Information sources and web definitions
A
polar vortex is a persistent, large-scale cyclone located near one or
both of a planet's geographical poles. On Earth, the polar vortices are
located in the middle and upper troposphere and the stratosphere. They
surround the polar highs and lie in the wake of the polar front. ...
http://en.wikipedia.org/wiki/Polar_vortex
From NPR: Washington Post weather editor Jason Samenow
described the polar vortex this way:
"We're
talking about a huge sprawling area of circulating cold air originating
from the North Pole. It's a low-pressure center, and typically during
the winter months it resides up there. At times, some tentacles of it
will slip southward and bring cold air outbreaks into the U.S., but this
year, we're seeing a huge chunk of it, most of it descending into the
U.S."
High-pressure
systems over Alaska and Greenland, Samenow added, are "allowing the jet
stream to dive south over the U.S. and also for this polar vortex to
drop south with the jet stream."
For a more visual image of what's happening, though, we suggest an analogy offered by science writer Andrew Freedman, "This
is air that is circulating the Arctic," Freedman said. "In the last
couple of days, it's sort of become lopsided — sort of like a figure
skater that has extended their arms and then tripped.
"You
know, when a figure skater pulls their arms in, they spin tighter and
tighter and faster and faster. But when they put their arms out, they
are a little bit slower and a little bit more wobbly and more prone to
fall or stop skating at the end of their routine.
"What's
happening now is that a piece of it is down on the other side of the
globe, but a piece of it kind of got lopsided and came down on top of
us."
The next logical question is why the vortex has weakened so much that a big piece of it has spun down over the U.S.
Along
with the effects that those high-pressure systems over Alaska and
Greenland are having, there's the possibility that climate change is
also a factor.
"More
and more Arctic sea ice is melting during summer months. The more ice
that melts, the more the Arctic Ocean warms. The ocean radiates much of
that excess heat back to the atmosphere in winter, which disrupts the
polar vortex. Data taken over the past decade indicate that when a lot
of Arctic sea ice disappears in the summer, the vortex has a tendency to
weaken over the subsequent winter."
From USA TODAY: A blast of brutally cold air - thanks to the polar vortex - is
roaring across the central and eastern USA early this week, sending
temperatures to levels not seen in 20 years.
The
polar vortex is a strong area of low pressure that usually wanders
around the Arctic throughout the winter. Counterclockwise winds around
the vortex occasionally push waves of intensely cold air into the USA.
MORE: Subzero temperatures settle across USA
The
vortex is strongest during the winter, and usually weakens or even
disappears in the summer. The vortex tends to move very slowly or even
stay stationary, and its position determines what part of the USA the
Arctic air will invade.
This week, the core of the vortex has
actually slipped all the way down over the USA. Essentially, the
bitterly cold air that's typically over the Arctic is now over the
central and eastern U.S.
The vortex was allowed to slip this far
south in part due to a strong "blocking" area of high pressure over
eastern Canada and Greenland.
Fortunately, this visit from the
Arctic is expected to be short-lived, as the vortex is forecast to
return to its proper location north of the border. This will allow
temperatures to return to average or even above-average levels by the
end of the week and the weekend.