- Published
Widespread snow driven by a polar vortex is rumoured to be heading for the UK this autumn and winter. But is that really the case?
Headlines have been suggesting it is down to the effects of El Niño but the truth is much more complicated.
For the rest of the autumn, El Niño actually increases the chance of wet, mild and potentially stormy weather for the UK.
Later in winter it could shift the odds towards cold and snow, something that can be influenced by changes in the polar vortex but none of this is certain.
What is a polar vortex?
The polar vortex is a ring of strong westerly winds encircling the North Pole every winter, around 10-30 miles (16-48km) above the Earth's surface.
It forms because the air high above the Pole becomes extremely cold and dense in winter. The contrast with warmer air further south drives the very strong stratospheric winds that make up the polar vortex.
This is all going on high up in the stratosphere, far above the troposphere - where our weather actually happens.
A strong polar vortex tends to encourage a strong polar jet stream in the troposphere.
This drives areas of low pressure from west to east, bringing mild air from the Atlantic across the UK with spells of wind and rain. The coldest air is essentially locked away to the north of us.
Sometimes, however, the polar vortex can wobble, weaken or even reverse direction.
Over a period of days and weeks this can work its way down through the atmosphere influencing our jet stream, causing it to weaken, become more bendy or reverse.
It can deliver colder northerly or easterly winds which can bring the UK its coldest winter weather
However, it does not always translate into changes in weather patterns at the Earth's surface.
Equally, periods of a meandering jet stream and cold winter weather often occur without significant changes in the polar vortex.
What does El Niño have to do with it?
El Niño - the naturally-occurring climate phenomenon in the Pacific Ocean - is continuing to grow and by some measures is already the strongest on record.
The heat released from the warm Pacific Ocean into the atmosphere impacts weather patterns around the globe.
In autumn it increases the chance of wet and stormy weather in the UK - something that is reflected in the forecasts for this season.
In late winter, the waves of energy released by El Niño can move upwards through the atmosphere making it more likely that the polar vortex becomes disrupted, increasing the chance of ice and snow.
However, the links are unclear and the evidence is mixed.
The so-called Big Freeze of 2009/2010 was, at the time, the coldest winter in three decades - and coincided with an El Niño.
However, 2006/2007 was also an El Niño winter and was unseasonably warm.
The so-called Beast from the East in 2018 actually occurred during a La Niña year, effectively the opposite of El Niño.
So while El Niño is already having big impacts on global weather, and may influence the polar vortex later in winter, snow and ice in the UK is far from guaranteed.
Current forecasts suggest no significant cold or snow is likely in the near future.
You can read the latest outlook for the next month here.
- Published3 September
- Published21 August