(NBC News) Most of the nation remains in the grips of the polar vortex.
In Chicago, the mercury reached 23 degrees below zero. Lake Michigan and the Chicago River are frozen.
Hospitals are seeing a rise in frostbite, which can happen in a matter of minutes.
At least nine deaths are being blamed on the weather.
Wednesday, police found a University of Iowa student dead behind a campus building. The wind chill was 51 degrees below zero at the time.
Meanwhile, some Michigan residents are being asked to turn down their thermostats after a fire at a natural gas plant threatened the local supply.
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