The purple and blue colors in the animation represent the coldest temperatures, which range from minus 40 F to minus 10 F, as measured about two to three miles above Earth's surface. The mass of cold air moves like a chubby finger that pushes aside regions of higher temperature, which are represented in the animation by the colors green, yellow and red.
The polar vortex is a low-pressure mass of cold air that swirls perpetually above Earth's polar regions. There's one in the Northern Hemisphere — the one responsible for the recent cold spell across the U.S. — and another in the Southern Hemisphere.
It's normal for the Arctic polar vortex to expand in winter, bringing colder air to lower latitudes. But sometimes the polar vortex weakens, and the fast-moving current of air known as the jet stream shifts, allowing the vortex to drift southward to cause unusually cold temperatures in those areas.
"That cold air that was stuck over the Arctic gets moved south, into the actual continental United States," David Kanter, an assistant professor of environmental studies at New York University, told NBC News in a video interview.
Paradoxically, extreme cold spells could become more frequent as the Arctic warms along with the rest of the planet.
"Arctic warming leads to a weaker jet stream, which leads to more extreme winters," Kanter said. So the notion that global warming would "rescue" us from cold weather extremes — something President Donald Trump alluded to in a recent tweet — is misguided. As Kanter put it, "We're going to see more extremes as the climate continues to warm."
The Aqua satellite has been orbiting Earth since 2002. Its instruments create a global map of atmospheric phenomena, including temperature and humidity, cloud heights and the concentrations of carbon dioxide and the other greenhouse gases that contribute to global warming.
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