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Why has it been so cold lately?

This spinning pile of cold air at the top of our planet is trying to sag southward

A moose walks across a field near the Manitoba/Saskatchewan border. It's not just humans who are tired of the cold weather, wildlife is likely looking for it to end as well.

One of the questions I have been asked several times over the last couple of weeks is: Just what is causing this outbreak of cold air we have been dealing with across the Prairies? Then, usually, added on to the end of the question is: If global warming is real, then why are we so cold?

The answer has to do with swirling eddies of air that are known as Rossby waves. These are named after the meteorologist Carl Rossby, who in 1938, mathematically described how the boundary between cold arctic air and the milder air to the south interact.

One way to think about Rossby waves is to picture the cold air over the poles or top of our planet as a big glob of goo. If you dropped a pile of goo on top of a ball, it would want to sag down, and the same holds true for the cold air in the polar regions — it wants to sag to the south. Now, things would be pretty straightforward if this was all we had to worry about, but as we all know, this simple picture is missing one important part: our planet is not just a ball simply sitting in place — it is spinning.

This spinning motion takes the atmosphere along with it, so our pile of cold air, or goo, is spinning around. So now we have a spinning pile of cold air at the top of our planet that is trying to sag toward the south. This sagging creates undulations, or waves, along the boundary of this cold air and these waves are what we call Rossby waves. One of the best ways to understand this is with a diagram. If you look at the diagrams I’ve included with this week’s article, you will see the development of a Rossby wave in three steps.

Figure 1. photo: Pearson Prentice Hall Inc.

In Figure 1 (above), we can see there are some slight undulations along the boundary between the cold and warm air. Over time these undulations become larger waves like we see in Figure 2 (below). These waves begin to develop into eddies and become areas of low pressure. In Figure 3 (at bottom), we see a very strong Rossby wave pattern, where one of the eddies of cold air, or areas of low pressure, is starting to break away from the main area of cold air. When this happens, we get what is called a closed upper low, and these can sit over an area and slowly spin down over a fairly long period of time, bringing with them cool, cloudy and generally miserable weather. This is kind of what we have seen so far this spring. A persistent lobe or Rossby wave has been sitting across much of Central and Eastern Canada, and while it wasn’t a cut-off low, it just didn’t move very much.

Figure 2. photo: Pearson Prentice Hall Inc.

At any given time, there will typically be between four and six Rossby waves at different stages of development in the Northern Hemisphere. These waves typically take a week or two to move across our region, but as I pointed out, they can sometimes get stuck or blocked. When this happens, we tend to see a long period of consistently cold or warm weather.

As for global warming or climate change, we always have to remember that you have to look at the planet as a whole, not just one region. The question is, how many regions are seeing colder-than-average conditions and how many are seeing warmer than average? Currently, the warmer-than-average areas are winning out over the colder-than-average ones.

Figure 3. photo: Pearson Prentice Hall Inc.

About the author

Co-operator contributor

Daniel Bezte

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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