EXPLAINER: Topsy-turvy weather comes from polar vortex

It’s as if the world has been turned upside-down, or at least its weather. You can blame the increasingly familiar polar vortex, which has brought a taste of the Arctic to places where winter often requires no more than a jacket.

Around the North Pole, winter’s ultra-cold air is usually kept bottled up 15 to 30 miles high. That's the polar vortex, which spins like a whirling top at the top of the planet. But occasionally something slams against the top, sending the cold air escaping from its Arctic home and heading south. It’s been happening more often, and scientists are still not completely sure why, but they suggest it's a mix of natural random weather and human-caused climate change.

This particular polar vortex breakdown has been a whopper. Meteorologists call it one of the biggest, nastiest and longest-lasting ones they've seen, and they’ve been watching since at least the 1950 s. This week’s weather is part of a pattern stretching back to January.

“It’s been a major breakdown,’’ said Jennifer Francis, a climate scientist at the Woodwell Climate Research Center on Cape Cod. “It really is the cause of all of these crazy weather events in the Northern Hemisphere.”

“It’s been unusual for a few weeks now — very, very crazy,” Francis said. “Totally topsy-turvy.”

RECORD COLD IN WARMER PLACES

Record subzero temperatures in Texas and Oklahoma knocked millions off the power grid and into deep freezes. A deadly tornado hit North Carolina. Other parts of the South saw thunder snow and reports of something that seemed like a snow tornado but wasn’t. Snow fell hard not just in Chicago, but in Greece and Turkey, where it’s far less normal. Record cold also hit Europe this winter, earning the name the "Beast from the East.”

“We’ve...