Much of the United States is currently in the grip of a polar vortex, a system of dense air producing plummeting temperatures and swathes of freezing weather, bringing some parts of the country to a halt and causing chaos in others.
A polar vortex is a circulating pattern of strong winds flowing around a low-pressure system, which normally sits over the Arctic during winter. The effect of such a vortex is to keep the cold air circulating within the polar region.
However, when the vortex breaks down or splits into two, it can send very cold air further south.
The weather conditions produced by this polar vortex have been so severe that even polar bears at the Lincoln Park zoo in Chicago were brought inside as the weather dropped to a record low of -26C, CNN have reported. Residents have been warned their skin may freeze if they leave their homes when wind chills plummeted to -53C.
Chicago's National Weather Service office reported that Monday's low of -26C at O'Hare International Airport beat a record set in 1884 and equalled in 1988. In Fargo, the largest city in North Dakota, temperatures sank below - 35C.
Forecasters predict 187 million people could feel the effects of the "polar vortex" by the time it spreads across the country. At least 13 people are thought to have died as a result of the extreme conditions already, including several road accidents, a man who succumbed to hypothermia in Wisconsin, and a worker crushed by a massive pile of road salt at a storage facility in Philadelphia.
The weather conditions produced in the polar vortex could also make temperatures drop to the coldest on record in the 21st century. The National Weather Service predicts temperatures will be 25 to 35 degrees below average for the Mid-Atlantic, Southeast to the Ohio, Tennessee Valleys and into the Upper Midwest today.
In pictures: 'Polar vortex' hits US Midwest and Northeast
In pictures: 'Polar vortex' hits US Midwest and Northeast
An AAA emergency technician assists a motorist on Bidwell Avenue in Buffalo, New York
Ice forms on the shore of the East River due to unusually low temperatures caused by a Polar Vortex in New York
A man braves the cold and walks along the shore of Lake Michigan as temperatures remain in the negative digits in Milwaukee, Wisconsin. A 'polar vortex' of frigid air centered on the North Pole dropped temperatures to the negative double digits at its worst
Snow is piled high along the street outside the front of a home in Indianapolis, Indiana. A deadly blast of arctic air shattered decades-old temperature records as it enveloped the eastern United States, canceling thousands of flights, driving energy prices higher and overwhelming shelters for homeless people
Jennifer Berry watches the sunset from a lifeguard chair at a beach on Lake Calhoun in Minneapolis. Shortly after daybreak in the Twin Cities, thermometers had inched their way up to anywhere from 8 to 13 below zero
Ice builds up along Lake Michigan at North Avenue Beach in Chicago, Illinois
Chicago hit a record low of -16 degree Fahrenheit this morning as a polar air mass brought the coldest temperatures in about two decades into the city
Ice builds up along Lake Michigan as temperatures dipped well below zero in Chicago, Illinois
Clouds of steam rise from the Mississippi river during -14 Degrees Fahrenheit (-25 degrees Celsius) weather, in Minneapolis, Minnesota
A view of the War Memorial in Indianapolis, Indiana. According to news reports, Indianapolis received about 30 centimeters of snow and the morning temperature was -9 degrees Celsius
A horse drinks water from a hole in a frozen water tank in Enid, Oklahoma. Record low temperatures were set in at least two Oklahoma cities as a frigid front moved into the state
Ron, a bison at Brookfield Zoo, is covered in snow and doesn't seemed phased by the frigid temperatures or snow blowing through the Chicago area. The zoo was closed due to the snowstorm and sub-zero temperatures
A woman walks through a gust of blowing snow in frigid cold temperatures though downtown Chicago, Illinois. A blast of Arctic air gripped the mid-section of the United States, bringing the coldest temperatures in two decades
Washburn High School's information board reads -14 Degrees Fahrenheit (-25 degrees Celsius), in Minneapolis, Minnesota. Schools in the area were closed due to the severe weather
Patrick O'Brien dressed up in a Spiderman superhero outfit goes for a run in -25 degrees Celsius weather, in Minneapolis, Minnesota
Snow is piled up high in front of Home Depot in the South Bay shopping center after a two day winter storm in Boston, Massachusetts
A man wears a face mask and heavy clothes while walking through downtown Springfield, in blowing and falling snow as a strong winter storm moves through the Midwest. Temperatures not seen in years are likely to set records in the coming days across the Midwest, Northeast and South, creating dangerous travel conditions and prompting church and school closures
A city snow plow clears a street of snow in an almost deserted downtown as strong winds and snow move through the Midwest in Springfield
A man prepares to remove his plow stuck in a snow bank as snow and wind swirls in St. Louis
A Delta airlines plane is seen taking off while the fleets other planes sit on the tarmac at JFK Airport, New York
Four homeless men warm themselves on a steam grate by the Federal Trade Commission, blocks from the Capitol, during frigid temperatures in Washington. A winter storm that swept across the Midwest this week blew through the Northeast, leaving bone-chilling cold in its wake
Nicholas Simmons warms himself on a steam grate with three homeless men by the Federal Trade Commission, just blocks from the Capitol, during frigid temperatures in Washington
New York City Department of Parks & Recreation employees clear out snow from Carl Schurz Park after an overnight storm dropped up to 7 inches of snow in New York City. The Northeast and Midwest regions of the United States were hit with a large amount of snowfall accompanied by blizzard-like winds and plummeting temperatures this week
A man falls while slipping on ice during freezing rain on Roosevelt Island, a borough of Manhattan in New York
A couple walks through the winter white-out on Swan Avenue in Webster Groves
Few cars drive on Interstate 44 in Fenton. Snow-covered roads and high winds were creating dangerous driving conditions from Missouri to Delaware ahead of a 'polar vortex' that'll bring below-zero temperatures not seen in years to much of the nation in the coming days
People carry bundles of warm weather wraps as they arrive outside Lambeau Field before the start of the NFL Wild Card playoff game between the San Francisco 49ers and the Green Bay Packers in Green Bay, Wisconsin
Joe (R) and Rick (L) Pecki wait for the gates to open outside Lambeau Field before the start of the NFL Wild Card playoff game between the San Francisco 49ers and the Green Bay Packers in Green Bay, Wisconsin. Temperatures are around 5 degrees (-15 C) and are expected to go lower throughout the next few days
An icy tennis ball was no deterrent for Laura Jorgensen's dog Wilber while they played in a dog park in sub-zero weather as the midwest braces for the coldest temperatures in nearly twenty years to arrive in later today in Minneapolis, Minnesota
James Schlafer and his wife Diana Schlafer of Minneapolis went for their daily four to five mile walk in sub-zero temps as the midwest braces for the coldest temperatures in nearly twenty years to arrive in later today in Minneapolis, Minnesota
Some scientists believe that warmer-than-average conditions in the Arctic can weaken the polar vortex, so causing colder-than-average conditions at lower latitudes. They think this could help to explain the extreme conditions of the 2009-2010 winter when the normal pattern of Arctic winds broke down causing a weakened polar vortex to allow a frigid body of air to move south. This brought record snow conditions to northern Europe, eastern Asia and eastern North America.
Polar vortices tend to strengthen in the winter and weaken in the summer and can span a distance of more than 1,200 miles, with the air circulating in an anti-clockwise fashion around the North Pole.
A polar vortex also occurs around Antarctica, and is a key feature of the winter weather of the southern hemisphere that results in the conditions necessary for the depletion of atmospheric ozone.