If you’ve been paying any attention to the news today (1/6/14), you might be forgiven for believing this warning: “Look out everybody!!! The Polar Vortex is coming!!!“. But relax – the Polar Vortex has been here since well before you were born!

Let’s examine the Polar Vortex!

Suppose you are on a spacecraft arriving at Earth, and you decide to park right over the north pole and look down. Imagine also that you can actually see the winds circulating around the atmosphere and around storms within the atmosphere. Depending on the altitude you look at, you’d see one of two things:

either: if you look at the air flow in the layer we call the stratosphere (see more below), you’d see a vortex of air circulating around the pole. Seen from above, the vortex would swirl counterclockwise on most days. Wind speeds could be as high as about 200 mph in the core of the flow! This is the phenomenon meteorologists call the Polar Vortex.

• The flow is usually not be in a straight line from west-to-east: rather, it would have one to three large “bends”, where the flow heads towards the northeast or southeast before flowing due east again.
• Meteorologists have determined that these undulations are caused by the Earth’s underlying terrain, especially by the big mountain ranges such as the Rockies. The flow undulations are also caused by the temperature gradients between the great oceans and continental land areas. The oceans are warm in winter relative to the land, and cold in summer relative to the land, and these temperature differences cause deflections in the air flow!
• The Polar Vortex has been studied and observed for some time now. It extends up to an altitude of around 50 miles above the land. In winter, the flow is counterclockwise (when viewed from above the pole), but in summer it reverses! Occasionally in winter it also reverses quite suddenly during an event known as a Stratospheric Sudden Warming.
• The stratosphere is the layer of the atmosphere from about 6-30 miles about the surface. At cruising altitude on a commercial airplane flight, you are flying in the stratosphere! It’s also the layer that contains the Ozone layer and the Ozone hole.

or: if you look at the air flow in the lowest layer of the atmosphere – the troposphere – you would again see a vortex of air circulating around the pole. This flow is not quite the same as the Polar Vortex, although the two are linked. We mostly refer to this as the Jet Stream.

• The Jet Stream flow is always west to east, whereas the Polar Vortex changes direction from winter to summer.
• Just as with the Polar Vortex, the flow has northward and southward meanders in it, large and small. The northward meanders are called ridges, and they bring calm, settled weather to regions under the ridge. The southward meanders are called troughs, and they are often associated with wet, windy, cloudy, stormy weather.
• The troughs and ridges generally move from west to east, but sometimes they get “stuck”. When this happens, our weather pattern also  gets stuck. For those of us in California, we have been stuck under a ridge for weeks (months??), and are experiencing an unprecedented dry fall and year as a result.
• In addition to sometimes locking into place and not moving, the troughs and ridges can sometimes be very strong (“high amplitude”). This causes excessive northward deflection of air moving “over” a ridge. When this happens over the north Pacific, for example, warm air can be diverted into Alaska, resulting in warmer-than-normal conditions for them. On the other hand, when there is a particularly deep trough there will be an excessive southward deflection of air moving “under” the trough. When this happens over the Midwest, as is the case right now, cold Arctic air can be diverted way south towards the Gulf Coast states.

The image below (taken from our earlier post)  shows an example of the Jet Stream flow in the troposphere. This was on Saturday 4 January 2014, and shows the winds in the upper troposphere. The view is from above the pole, which is in the center of the picture (with the USA below it in the picture). The red and purple shading shows where the strongest winds are, and you can follow around the hemisphere to see where the Jet Stream is located. Note the very strong winds over the Atlantic and the Pacific! The very cold outbreak east of the Rockies is associated with the troughs indicated there.

So in summary, to most meteorologists, the Polar Vortex refers to air flow up in the stratosphere and above. It’s been there for as long as our atmosphere has been behaving the way it does today – so certainly for tens of thousands of years! It absolutely does behave somewhat differently from year to year, and has an occasional “breakdown”! But it’s not something new that just developed over the past few weeks…so relax!!!

Alison Bridger, 6 Jan 2014