A blast of Arctic air has swept through the United States, producing the coldest temperatures in two decades. Temperatures are expected to plummet to minus 19 degrees Celsius from the Midwest to the Southeast. Authorities have put about half of the nation under a wind chill warning or cold weather advisory. Homeless shelters have received more people and made provisions for extended hours, and cities have set up free transportation to the sites. The bitter cold has also forced many schools and businesses to close. Electricity suppliers are struggling to keep up with surging demand.