Tens of millions of Americans braved Arctic-like temperatures on January 31 as low as minus 49 degrees Celsius that paralysed the U.S. Midwest and were blamed for 21 deaths. News agencies said warmer than normal weather was on the way, but that oﬀered little comfort to the homeless enduring a chill that caused frostbite in minutes. The cold has been blamed on a phenomenon called the 'polar vortex'. Meteorologist Mark Chenard said a polar vortex refers to the upper level jet stream that circulates around both the North and South Poles, keeping the coldest air there. When that jet stream occasionally weakens and buckles, he told Reuters , it disrupts weather patterns — bumping warmer air into Alaska and pushing cold winds down into the U.S. Midwest and East Coast. Pictures show ice ﬂowing in the Merrimack River as it heads to the Atlantic Ocean in Newburyport, Massachusetts, and the impact of freezing temperatures on the Horseshoe Falls in Niagara in Canada.