National Post

Dec. 28, 2000

Weathermen love the wind chill factor -- it adds scare value to their broadcasts. Minus 10 degrees doesn't sound so bad. Add 40 km/h wind gusts, though, and suddenly it becomes "minus 28 with the wind chill."

But it turns out the wind chill formulae scientists have been using are wrong -- by as much as six degrees. "We're probably scaring people a little bit," Joe Shaykewich of Environment Canada disclosed earlier this month. "It's slightly overstated. So we'll have to revamp that."

I have a more radical idea: Dump the wind chill factor altogether. For the most part, all it does is confuse us.

The theory behind wind chill goes like this: When our bodies are exposed to the cold, the amount of heat that's lost is proportional to the difference between the air temperature and our skin temperature multiplied by something called a "convection heat-transfer coefficient." In still air, the coefficient is low -- in the range of 25. So if the outside temperature is -10° and we assume a person's surface temperature is 37° (actually, it's slightly lower but never mind), then the temperature difference will be 47°, and the heat loss for a square metre of exposed flesh (about the amount exposed by, say, an extremely large topless football fan sitting in an outdoor stadium) will be 1,175 watts -- about one-and-a-half horsepower.

But when the wind starts to blow, the heat transfer coefficient goes up -- because the layer of warm, slow-moving air protecting one's skin becomes thinner. When the wind speed hits 40 km/h, the heat transfer coefficient rises from about 25 to almost 35. At that level, heat loss is 1,625 watts -- almost two horsepower.

But no one watching television wants to hear about "watts per square metre" (although that is one way Environment Canada, to its credit, reports wind chill data). So networks dumb things down by presenting wind chill as if it were a temperature. They do this by using the heat transfer coefficient for still air (about 25) and then finding an imaginary outdoor temperature that yields the correct level of heat loss. In this example, for instance, the "wind chill" is about -28°, since a skin-air temperature difference of 65° (the difference between 37° and -28°), multiplied by 25, yields 1,625 watts.

That's fine in theory. But no matter how often weathermen remind us wind chill is not a measure of actual air temperature, that's how many people take it. And it's hard to blame them. Although the experts call it a "factor," it's quoted in Celsius degrees, the language of thermometers. Millions of Canadians suffer under the delusion that wind makes the air -- not just their bodies -- colder.

"But even if people are misinformed, where's the harm in it?" you may ask. Good question, and one that leads me, in these final paragraphs, to my middle-aged mother, whose understanding of technical issues I take as a crude gauge of popular comprehension.

Before she bought her current vehicle -- a rugged SUV that would start if it were entombed in polar ice -- my mother would trudge outside nightly to plug in the block heater on her station wagon.

"Why bother?" I would ask. "It's only going down to minus 10 tonight."

"Because they said on TV it'll be minus 28 with the wind chill."

"But mom," I would protest, "that doesn't mean your engine block will cool below minus 10. It just means warm objects lose heat at an accelerated rate."

"Minus 28." she would say. "Minus 28 degrees!"

"But ..."

"They said it on TV!"