A persistent cold snap  across the U.S. has already affected millions of people, and resulted in some incredible situations (such as a Chicago warehouse getting covered in instant ice even as fire fighters battled to put out a fire inside it--check out the mindboggling images ).
Wonder how it got this way? This video from the National Oceanic and Atmospheric Administration posted on NPR  shows how the cold air descended throughout the country. It's all about cold lumps of air that keep sinking down from the North. But the video demonstrate this in a sort of chilly no-frills infographic that really needs no explanation:
But of course this is the age of the social web, and you don't need a weather report or any super-high-tech meteorological equipment to know it's been very cold. Just check out social trends like #cold , #freezing , and #ice  on Twitter. The photos you'll find there will be enough to make you stay inside and huddle round a cup of hot chocolate!
[Image: Flickr user dgreichert ]