| Brrrace yourself for a change Weather service seeks to revamp wind-chill formula 06/04/2001 By GUY GUGLIOTTA / The Washington Post
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 Mr. 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 Mr. 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.
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.
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," Mr. 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," Mr. Bluestein said.
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 "Commission 6" to
develop what Mr. Schwerdt, a member of the panel, called "a continuous
spectrum" that would describe comfort levels at all temperatures and
weather conditions.
AccuWeather, the Pennsylvania-based commercial forecaster, has already
done this, and it 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.
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.
So while AccuWeather has opened "indirect contacts" with Commission 6
looking for a single-index solution, Mr. Steinberg said, the Weather
Service-led Action Group is focusing, at least for the moment, on wind
chill, and has its hopes riding on Mr. Osczevski and Mr. Bluestein.
In the mid-1990s, Mr. 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," Mr. Osczevski said. Mr. Bluestein
had reached the same conclusion by modeling the whole head as a
cylinder. Mr. 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 the two men to work together in a wind
tunnel with a compromise device – the windward side of a cylinder – and
put it at face height, 5 feet off the ground.
The pair will test the resulting index by measuring skin temperature and
heat loss in the faces of human volunteers.
Mr. Bluestein acknowledged that the sun can have a significant effect on
wind chill, and he predicted that he and Mr. 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," Mr.
Bluestein said.
"I'm just particularly interested in cold."
Distributed by Los Angeles Times-Washington Post News Service
|
