The polar vortex is at it again. The weather phenomenon responsible for bone-chilling cold fronts settling over parts of North America and Europe, including those that normally experience mild winters, has held much of the central United States in its icy grip the past few weeks. In mid-February, a blob of Arctic air moved as far south as Texas, causing extreme cold and heavy snowfalls, loss of life, traffic chaos and power outages for millions. According to the US National Weather Service, more than 100 million people over a distance of 1.6 million square kilometers were under winter storm warnings. On Feb. 16, some three-quarters of the continental USA was covered in snow, the greatest extent on record in the database which goes back to 2003.