The polar vortex of ultra-cold air usually spins around the North Pole, but it sometimes ventures south into the U.S., Europe and Asia. Some experts say such cold air outbreaks are happening more frequently, paradoxically, because of a warming world.

Texas is preparing for severe winter weather. An area stretching from Texas to Tennessee braced Wednesday for the possible arrival of freezing rain and snow even as some other parts of the U.S. hit by the plunging polar vortex this week began to shake off its icy effects.