A polar vortex is a large pocket of very cold air, typically the coldest air in the Northern Hemisphere, which sits over the polar region during the winter season.
Now the desperately cold weather has found its way to the Midwest and Northern Plains of the United States, as well as interior Canada.Temperatures has now dropped to their lowest levels since at least 1994. 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 map is borrowed from Nasa´s earth observatory and shows air temperatures at 2 meters (around 6.5 feet above the ground) at 09:00 Universal Time (4 a.m. Eastern Standard Time) on January 29, 2019, as represented by the Goddard Earth Observing System Model. GEOS is a global atmospheric model that uses mathematical equations run through a supercomputer to represent physical processes. The animation shows the same model data from January 23-29.
The figures above are not traditional forecasts, but a reanalysis of model input—that is, a representation of atmospheric conditions on those days. Measurements of temperature, moisture, wind speeds and directions, and other conditions are compiled from NASA satellites and other sources, and then added to the model to closely simulate observed reality. Note how 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.
Source: Earth Observatory