Weather index tends to exaggerate; service considers new model

By Dennis Cauchon, USA TODAY

February 11, 2000 - Mechanical engineer Maurice Bluestein went outside to shovel snow a few years ago on a bitterly cold day, a -25 degree temperature and a -65 degree wind chill factor.

Current wind chill index


Click here or on image

While standing in the driveway, Bluestein noticed that his exposed skin wasn’t freezing in 15 seconds, as it should have if the wind and the cold were the equivalent of –65 degrees.

Bluestein had discovered what scientists who study weather have known for years: the wind chill factor is wrong.

The wind chill index, used by the National Weather Service since 1973, significantly over-states how cold it feels.

The heat index, which combines temperature and humidity, has similar problems, scientists say. It exaggerates how hot it supposedly feels in the summer.

After more than a decade of criticism from scientists, the National Weather Service says it is evaluating whether to change the formula used to calculate the wind chill index. The common wind chill charts are based on this formula.

The weather service will examine a new wind chill index created by Bluestein, as well as other models created by half a dozen other researchers.

Bluestein admits he’s no weather expert but, as a mechanical engineer, he does know a lot about heat transfer.

"It didn’t take me long to realize it wasn’t as cold as the chart claimed it was," says Bluestein, a professor at Purdue University-Indiana University in Indianapolis.

The problem

Theoretically, the wind chill index is supposed to measure the rate at which the body loses heat when exposed to cold and wind. This index was created as a public health tool to reduce hypothermia, frostbite and other cold-related ailments.

As a practical matter, the wind chill index is supposed to tell people how warmly to dress, a crucial decision for people who spend long periods outdoors such as construction workers or skiers.

But scientists say the wind chill index is at best a rough estimate. It doesn’t take into account big differences between heat loss in the sun and heat loss in the shade, for example.

Weather revisions may warm up cold

The current wind chill estimate when the temperature of air is 10 degrees above zero, and a proposed revision:

Wind speed (mph)

Current wind chill

Proposed wind chill

5

6

10

10

-9

0

15

-18

-8

20

-24

-14

25

-29

-19

30

-33

-23

35

-35

-26

40

-37

-29

Source: Maurice Bluestein and Jack Zecher, Indiana University-Purdue University at Indianapolis

In addition, the current wind chill index is based on research conducted in the Antarctic in the 1940s. That study measured how long it takes cans of water to freeze at different temperatures and wind speeds.

But human skin freezes at a different rate than water. Even different parts of the body – the face versus the hands, for example – freeze at different rates.

Finally, the official wind measurements used today are taken 33 feet above ground, where wind blows much faster than it does on the ground.

The new models vary in what is the best way to estimate the wind chill factor, but they all agree that the current index overestimates the problem.

The modeled developed by Bluestein and his colleague, Jack Zecher, makes use of what they say is a more accurate estimate of "the thermal properties of the skin" and of "modern heat transfer theory."

Get rid of it

"This whole romance with the wind chill factor is just a bunch of hype so the TV weatherman can scare you," says Edwin Kessler, former director of the National Severe Storms Laboratory in Norman, Okla.

He says crying wolf with exaggerated wind chill factors can cause people not take the threat of cold seriously.

Development of the
wind chill index

The term "wind chill" goes back to the Antarctic explorer Paul A. Siple, who coined it a 1939 dissertation, "Adaptation of the Explorer to the Climate of Antarctica."
During the 1940s Siple and Charles F. Passel conducted experiments on the time needed to freeze water in a plastic cylinder that was exposed to the elements. They found that the time depends on how warm the water is at the beginning, the outside temperature and the wind speed.

Consider two days with a wind chill of 20 degrees. One day is breezy and 35 degrees; the other is windless and 20 degrees.

"If you dress the same on these days, you’ll pay the price. At 20 degrees, you can get frostbit but at 35 degrees you can't, no matter how hard the wind blows," Kessler says.

Kessler and others want to abandon the wind chill index in favor of a more scientifically accurate warning system.

"The wind chill index is like asking me how tall I am and my answer is 150 pounds. It doesn’t accurately measure anything," says Dennis Driscoll, a meteorologist at Texas A&M.

But Canada switched to a different system – watts per square meter – in the 1970s and the public still refuses to accept it.

"Weathermen just convert it themselves. The public wants these wind chill equivalents, not what we want to give them," says Bruce Paruk, a Canadian government meteorologist.

Canada is considering going back to the old wind chill index, using the newer models.

The National Weather Service review of the wind chill index will take six months to a year, says Jannie Gibson, a meteorologist at the service.