Before wrapping up our weather school look at winds and working our way into general atmospheric circulation, I figured I should try to give a quick explanation of what has been driving our recent bout of cold weather. The jet stream — a narrow band of rapidly flowing air in the upper atmosphere, which usually sets up between arctic air to the north and subtropical air to the south — has become convoluted. That is, instead of mostly flowing from west to east, it has developed large loops. One of these loops has been arching northward along the west coast of North America and allowing mild air to work its way northward. East of the northern bulge, the jet stream dives southward, creating a trough of cold air over central and eastern North America. Move a little farther east and we get another northward bulge. This pattern might sound familiar; it is a type of Omega block, which tends to be fairly stable. Hopefully the signs that it is breaking down are true and we will see warmer weather move in soon.
Winds of the world
Now, back to weather school. After the last couple of weather school lessons on wind, we now have a basic understanding of what drives our winds. With this knowledge we are ready to start piecing together the bigger picture of global winds, or what is referred to as general atmospheric circulation.
We now know that on Earth the strong sunshine along the equatorial regions results in warm rising air and a region of low pressure. This is known as a thermally created area of low pressure. At the poles, the low amount of sunshine results in cold heavy air that sinks, and results in an area of high pressure — again, this is known as a thermally created high. Now, if the Earth didn’t rotate, our lesson would be coming to an end. These differences in pressure would result in a global pressure gradient force between the poles and the equator and the winds would simply blow from the poles toward the equator.
Fortunately, our planet does rotate, and this ends up creating a much more complex pattern of global winds. As we learned last week, the rotation of Earth causes winds to be deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. So, on our simplified Earth, the flow of air moving from the North Pole toward the equator would be deflected to the west, eventually resulting in all the winds blowing from east to west (easterly winds).
If you live in the Arctic or in the tropics you would have no problems with this picture, because in these regions the prevailing winds are easterly. Here in our neck of the woods, we have to kind of scratch our heads, because our winds don’t seem to follow this simple Earth scenario — our prevailing winds blow from west to east (westerly winds).
So, what’s up? Obviously, something more is going on with the atmosphere. As most of us know, our Earth isn’t a simple place. This pretty much holds true no matter where you look — and the atmosphere is no exception. Our simple picture of thermally induced low pressure at the equator and high pressure at the poles needs to be added to by a couple more regions of low and high pressure. These two new regions are what are known as dynamically induced areas of high and low pressure.
Dynamic high- and low-pressure regions are created not by intense heating or cooling, but by mechanically forcing air to either rise or sink. The first of these dynamically created regions are the subtropical highs — the desert regions of the Northern Hemisphere. As warm air rises at the equator, this rising air eventually has to start spreading out. As it flows northward it gets mechanically, or dynamically, pushed downward. As it is compressed, this air heats up. Once this air reaches the ground, it spreads out once again with some air returning toward the equator and some heading north toward the Poles. It is this northerly flow of air out of the subtropical high that gives us our westerly winds.
The second dynamically produced region is an area of low pressure known as the subpolar low. This region of general low pressure is located around 60 degrees latitude. Much like the subtropical high, this area of low pressure is formed when the air flowing northward from the subtropical high (the westerly winds) bumps up against the southward-flowing polar air. This forces the air to move upward, creating low pressure.
I have included a side-view image of these four different general areas of high and low pressure to help you visualize just what is going on. In our next lesson we will focus on the subtropical highs and the polar lows, to try and understand just how these combine together to create the weather we experience on a day-to-day basis, but for next issue, we’ll begin our yearly review of thunderstorms.