As we slowly move toward the heart of winter, I felt it was a good time to take a quick look at wind chill. When we indicate the temperature outside, we are talking about the actual temperature of the air, but depending on the conditions, this may or may not be a true indication of what it really feels like. For most of us, the apparent temperature gives us a much better snapshot of what the temperature conditions are really like.
When we talk about apparent temperature we are taking into account the water vapour that is in the air, wind speed, and the actual air temperature. In the winter, we call this measurement wind chill. Even taking these measurements into account, there is still a variable that can’t be accurately measured: your body’s ability to maintain its own internal temperature. Every person is different: some people are naturally a little warmer than others, and some people can produce more internal heat than others. If our internal temperature rises or falls too much, we can run into medical difficulties.
If we look at the effect of cold temperatures on the human body, one of the first things our bodies do is contract. This pulls blood away from the extremities of our body, conserving it to help keep our core temperature warm. This leads to a couple of things: first, we run a greater risk of frostbite due to a lack of blood supply, and second, we experience an increase in urine output (now you know why you have to go to the bathroom when you get cold).
These are just the effects of cold on the body, but as almost every Canadian knows, when you add in the wind when it’s cold, everything changes. The explorer Paul Siple first introduced the idea of a wind chill factor in 1939. The wind chill factor indicates the enhanced rate at which the body will lose heat to the air. Our bodies help to keep us warm in the winter by trapping a thin layer of air near the surface of our skin. When it is windy this thin layer is taken away and additional heat from our bodies is released to try and recreate this layer. This process repeats itself over and over; the higher the wind speed and the colder it is, the faster it goes. In addition to this, moisture from our bodies is being evaporated – a process that uses up more heat from our bodies.
A formula was developed to calculate the rate of heat loss, and wind chill readings were added to weather forecasts around 1970. In 2001, both the National Weather Service (U. S.) and the Meteorological Service of Canada revised the wind chill formula, as the old formula tended to overestimate the heat loss.
The things that still can’t be built into the formula for calculating wind chill include a person’s physical activity, the sun’s intensity, and the protective clothing being worn. All of these things can decrease the cooling effect of those cold winter winds.
Looking at the table of wind chill values, we can see that for conditions to become really uncomfortable, we have to wait for temperatures to fall below 18C, no matter how windy it is. Things start to become serious when the temperature reaches around 23C, with wind speeds of around 40 km/h. At these conditions we could expect frostbite to occur within 10 minutes.
Let’s hope that we won’t have to deal with too much wind chill this year.