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What in the world is a polar vortex? On Earth, it’s a large area of low pressure and extremely cold air that usually swirls over the Arctic, with strong counter-clockwise winds that trap the cold around the Pole. But disturbances in the jet stream and the intrusion of warmer mid-latitude air masses can disturb this polar vortex and make it unstable, sending Arctic air south into middle latitudes.

That has been the case in late January 2019 as frigid weather moves across the Midwest and Northern Plains of the United States, as well as interior Canada. Forecasters are predicting that air temperatures in parts of the continental United States will drop to their lowest levels since at least 1994, with the potential to break all-time record lows for January 30 and 31. With clear skies, steady winds, and snow cover on the ground, as many as 90 million Americans could experience temperatures at or below 0 degrees Fahrenheit (-18° Celsius), according to the National Weather Service (NWS).

The Goddard Earth Observing System Model above shows this air temperature movement at 2 meters (around 6.5 feet above the ground) from January 23-29. You can see some portions of the Arctic are close to the freezing point—significantly warmer than usual for the dark of mid-winter—while masses of cooler air plunge toward the interior of North America.

Science Behind the Polar Vortex / Credit: NOAA

Meteorologists predicted that steady northwest winds (10 to 20 miles per hour) were likely to add to the misery, causing dangerous wind chills below -40°F (-40°C) in portions of 12 states. A wind chill of -20°F can cause frostbite in as little as 30 minutes, according to the weather service.

Not sure how cold that is? Check out the low temperatures on January 30, 2019 in some of the coldest places on Earth—and a planetary neighbor:

 -46°F (-43°C) – Chesterfield, Newfoundland

-36°F (-33°C) – Yukon Territory, Canada

-33°F (-27°C) – Fargo, North Dakota (Within the Polar Vortex)

-28°F (-18°C) – Minneapolis, Minnesota (Within the Polar Vortex)

-27°F (-33°C) – Amundsen-Scott South Pole Station, Antarctica

-24°F (-31°C) – Chicago, Illinois (Within the Polar Vortex)

-15°F (5°C) – Barrow, Alaska 

-99°F (-73°C) – Mars

Learn more about the science behind the polar vortex and how NASA is modeling it here: https://go.nasa.gov/2Wtmb43.