Polar vortex delivers Arctic blast to U.S. Midwest

The Midwest was the hardest hit region, as temperatures plummeted below zero Fahrenheit (minus 17.8 degrees Celsius).By mid-afternoon, the mercury was hovering at 6F in Chicago (minus 14C), 12F (minus 11C) in Detroit and minus 22F (minus 30C) in Fargo, North Dakota.The brutal chill was caused by the polar vortex, a mass of freezing air that normally spins around the North Pole but has made its way south into the United States.