The polar vortex engulfing the US has left 21 people dead. Here’s why events like this might be getting more common.

AP The polar vortex swept parts of the US Midwest and East Coast this week, forcing millions of Americans to contend with subzero temperatures.  The polar vortex is an area of circulating cold air that rings the planet’s North and South poles. Sometimes that vortex destabilizes, sending surges of Arctic air south. Windchill temperatures were as […]