Mohawk Valley Astronomical Society

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The Polar Vortex

by Perry Pezzolanella

A term emerged last winter that was thought to be new since hardly anyone had heard of it. The polar vortex was thought to be a newly created phrase to describe a huge, frigid, windy arctic air mass that settled deep into the south during early 2014 covering much of the U.S. It is hardly a new term; it is actually quite a regular phenomenon on Earth, and most of the other worlds in the Solar System that have an atmosphere.

The Earth has polar vortices because of its seasons and warm oceans. As one of the poles tips into the long night lasting up to six months, it grows cold fast. A dome of cold air, which is heavy, builds up over the dark pole and eventually flows outward. It butts up against the polar jet stream, which tends to hold the cold dome in place, but storms carrying heat and moisture from the oceans to the south energize the polar jet stream. This causes it to buckle as the contrast between the cold and warmth intensifies and creates a huge polar low pressure at mid and upper levels of the atmosphere drawing down the intense cold from the far north and sending it deep into the south. There are two polar vortices in the north. One is near Baffin Island and the other is in northeast Siberia. They often stay put more or less, but if the polar jet stream gets strong, it will begin to fluctuate causing a risk of dangerous cold heading south into areas that are not accustomed to it. Polar vortices do not necessarily contain huge storms or blizzards and are often sunny and dry, but the strong winds can create ground blizzards and lake effect snowstorms.

All worlds that have atmospheres have polar vortices because they rotate, which causes a temperature difference, which in turn puts the atmosphere in motion. Mercury has no true atmosphere and the few molecules it has do not collide, therefore there is no wind, which means no circulation. Venus has beautiful polar vortices, which are usually double over each pole, although they merge at times into a single vortex. The thick clouds and dense atmosphere readily reveal the paths of the clouds around the vortices, which gives them a true sense of depth. Mars has a huge, strong polar vortex over each pole in spite of its thin atmosphere. Thin clouds are often seen flowing around the vortices and small dust storms occasionally develop from the strong, cold winds blowing off the ice caps.

The giant gas planets, Jupiter, Saturn, Uranus, and Neptune, have deep atmospheres, rapid rotation, and internal heat in addition to the feeble heat from the Sun that together help create huge polar vortices that could easily swallow the Earth. Saturn has a classic polar vortex that appears like a hexagon due to a series of storms swirling around its perimeter. The white clouds that form clearly define the outer edge of the vortex. The center of the vortex is an obvious deep hole in Saturn's thick clouds where the clouds rotate faster than the outer ones. Titan also has a polar vortex that is similar to Mars' but smaller and located over its south pole. The clouds rotate fast compared to Titan's rotation and only formed recently as the southern hemisphere headed into winter, which will last over seven years.

Polar vortices are nothing new and have occurred as long as there have been atmospheres. Triton has a very thin atmosphere, so it should have them, but we haven't detected any. Pluto has an atmosphere thicker than Triton and strong winds were recently discovered near the poles, which mean it too should have polar vortices. Hopefully, unlike Triton, Pluto will demonstrate thin clouds that will define them when New Horizons arrives later this year. When we shiver during an unusually cold and blustery spell during winter and it feels like the world will freeze over solid, we can blame the polar vortex. It is comforting to contemplate that there are other worlds experiencing the same phenomenon.