Works in Progress

How do migrating birds know where to go?

By Karen Wright|Wednesday, January 01, 2003
Windchill used to be a little like the weather itself. Everybody complained about it, but nobody did anything. Adopted by the U.S. National Weather Service in 1973, the wind-chill temperature was supposed to reflect the cooling effect of strong winds on cold days. But somehow it never seemed to reflect reality.

Environmental physicist Randall Osczevski of Defense R&D Canada questioned the index after finding that windchills in Toronto rivaled the still-air temperatures he'd experienced doing field research in the Arctic. "It didn't feel like -40 degrees to me, and I'd lived at a place where it became -40 without the wind," he recalls. Biomedical engineer Maurice Bluestein of Indiana University-Purdue University became a doubter when wind-chill estimates reached 65 degrees below zero in Indianapolis. Bluestein started shoveling snow and was soon shucking clothes to keep from overheating. "But they'd said, 'Don't go out, or the wind will freeze the skin on your face,'" he says.

Other experts complained about the index, too, but Osczevski and Bluestein did something about it. The two researchers have formulated a new index, recently adopted by the U.S. and Canadian weather services, that will warm up windchills by as much as 20 degrees Fahrenheit.

The wind-chill temperature is, in theory, the temperature in still air that would cause the same rate of heat loss from exposed skin as that brought about by prevailing winds. Under the deposed wind-chill index, for example, a 10-mile-per-hour wind on a 20-degree day has the same chilling effect as a calm 3-degree day. Hence the wind-chill temperature is 3 degrees.

That old formula derives from measurements made in the 1940s. To determine rates of heat loss, scientists sealed a thermometer in a plastic bottle of water and timed how long it took the water to freeze under a variety of wind speeds and air temperatures. The freezing times were later converted into a chart of temperature equivalents that was challenged from the start. One complaint was that the thermal properties of a plastic bottle are unlikely to resemble those of living flesh. But little was known about heat transfer in human skin—until Osczevski came along. In the early 1990s, he'd taken to sitting with his head in a refrigerated chamber until his skin temperature came close to the freezing point. Any attempt to revamp windchill, he argued, should start with the face, which is the body part most often exposed and therefore most vulnerable to frostbite.

In 2000 Osczevski and Bluestein won a dual commission from Canada and the United States to revise wind-chill calculations. They created a mathematical model of heat transfer in the face and supervised wind-tunnel tests on human subjects. A dozen volunteers braved icy winds with thermal sensors on their cheeks and rectal probes monitoring their core temperatures. Their sacrifice yielded a kinder, gentler windchill. A 20-degree day with a 10-mph wind, for example, now has a wind-chill rating of 9 degrees instead of 3.

"At lower wind speeds, there's not a big difference between the old and the new index, but at higher wind speeds, it's actually quite a bit warmer," says Mark Tew, the senior meteorologist at the National Weather Service in charge of the new chart.

Now Osczevski and Bluestein are part of an international effort to develop an all-weather, whole-body heat-transfer formula called the universal thermal index. The index would take into account factors such as sunlight, humidity, clothing, and body fat that affect the feel of the weather. The current wind-chill chart, in contrast, assumes certain extreme conditions: It's calibrated for a person with chubby cheeks walking against the wind in an open field at night. "It's sort of a worst-case scenario," says Osczevski.

Osczevski is also devising ways to measure marine windchill. Back at the wind tunnel, his colleagues mimic ocean spray by spritzing some hardy volunteers with cold water every 15 seconds.
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