Just How Cold Is Cold?

Feb. 8, 2007 — -- In 1939 a man named Paul Siple wrote a Ph.D. thesis which, for the first time in history, included the term "wind chill" as a scientific concept.

Siple knew all about chill winds. He went to Antarctica on six expeditions, twice with the legendary explorer Richard Byrd. He devised a formula to suggest how much time it takes for exposed skin to become frostbitten; meteorologists have included it in winter forecasts since the 1970s.

But nobody's been happy with it. Ski-run operators complained it scared people away. Scientists complained that it wasn't really ... well, scientific.

So in 2001, teams at Defence Canada and the National Weather Service in the United States decided it was time to rethink wind chill.

Wired for Tests

In a refrigerated wind tunnel at the Defence and Civil Institute of Environmental Medicine near Toronto, 12 volunteers were wired up with temperature sensors, and made to walk on treadmills for 90 minutes -- or until their faces began to show the first sign of frostbite.

"We fixed a number of sensors on their face," said Dr. Michel Ducharme, who oversaw the research. "We were measuring skin temperature at five different locations, and we were looking also at heat loss at different locations on the face."

The volunteers were not exactly comfortable. One sensor was inside their mouths. They also had rectal thermometers to measure core body temperature.

During one trial, they were splashed in the face with water every 15 seconds. Cold water.

Bare Skin and Thermodynamics

The problem with wind chill, dating back to Siple's work in Antarctica, was that it was more of a concept than an actual measurement. Siple's original research was done with a plastic container filled with water and hung from a pole in the Antarctic chill.

For scientists who study thermodynamics, there is something real going on. When something cold (an icy Arctic blast, for instance) comes in contact with something warmer (a street lamp, say, wavering unsteadily in the wind), the warm thing transfers some of its heat to the cold thing.

In human biology, there's an issue too. The body has ways of protecting itself; blood vessels constrict so that less heat will be lost. This mechanism is less effective in the face than the rest of the body, and works less well when you're exerting yourself and your body needs extra blood flow.

So how to turn this into a number we call wind chill?

"It's a feeling, OK?" said Ducharme. "It's providing a temperature that you will feel like if there's no wind."

Ducharme and his team thawed out their volunteers, went through the data and came up with a long formula to calculate wind chill.

The actual formula looks something like this: T(wc) = 35.74 + 0.6215T - 35.75(V0.16) + 0.4275T(V0.16), where T(wc) stands for the wind chill in degrees Fahrenheit, V is the wind speed in miles an hour, and T is the actual temperature.

What does it mean to you? Say the temperature is zero degrees Fahrenheit and the wind is howling at 20 mph. That will feel the same as if the temperature around you were -22, but the wind was still.

"It's a guide, actually," said Ducharme. "We should not be scared by that number, because that number means nothing if you're perfectly dressed and you don't have any bare skin exposed to the environment."

In other words, after years of debate and experiment, the bottom line is that your mother was right. Wear your hat.