(NEWSER– A weather phenomenon called a "polar vortex" has tens of millions of Americans shivering in the lowest temperatures in nearly 40 years, and forecasters believe it's going to get colder until nearly half the country is in below-zero conditions by Wednesday.

  • "The coldest temperatures in almost two decades will spread into the northern and central US today," the National Weather Service says. "Combined with gusty winds, these temperatures will result in life-threatening wind chill values as low as 60 degrees below zero."
  • Minnesota has closed all its public schools today and cities including St. Louis and Chicago have followed suit, saying it is too dangerous to send kids out in conditions so cold that exposed skin can become frostbitten within minutes, Reuters reports.