Last time in weather school we started to examine why our particular part of the world has such changeable weather. We looked at general global circulation patterns and then examined the zone where westerly winds bump up against easterly moving polar winds, creating giant eddies of swirling air.
This week we are going to continue our look at these swirling eddies of air and tie in something that is known as Rossby waves. Rossby waves are named after the meteorologist Carl Rossby who, in 1938, mathematically described how the boundary between cold arctic air and 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 it is these waves that we call Rossby waves.
One of the best ways to understand this is with a diagram. If you look at the diagram I have included with this week’s article, you will see the development of a Rossby wave in three steps.
In the first image 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 the second image. These waves are beginning to develop into eddies, and are becoming areas of low pressure. In the final image 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 – but more on that in a future article.
At any given time, there will typically be between four and six Rossby waves at different stages of development in the Northern Hemisphere. In our next lesson we will continue to focus in on what causes our weather because, believe it our not, it is much more complicated than simply watching out for Rossby waves.