Air temperature is an important factor in how hot or cold you feel, but it’s not the only one. A few months ago, I posted a piece about how to calculate humidex, but given that it’s winter here in Ottawa, it’s highly unlikely that you’re going to have to worry about high humidex days for a few months. Wind chill, on the other hand, is something that you are almost certainly going to have to contend with in the next few months.

So, what exactly is wind chill and how is it calculated?

Wind chill is the perceived air temperature on exposed skin due to wind. The faster the wind, the higher the rate of heat loss from your skin and the colder it feels even though the actual temperature measured on a thermometer remains the same. Wind chill is only defined for temperatures at or below 10 °C and wind speeds above 4.8 kilometres per hour. The model currently used by Environment Canada is represented by this formula:

[math]T_{wc} = 13.121 + 0.6215T_{a} – 11.37V^{+0.16} + 0.3965T_{a}V^{+0.16}[/math]

Where:

• [math]T_{wc}[/math] is the Wind Chill Index based on degrees Celsius
• [math]T_{a}[/math] is the actual outside air temperature in °C
• [math]V[/math] is the wind speed 10m above the ground in km/h. (10m is the standard height for an anemometer.)

So, when it’s -5°C out with a 5 km/h breeze, it feels like it’s -7.2°C. But, when it’s -5°C out and there’s a 40 km/h breeze, it feels like it’s -14°C. Yesterday at lunch it was -17°C with a 28 km/h wind, so it felt like it was -28°C on a calm day. (If you want to play what-if, check out Environment Canada’s Wind Chill Calculator page.)

One of the biggest dangers a high wind chill index poses is an increased chance of frostbite for skin that isn’t covered up, particularly when wind chill values go below -27. I had a touch of frostbite on one of my cheeks several years ago and even today that little patch is still a bit more sensitive to the cold. Some people who have had more severe cases of frost bite have ended up losing toes and fingers as a result.

So, dress warmly, stay dry and stay out of the wind as much as possible and you’ll survive the cold weather.