The Polar Vortex: Good riddance!

I was in Chicago earlier this week, and it was freezing cold once again. My friends in Illinois have had a miserable winter and it wasn't letting go easily. As I started to look at why it was so cold there mid-April, I discovered that I had a post that I started in the winter and didn't finish. It was about the "polar vortex," and I realized that I don't know very much about this thing. So, belatedly, here's my introduction to myself about the polar vortex. For more details and the references from which I took this material, see Skepticalscience, an excellent resource on climate, and the weather.com post here.
     We live in a portion of the atmosphere called the troposphere, and most of us have heard of the stratosphere, the layer of the atmosphere above the troposphere. The boundary between the two is the tropopause, and it's altitude varies considerably with the seasons. We in North America also live in the mid-latitudes, a region of mild temperatures that extends very very roughly between 30 and 60 degrees latitude. North of this (in the northern hemisphere, the reverse in the southern hemisphere) is the very cold polar air. The boundary between the two is the Polar Front, a collision zone between the warm moist and cold dry air.

Typical polar vortex position on the edge of the
polar high (not shown). Graphic from weather.com.

     The collision zone between the two big air masses results in very high winds at high altitudes at the top of the troposphere. In the northern hemisphere, this is referred to as the Polar jet stream. It is strongest in the winter when the temperature contrast is the greatest between the polar air (because there is no sunlight) and the temperate air of the mid-latitudes (which may seem cold, but is nothing like the Polar air!)

Distorted polar vortex (from same weather.com site as above.

      The polar regions are areas of high atmospheric pressure covering the north and south poles. On the borders of the polar highs are polar vortices (sometimes called polar cyclones or polar lows). These are permanent areas of cold, low pressure in the upper atmosphere that draw their energy from the temperature difference between the cold polar air and the warmer air of the midlatitudes. They are, therefore, strongest in the winter.    There are typically two polar vortices in the northern hemisphere--one near Baffin Island and the other over northeast Siberia.The air in them spins counter-clockwise. (Only one of these is shown in the graphics here.)
    The polar vortex is contained by our jet stream (typically at around 35,000 feet altitude where airplanes fly).  The jet stream is normally rather loopy, an instability known as Rossby waves, that arises because the Coriolis effect has a different magnitude at different latitudes. When one of loopy parts of the jet stream tokes an unusually deep plunge southward into the midwest, it brings air from the polar vortex and freezing temperatures southward. Note in both of the graphics that the main location of the low pressure zone stayed up north over Baffin Island, it's permanent home.
     The cold air aloft in the polar vortex sinks to the ground, displacing the lighter warmer (winter) air normally there. Rossby waves migrate, typically to the east, and so the disturbance moved on out of the midwest. Things  warm up, and Chicago goes back to balmy 30+ degree nights in the winter!