AILSA CHANG, HOST: Thunder sleet, single-digit temperatures in Texas, record-setting deep freezes - much of the country is witnessing some bizarre and historic winter weather, but why? Well, it has to do in part with something called the polar vortex, a mass of cold air spinning over the Arctic and up into the stratosphere. It strengthens in the fall and normally stays stable over the North Pole, keeping cold air penned in over the Arctic. Dr. Michael Ventrice, a meteorological scientist at IBM, explains it like this. MICHAEL VENTRICE: The polar vortex wants to sit over the North Pole. It feels like it's comfortable there. That's equilibrium.