What is sudden stratospheric warming?

The current cold snap in Britain is soon set to come to an end, with the frost, freezing fog and sub-zero temperatures of recent days giving way to milder conditions as bands of high pressure return.

But the rapid westerly winds currently roaring above the Arctic could be about to slow down, The Times reports, potentially opening up the possibility of their switching direction and turning easterly.

That eventuality, should it arise, could in turn open the door to a new “Beast from the East” bringing heavy snow or even blizzards to the British Isles in February, recalling the bitter winter storm that slammed the UK in early March 2018 and caused 17 fatalities.

According to the Met Office, any weakening of those westerly winds, known collectively as the Polar Night Jet, over the North Pole occurs when natural weather patterns or disturbances lower in the atmosphere disrupt their flow and cause the jet to “break just like waves on the beach”.

When that happens, “cold air then descends very rapidly in the polar vortex and this causes the temperature in the stratosphere to rise very rapidly, as much as 50­C over only a few days”.

This phenomenon is known as sudden stratospheric warming (SSW) and typically takes place “between 10km and 50km above the Earth’s surface”, meaning we do not notice the warming effect on the ground.

The forecaster continues: “As the cold air from high up in the stratosphere disperses, it can affect the shape of the jet stream as the cold air sinks from the stratosphere into the troposphere.

“It is this change in the jet stream that causes our weather to change.”

If the jet stream “snakes”, large areas of blocking high pressure can form over the North Atlantic and Scandinavia, condemning northern Europe to a long period of cold, dry weather while the south of the continent remains mild.

The good news is that the interval of several weeks between the sudden stratospheric warming taking place and its beginning to impact our weather is long enough that it can be reliably tracked with satellites, according to the Met Office.

The forecaster also stresses that this process does not necessarily take place every year and does not always affect our weather when it does, although the prospect of it occurring next month leaves spring feeling a very long way away.

Despite some excitable early forecasts, there is currently some doubt as to whether the Arctic will actually experience SSW in February.

Met Office weatherman Alex Deakins said on Monday: “The chances of a sudden stratospheric warming is currently 20 or 25 per cent.

“The most likely scenario at this stage is for the polar vortex to weaken, but computer models show an SSW is at least a possibility.”

Dr Amy Butler, an atmospheric scientist at the US National Oceanic and Atmospheric Administration, meanwhile wrote on Twitter: “Latest forecasts suggest a major SSW is off the table for now. Just not getting the amplitude of planetary wave breaking that you typically see before a complete reversal of the polar vortex winds.

“Still, will be interesting to see how things evolve in Feb!”