What is a polar vortex, and why is it here?
A mass of frigid air brought on by a "polar vortex" invaded the U.S. on Monday, bringing bitter cold to the Midwest and eastern United States. Read more weather coverage.
The polar vortex that is chilling us this week is a band of low pressure Arctic air normally centered around the North Pole.
It is usually held in place by the jet stream, a river of wind 15,000 to 20,000 feet above the Earth that divides cold air from warm air, bending around high- and low-pressure weather systems.
This week, a high-pressure system nosed in from western Canada and pushed the jet stream, and a portion of the polar vortex, much farther south than normal.
That brought the center of the vortex well into North America and caused temperatures in the Midwest and eastern United States to dive below zero.
How the polar vortex affects weather around the nation
By 7 a.m. Tuesday, temperatures should be at their lowest point — single digits in D.C. The average low for Jan. 7 in D.C. is 28.7 degrees. Temperatures should return to normal by the weekend.
Lowest temperature ending ...
Tuesday, 7 a.m.
Wednesday, 7 a.m.
Thursday, 7 a.m.
SOURCES: Jason Samenow of Capital Weather Gang, meteorologist Brian Lasorsa of the National Weather Service, NOAA. GRAPHIC: By Bonnie Berkowitz, Richard Johnson, Katie Park and Gene Thorp - The Washington Post. Published Jan. 6, 2014.