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Understanding wind chill and the polar vortex

Cold air now has a near-continuous path southward… and we’re right in that path

We are currently experiencing some of the coldest temperatures in our region since the winter of 2014, and, depending on just how cold it gets during the first part of this week, we may have to go all the way back to the winter of 2007 since we’ve been in a similar deep-freeze. February 2007 saw 16 days with overnight lows colder than -30 C, with several locations reporting temperatures falling below -40 C on one or two nights, but I digress. Since it’s cold, I thought I would cover a couple of cold topics that have hit the news over the last week or two. The first topic is wind chill.

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. 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, as 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 lost around 1970 and in 2001 the wind chill formula was revised into what we see and hear about today.

Factors 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 and this is where the problem seems to arise. Some people are arguing that wind chill values are not very good due to these variables. While you could take this argument, I do believe that wind chill values have a place in helping to determine just how cold it feels outside. What I have an issue with is in how the media uses and reports wind chill.

The biggest problem is that they often don’t really understand how wind chill works and they tend to apply wind chill to inanimate objects like your vehicle. It just doesn’t work that way. Objects can only get as cold as the air temperature. If the wind chill indicates that it feels like -45 C but the air temperature is -25 C, then the coldest an object can get is -25 C; this includes people. What the -45 C means is that you will be losing heat from exposed areas at a rate equivalent to an air temperature that is -45 C, but once you hit -25 C the object cannot get any colder. So, your car might cool off quicker, but it won’t drop below what the air temperature is.

Split vortices

The second cold topic I want to revisit is the polar or arctic vortex. Just like last winter, and the winter of 2013-14, this weather phenomenon is making its presence felt once again across much of Eastern and Central Canada, along with the north-central and northeastern U.S.

A polar vortex is a large area of circulation (low pressure) in the upper atmosphere that is centred near both poles and tends to be the strongest in the winter. The counterclockwise flow around this region in the Northern Hemisphere means that the atmosphere is flowing from west to east. The stronger the air flowing around the vortex, the more circular the vortex tends to be. If the flow weakens, the shape of the vortex tends to get distorted and we start to see large ridges and troughs form. In some cases, these troughs and ridges can become elongated enough that the vortex breaks into two separate circulations; this is what has happened this year. It happened because of a sudden stratospheric warming event that occurred over the high Arctic in late December, which saw temperatures high above the Arctic spike by more than 50 C. This temperature spike then propagated downward over the first couple of weeks of January and disrupted the vortex, causing it to split.

One of these two vortices has taken up residence over northern Quebec, allowing cold air to have an almost continuous path to drop southward. The unfortunate part of this setup is that Manitoba is almost right in the centre of the cold path. The big question is, just how long will this setup last? The answer: Hard to say. In previous years this type of setup has lasted for up to two months. The current mid- to long-range forecasts call for it to break down in early February, but it will take until late February for the colder-than-average air to either warm up or get scoured out of western North America. So be prepared for more cold temperatures, as winter is not quite done yet!

About the author

Co-operator contributor

Daniel Bezte is a teacher by profession with a BA (Hon.) in geography, specializing in climatology, from the U of W. He operates a computerized weather station near Birds Hill Park.


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