by the National Weather Service since 1973. Wind chill information was seen as a public health tool that would help reduce frostbite, hypothermia and other effects of exposure.

The service, on its Web site, defines wind chill temperature "as the temperature that it feels like outside to people and animals."

The agency goes on to explain that "wind chill is based on the rate of heat loss from exposed skin caused by combined effects of wind and cold. As the wind increases, heat is carried away from the body at an accelerated rate, driving down both the skin temperature and, eventually, the internal body temperature.

"Therefore, the wind makes it feel much colder."

In theory, skiers and others who spend a lot of time outdoors in the cold can use wind chill forecasts to help decide how much clothing they need to wear.

In addition, wind chill temperature charts contain information about how long bare skin can be exposed to various wind and temperature combinations before frostbite sets in.

The original "wind chill temperatures" chart was based on the "Siple and Passel Index," named after Antarctic explorers James Siple and Charles Passel, who in 1945 conducted research on "connective heat loss" in the Antarctic.

Their experiments involved finding out how long it took cylinders filled with warm water and suspended from poles stuck in the ice to freeze at various temperatures and wind speeds. Data collected by Siple and Passel was used by other scientists to develop a formula for calculating "wind chill temperatures."

Apparently, however, that formula significantly overstates the effects of wind and cold.

Some modern scientists apparently have long recognized the old system's shortcomings, based on their own personal experiences in conditions the charts said should be producing frozen skin.

Representatives of the U.S. and Canadian meteorological services agreed a couple of years ago to form a "Joint Action Group for Temperature Indices," that would come up with a more accurate system. Representatives of seven government agencies and universities helped develop the new formula.

A news release announcing the change boasts that the new index will "incorporate modern heat transfer theory, i.e., heat loss from the body to its surroundings during cold and breezy windy days."

And it points out the new index is "based on a human face model" - not plastic cylinders full of water.

Indeed, several human volunteers apparently spent considerable time in a chilled wind tunnel at Canada's Defense Civil Institute of Environmental Medicine with wires and sensors hooked up to their bare skin so researchers could study the effects of cold and wind in numbing detail.

"Clinical trials have been conducted and the results of those trials have been used to verify and improve the accuracy of the new formula," the National Weather Service says.

In other words, researchers measured how fast the temperatures of the exposed skin dropped and used that information to fine-tune the new formula.

The agency also announced several changes in its methodology to further humanize future calculations of wind chill temperatures.

Wind speeds used in figuring wind chill temperatures will henceforth be calculated at 5 feet above ground, or about the level of a typical human face. The old system used wind speeds measurements taken at 33 feet above ground, the height of a standard wind gauge.

The new formula assumes a "worst-case scenario" for solar radiation - a clear night sky.

Most forms of outdoor recreation occur during daylight hours, when solar radiation would further temper the effects of wind chill.

Researchers are currently working on adjustments to the calculation model that will allow adjustments for sunny, partly sunny and cloudy conditions.

Those adjustments are expected to be added sometime by the end of 2002.

One result of the new wind chill temperature recommendations is that frostbite clearly should be less of a concern at temperatures above zero degrees Fahrenheit.

Under the previous guidelines, our example of being out in winds of 25 miles per hour at 10 degrees would have fallen into the category: "Frostbite likely. Outdoor activity increasingly dangerous."

Being outside at 15 degrees Fahrenheit in a wind of 15 miles per hour was listed as "frostbite possible."

The new index indicates that winds have to exceed 50 miles per hour before frostbite becomes a concern in 10-degree weather - and then it takes 30 minutes of exposure for frostbite to occur.

At zero degrees Fahrenheit, however, the same frostbite warning kicks in when the wind hits 15 miles per hour.

Copies of the new gentler, warmer wind chill index chart (complete with the new frostbite exposure time guide) and a chart comparing the old and new wind chill temperatures for various wind speeds on a 5-degree day accompany this article.

A calculator that will provide the old and new wind chill factors for any combination of temperatures and wind speed entered is available on the Internet at: www.nws.noaa.gov/om/windchill.

In dressing for cold weather, remember that air warmed by the body and trapped within your clothing provides the best protection against the cold. Therefore, loose-fitting, lightweight clothing in several layers is recommended. Outer garments should be tightly woven, water repellent and fitted with a hood.

One downside of the new system: Your adventures will no longer sound quite as scary.

People returning from outdoor excursions to Alaska, for example, used to be able to honestly talk about encountering wind chill temperatures of minus 60.

Now, the same conditions will be equivalent to a measly 37 below zero, barely cold enough to cause frostbite in 10 minutes.

CAPTION(S):

Motorists often must brave wind and snow while chaining up for a trek over the Cascades. THOMAS BOYD / The Register-Guard Coastal storm watchers in Oregon often endure high winds and chilly temperatures to get their fill of waves crashing onto shore.