A blast of icy polar air brought dangerously low temperatures to the U.S. Midwest on Wednesday, causing at least three more deaths while halting mail services and forcing residents who pride themselves on their winter hardiness to huddle indoors.

Andrew Orrison, a meteorologist with the National Weather Service, said some of the coldest wind chills were recorded in International Falls, Minnesota, at minus 55 degrees Fahrenheit (minus 48 Celsius). Even the South Pole was warmer, with an expected low of minus 24 F (minus 31 C) with wind chill.

Classes were canceled for Wednesday and Thursday for students across the region, including Chicago, home of the nation's third-largest school system, and police warned of the risk of accidents on icy highways. Michigan said all state offices would remain closed through Thursday.