What causes a polar vortex?
6 January 2014 Last updated at 15:07 GMT
Parts of America are preparing for potentially record-breaking low temperatures, caused by an icy blast of air known as a polar vortex.
A winter storm has already covered north-eastern states and parts of Canada with snow.
Weather expert Simon King told Newsround how a polar vortex is formed - and how it's affecting the weather in the UK.