If you think you understand wind chill, think again

As a kid growing up in the central Alps of Switzerland I knew three winter temperature conditions: not too bad, cold, and bloody cold. When I experienced my first 40 below in Fort Nelson, where Fahrenheit meets Celsius, I convinced myself that once the temperature hits 40 below it doesn’t make any difference anymore. I abandoned that assumption long before I experienced my all-time low, -54 C in Dawson City. All these experiences were before the invention of the wind chill factor.

We have standard definitions and measurements for temperature and wind speed; why not leave it at that in our weather reports? I was curious to find out how, using the two measurements of wind and temperature, a wind chill factor is determined.

The explanation is quite simple: the wind chill factor is the rate of heat transfer a person experiences when walking bare-faced in an open field, facing a wind speed of 1.4 metres per second at face height. Anyone who finds this explanation to be too vague may apply the standard wind chill formula used by Environment Canada: Twc = 13.12 + 0.6215Ta – 11.37v+0.16 + 0.3965Tav+0.16. Does that help?

Temperature reports given in wind chill measurements are problematic. The face height used in the formula is not specified. Do people who are more than six feet or less than 5 feet tall experience different wind chills?

What adjustments should people make when they are walking on a downtown sidewalk, not in an open field? What if the wind is not blowing at a steady rate into your face, but swirling and gusting? The most useful piece of information I found in my wind chill research was that “wind chill has no universally agreed upon standard definition or measurement.”

That explanation inevitably leads to the question why. Why report wind chills when the official formula has meaning only for a person who is 1.78m tall and who is walking into the wind in an open field? Would a weather report announcing a temperature of -15 C with strong, occasionally gusting north-northwest winds leave people puzzling, unsure as to what to wear? Light jacket or parka? Sandals or lined boots?

I assumed that people in Australia are spared such problems; then I discovered that Australia too has invented a temperature indicator unique to its climate. Australia has developed an apparent temperature (AT) index. AT is a mathematical model using temperature and humidity. The official formula is AT = Ta + 0.33e – 0.7v – 4.00. The measurement reveals the amount of discomfort an adult person experiences when walking outdoors, in the shade.

As a kid I had to learn to anticipate comfort (or discomfort) based on degrees Celsius and kilometer per hour wind speeds. On landing in Canada I had to adjust my preparations for what I would experience on stepping outside to degrees Fahrenheit and miles per hour. By the time I felt comfortable with Fahrenheit and miles reports I had to relearn Celsius and kilometers.

When I reached the age of retirement, instead of life becoming easier and simpler, along came wind chill. I give up. Time for me to revert to my original winter temperature assessments: it is not too bad, cold, or bloody cold. Remembering my Yukon days, early February days in Terrace were not too bad; still a long away from bloody cold.

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