Polar Vortex demystified: U.S. bitter cold, explained
Wednesday, December 27, 2017, 12:43 - With bitterly cold air on tap for much of the United States, you may hear a weather term cropping up that has received a lot of publicity in recent years – the Polar Vortex.
It may sound like something out of science fiction, but don’t panic. The polar vortex is a real feature of our atmosphere, and understanding what it is – and more importantly what it isn’t – will help you understand your forecast better, and prepare for winter’s coldest days.
The first thing to know is that the Polar Vortex is nothing new. It is a fundamental and persistent feature of our atmosphere, a part of what meteorologists call the “global circulation”. This circulation is the planetary heat pump that moves extra energy from the tropics towards the poles, and keeps the earth’s temperatures in balance.
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Editor's note: This article originally ran in December 2016, but has been revived to help you cope.
The global circulation is responsible for much of our weather and climate, and it explains a lot of basic meteorology questions including, “why is the Sahara Desert so dry,” and “why do weather systems move from west to east in North America?” The polar vortex is just one part of this larger global circulation system, and it can play a big role in some of our coldest winter weather.
In specific terms, the polar vortex is a large area of low pressure and cold air surrounding the earth’s poles (both of them – the south pole has one too). It is always there, but it weakens in the summer and strengthens in the winter. The “vortex” part refers to the counter-clockwise circulation of air around this large area of low pressure.
The polar vortex is an upper level feature, which means that here at the surface we don’t experience it directly – only its effects. When forecasters analyze the polar vortex, we are looking at parameters high in the atmosphere, several kilometers above the surface. Forecasters can recognize particular patterns based on the polar vortex’s strength and position, which can give us important information about weather conditions days or even weeks ahead.
When the weather pattern is quiet, the polar vortex keeps the coldest air bottled up near the pole, leading to widespread seasonal or even mild winter temperatures. But many times during a typical winter, an amplified weather pattern will develop that causes a piece of the polar vortex to migrate southward. This can happen anywhere in the hemisphere, but when a part of the polar vortex moves into North America, it can set the stage for an outbreak of very cold air across the continent.
This week's event is an excellent example of how the position of the polar vortex can help set up a cold air outbreak. A conveyor belt delivers bitterly cold air, which surges south across the continent, preceded by a powerful arctic front. This can lead to temperatures more than 30°F below seasonal, and strong winds that drive wind chill values even lower. As long as this piece of the polar vortex remains in place, the door remains open for reinforcing shots of arctic air to flow in from the north. A long-duration polar vortex event can lead to some of our most severe and prolonged cold air episodes.
Although these cold air outbreaks are impactful, they are not uncommon. To varying degrees they happen every winter across North America, Europe, and Asia. Though some of these events are particularly memorable, such as January 2014, they represent a normal and predictable part of typical winter weather.
In summary, the polar vortex is a normal part of the global atmospheric circulation. It is an upper level feature that is an expected part of winter weather, and it contributes to some of our coldest periods across North America and elsewhere. It is not a new development, despite the recent publicity, and it is not something we experience directly at the surface – only its effects.
Watch below: An Arctic air mass will produce wind chills of -20 degrees or colder across the northern Plains over the next few days. This Arctic air will move down the Plains and into the Ohio and Tennessee Valleys into the mid-Atlantic and Northeast by the end of the week
So, the next time you hear the term “polar vortex” don’t be alarmed. Usually it simply means that a period of very cold weather is on the way, and that you should begin to make typical preparations and take typical precautions. These might include refilling emergency supplies, checking in on neighbors and relatives, bringing pets indoors, and being ready to limit your time outdoors once the cold air arrives.
(with information from the National Weather Service)