Frigid air that normally stays trapped in the Arctic has escaped, plunging deep into the United States for an extended visit that is expected to provoke teeth-chattering but not be record-shattering.
It's a cold air outbreak that some experts say is happening more frequently, and paradoxically, because of a warming world. Such cold air blasts have become known as the polar vortex. It's a long-established weather term that's become mainstream as its technical meaning changed a bit on the way.
