Polar vortex warning as US freezes overPublished30 January 2019Image source, AFPImage caption, Millions of people in the US are being warned not to go outside as the nation faces some of the coldest weather for a generation. A state of emergency has been declared in Wisconsin, Michigan and Illinois, and even in Alabama and Mississippi, which are further south where the weather is usually milder.Image source, ReutersImage caption, Judah Cohen, a storm expert for Atmospheric Environmental Research, explained how the sub-zero temperatures across the US are due to a sudden warming above the North Pole which forced a polar vortex - a spinning pool of cold air - to head south.Image source, AFPImage caption, The sub-zero temperatures had already blanketed parts of Canada, where the famous Niagara Falls even froze over! Now, millions of US citizens are being affected by the cold snap with companies and schools closing, and people being told to wrap up warm and stay inside where possible.Image source, AFPImage caption, In the state of Iowa, people have to been told to "avoid taking deep breaths and to minimise talking" if they go outside. In some places, it is set to be so cold that the National Weather Service has said that frostbite could be possible after just 10 minutes of being outside.Image source, WLS/ABCImage caption, Chicago is set to be one of the worst affected cities by the freezing temperatures, with much of the Chicago River freezing over. Officials are saying that it could get as cold as -33C, but winds will make it feel even colder - more like -45C. Temperatures in the city are expected to drop so much that it is due to be even colder than Antarctica!Image source, Getty ImagesImage caption, In this picture, airport staff can be seen working to remove ice from a plane in Chicago. More than 1,100 flights into or out of the US on Tuesday were cancelled, including some to Georgia in Atlanta, which is where the Super Bowl is due to take place on Sunday.More on this storySee Niagara Falls frozen overPublished24 January 2019Has it snowed where you live?Published30 January 2019Your snowy pics!Published1 February 2019