The recent extreme freeze in North America – reaching -51 degrees celsius including wind chill – has provided some astonishing photographs and videos. Not least these amazing shots of the frozen Niagara Falls. It’s also seen the brilliantly scary phrase ‘polar vortex’ enter the public consciousness via an awe-struck media. Unfortunately, as New Scientist points out, the polar vortex is a normal winter wind system in the stratosphere above the weather. The cause of the terrible conditions is, rather, a weakened jet stream.