Certain things in winter are expected. It’s going to be cold, so we need to make sure we have our hats, gloves and boots at the ready. And some days will probably be so cold that it would be easier — and safer — to just stay indoors.
When the latter happens, the polar vortex often takes the blame. But what is the polar vortex, and why does it make weather headlines from time to time?
Put simply, the polar vortex is a large area of low pressure and cold air that can be found at both the North Pole and the South Pole. That means there are actually two polar vortexes, or vortices. There is one at the North Pole and one at the South Pole. The one we hear about here in Illinois is the Arctic polar vortex, which is found at the North Pole.
The air in the polar vortex sits between 10 miles and 30 miles above Earth's surface. The air is always moving, in a counterclockwise direction. Most of the time, that cold air stays up above the North Pole. But from time to time, conditions allow the cold air to get pushed south. This happens because of the polar jet stream, a band of strong wind in the upper atmosphere.
Words to know
Counterclockwise: In the opposite direction of the way the hands on a clock move.
Phenomenon: A rare or unusual fact or event.
Stable: Not likely to give way or overturn.
Vortex: A mass of whirling air or fluid.
The polar jet stream is high above Earth’s surface, but not as high as the polar vortex. The winds in the jet stream are about 5 miles to 9 miles above Earth's surface, compared to 10 miles to 30 miles for the polar vortex.
Normally, the polar vortex remains stable, keeping the air in place. When the polar vortex is unstable, the jet stream isn't strong enough to keep the polar vortex in place. This is when we start hearing about the polar vortex here in the United States.
When the polar jet stream is weakened by instability in the polar vortex, that cold, Arctic air that normally remains above the North Pole pushes south. Sometimes it reaches as far south as northern Illinois and even points beyond. During these periods, areas affected by the polar vortex experience much colder temperatures than normal for the time of year.
The United States is not the only part of the world that sometimes experiences the unusually cold air associated with the polar vortex. It can also affect parts of Europe and Asia.
It might seem like the polar vortex is a recent phenomenon because it has been popping up on the news and on social media pretty regularly for the past several years, but in reality it's nothing new. Even the term "polar vortex" isn’t new. Its first known use was in 1853. The term has grown in popularity more recently, however, bringing new attention to a weather phenomenon that's been happening for a long time.
When frigid temperatures are forecast, preparation is key. In the Chicago area, the National Weather Service issues a wind child advisory when predicted wind chill values are -25 degrees or colder. A wind chill warning is issued when predicted wind chill values are -30 degrees or colder.
The wind chill is a temperature index that takes air temperature and wind speed into consideration to describe what the temperature feels like. This is why it is also sometimes called the feels-like or real-feel temperature. When the air temperature is 10 degrees with a wind speed of 5 mph, the wind chill is 1 degree. When the air temperature is 10 degrees with a wind speed of 20 mph, the wind chill is -9 degrees. When the air temperature is 0 degrees with a wind speed of 20 mph, the wind chill is -22.
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