The Northern Hemisphere must brace itself for bone-rattling chills. Due to polar vortex icy blasts are threatening to sweep across North America, Europe and Asia starting in late January. The polar vortex is caused by the warming of the stratosphere at high altitudes above the North Pole, pushing cold air southward. The same weather pattern had earlier triggered the 2014 cold snap. The cold has already descended upon Western Europe and China. Paris has been 3.5 degrees below normal and Madrid 6.9 degrees cooler, while Beijing temperatures fell to a record low of minus 9 on Thursday. A wave of deep cold could give the Great Lakes and East Coast their first real blast of frigid winter weather marking a big shift for the U.S., where winter has been a bit lackluster so far.