Sudden stratospheric warming

A sudden stratospheric warming (SSW) is an event in which polar stratospheric temperatures rise by several tens of kelvins (up to increases of about 50 °C (90 °F)) over the course of a few days. The warming is preceded by a slowing then reversal of the westerly winds in the stratospheric polar vortex. SSWs occur about six times per decade in the northern hemisphere, and about once every 20-30 years in the southern hemisphere. Only two southern SSWs have been observed.