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What is a polar vortex?

The bitter chill gripping North America is a result of Arctic air that has spilled southwards, and global warming may be a cause.

Relief on the way for shivering Americans

The brutal chill came as people in the Great Lakes region were still digging out from two massive snow storms which dumped more than two feet of snow.

North America`s deep freeze loosening its icy grip, leaves 21 dead

The deep freeze that has paralysed life in North America and left hundreds of residents here shivering for days is fast loosening its icy grip.

Polar freeze grips United States; over 4,390 flights cancelled, business disrupted

A blast of Arctic air gripped the vast middle of the United States on Monday, bringing the coldest temperatures felt in two decades, causing at least four deaths, forcing businesses and schools to close and canceling thousands of flights.

`Life-threatening` cold bites US Midwest, heads east

As the Midwestern United States shivered through the region`s lowest temperatures in two decades and forecasters warned that life-threatening cold was heading eastward, officials in Chicago and other districts said schools would be closed on Monday.

Deadly tornadoes, storms hit US Midwest; 6 killed

Powerful tornadoes struck US Midwest on Sunday killing at least six people and injuring hundreds others with the death toll reportedly expected to rise further.

Storm hits US Midwest, Deep South; 3 dead

A powerful storm has unleashed tornadoes and peeled the roofs from homes in the US Deep South and heaped snow on the Midwest, killing three people.