What is a polar vortex? The arctic winds that brought cold air and chaos southwards

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. TheNational 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.