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| Courtesy of ABC News |
It’s
that time again. What time is that you may ask? The time of the year when extremely
cold air that comes into our region is labeled a Polar Vortex. Around this time of year, the media can
sometimes go into a frenzy, repeatedly using this term. However, many times the
word is not used correctly. So what exactly is a “Polar Vortex”?
First off, the Polar Vortex is a low-pressure system that continuously circulates
year round in the Arctic and Antarctic regions. It is worth noting that the
Polar Vortex in-fact exists during the summer months; it’s just not as strong
as it is in the winter. It extends from the middle troposphere (0-11km) into
the stratosphere (12-50km) and rotates counter-clockwise in the Northern
Hemisphere and clockwise in the Southern Hemisphere. The reason why you hear
about it in the winter is because the center of low-pressure
circulation strengthens in the wintertime. Occasionally a portion of the Vortex
will migrate southward, which was the situation last week. In that scenario,
the Polar Vortex weakened allowing a portion to migrate from the north into
regions further south and gets absorbed into the jet
stream. When this happens, portions of the United States experience dramatic episodes
of cold air outbreaks. A further extreme situation is when the circulation of
the Polar Vortex starts to collapse. As it
collapses large portions of cold air break free and affect our region. This is
what occurs when we have historic/record breaking Arctic air outbreaks, like we
had last January. High winds can often accompany a
Polar Vortex.
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| Courtesy of NASA |
A large part of forecasting these Arctic air outbreaks is looking at the
positioning of the jet stream. In the wintertime, the polar jet will dip
further south than normal. The strengthening of the
jet stream brings frigid air into a region of traditionally warmer air. This is
a significant factor in the deepening and strengthening of low-pressure
systems. These downward dips in the jet stream accompanied with polar Arctic air
are important ingredients in fueling winter storms. During this event, the
jet stream acts as a boundary between the cold, polar air masses in the north
and the southern tropical air masses. The warmer air mass to the south is
moist; and when it forms a front along a cold, dry air mass, strong winter
storms are possible. The warm, moist air mass rises
over the cold front allowing the moist air to cool. This is a key ingredient of
the precipitation process. When precipitation begins to fall, the colder air
mass at the surface provides the cold, frigid temperatures needed for frozen
precipitation to develop in the form of snow and frozen rain.
Hopefully, this article was able to
clear up some misconceptions you may have had about this unique, but often
misunderstood weather pattern. Every cold snap is not a product of a Polar
Vortex. It is very possible for our region to experience a cold blast without
it being accompanied by a Polar Vortex. It’s often that we are just under the
influence of a polar Arctic air mass. Finally, when you do hear a forecast with
the Polar Vortex, remember there can be varying extremes of air circulation. A
weak vortex is much different than a collapsed Polar Vortex where a cold air
mass is very large and temperatures significantly drop. Don’t always assume the
worst, but it is still prudent to be prepared for extreme cold weather during
the winter months!
-Katie
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