January 15, 2025
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How the polar vortex could bring Arctic blasts to the US
Large-scale weather patterns can carry blasts of icy air away from the polar regions; here’s how it works
A woman walks on a pedestrian bridge with her face covered to protect herself from the cold on January 31, 2019 in New York.
Johannes Eisele/AFP via Getty Images
Winter is supposed to be cold, and in fact, if it weren’t for global warming, it would be the season Colder than usual in the US in recent years. However, when freezing temperatures and strong winds combine day after day, it can be hard not to wonder. why the conditions must be quite so cold. But if you keep your brain unfrozen enough to be curious, the answer offers interesting insights into the weather systems that rule our planet.
“Thinking about how the middle of the country or the Gulf Coast states get cold air is not just about thinking about what’s happening locally,” says Andrea Lopez Lang, an atmospheric scientist at the University of Wisconsin-Madison. “You really have to zoom out and take a big-picture view.”
This far-sighted approach is focused on the Arctic and driven by two atmospheric phenomena. The first is the polar jet stream, a huge current of air that circles the globe at 50 to 60 degrees north latitude within the low-altitude troposphere, the same layer of the atmosphere where most weather spreads. To the north of the polar jet stream is colder air; to the south of it is warmer air.
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When the polar jet stream is aligned around the Arctic, the continental US and other mid-latitude areas stay relatively warm because it keeps cold air trapped in the north. But as the jet stream meanders, drifting north and south, as it circles the Earth, the teeth-rattling air can envelop it farther south, and local temperatures drop dramatically in what is sometimes called “”arctic blast“.
The polar jet stream exists year-round and can cause winter cold on its own. But in winter, it is joined by a second atmospheric phenomenon called the polar vortex. This vortex of frozen air, located more than 10 miles above the Earth’s surface, strengthens each year during the coldest months of the Northern Hemisphere. And while the play of the polar vortex isn’t involved in all cold air outbreaks, the two atmospheric phenomena can interact in ways that can affect them. especially frozen ones.
Under normal conditions, the polar vortex rotates tightly over the Arctic, and the polar jet that passes below and south of the polar vortex remains relatively smooth. “If the polar vortex is left alone, nothing bothers it, it’s a nice, fast, quiet rotation (and) the cold air is near the center of that rotation,” says Judah Cohen, a climate scientist at Atmospheric and Environmental. Research
But sometimes the polar vortex gets confused—slowing down or drifting or even breaking in two—and that destabilized air mass can cause the polar jet stream to develop particularly large meanders along its path, allowing extremely cold weather to circle further south than usual. This factor was in February 2021, for example, when freezing temperatures in Texas it lasted more than eight days, caused widespread power outages and killed more than 200 people.
Regardless of whether the polar vortex contributes to a specific occurrence of cold air or whether that event is the work of the polar jet stream, these freezing packs sometimes coincide with winter precipitation. “There are a lot of components that go into making a big snowstorm,” says Lopez Lange. One of these factors is cold, of course, but another is the water vapor available to freeze snow.
Curiously, there is also a polar vortex in the Southern Hemisphere, but this system does not experience the disruptions characteristic of the northern vortex. “These events never happen in Antarctica,” says Aditi Sheshadri, an atmospheric scientist at Stanford University.
And in the Southern Hemisphere, with its polar vortex and polar jet stream, the absence of disturbance of these polar vortices may explain what causes the phenomenon in the Northern Hemisphere. The South Pole is located on the continent of Antarctica, surrounded by the Southern Ocean. In contrast, in the north of the Arctic sea ice is surrounded by alternating sea ice and oceans: North America, Atlantic, Eurasia, Pacific. The Earth’s atmosphere acts differently over land and ocean, creating large-scale atmospheric waves in the Northern Hemisphere that, when the circumstances are right, have the potential to break up the polar vortex, causing more outbreaks of extreme cold air.
As it affects everything else on Earth, climate change is contributing to cold weather outbreaks in some clear ways and in some ways that scientists are still working to understand. What is certain, researchers say, is that winters are getting milder and colder extremes in the 20th century.
The models are unclear whether or not climate change is making polar vortex disruptions more common. Cohen notes that cold extremes have become more frequent since 2000, when the rate of Arctic warming picked up steam, and he believes they are linked to the way melting ice shapes the atmosphere. “In this period of Arctic change, extreme winter weather and severe winter weather have been surprisingly resilient,” he says.
