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