North polar vortex. Satellite image showing the potential for a sudden stratospheric warming (SSW) in the North polar vortex, an area of low pressure found over each pole. Areas with a high potential are white, those with a lower potential are darker. SSWs occur when there is rapid warming (at least 25 degrees Celsius in one week) and the polar vortex wind reverses in direction from westerly to north-easterly. This brings colder than average temperatures to lower latitudes. This system progressed into a SSW that brought very cold weather to North America in January 2013. Image obtained by the GEOS-5 data assimilation system on 7th January 2013.