What is the polar vortex and how does it impact UK weather?

The polar vortex is a fast-moving wind high in the atmosphere that traps very cold air above the North Pole.

Although the polar vortex is a very normal feature of winters in the northern hemisphere, changes in its behaviour can impact the weather we experience on the ground.

If the polar vortex rapidly weakens or changes position, this can lead to something called a 'sudden stratospheric warming' (SSW) which in turn can trigger colder, wintry weather in the UK, although the link is not straightforward.

Whilst SSW events are relatively common during our winters, not all of them bring harsh wintry weather like we saw in the 2018 Beast from the East.

Meteorologists watch a variety of weather and climate drivers to forecast how the polar vortex and any SSWs may affect what type of weather we get.

The polar vortex consist of a ring of strong westerly winds encircling the North Pole every winter, around 10-30 miles (16-50km) above the Earth's surface in the stratosphere.

It forms because as the North Pole tilts away from the Sun, the air high above becomes extremely cold and dense. The contrast with warmer air further south drives the very strong stratospheric winds.

This is all going on high above the troposphere where our weather actually happens.

A strong polar vortex tends to encourage a strong polar jet stream, which generally brings us mild and unsettled weather as Atlantic low pressure systems are driven in.

In this situation, the polar vortex essentially keep the coldest air locked away to the north of us.

Sometimes, however, the polar vortex can wobble, weaken or even reverse, disrupting the usual westerly wind flow.

This can lead to that cold air trapped above the pole sinking downwards in the atmosphere. This rapid descent causes the air to compress, generating an immense and rapid temperature increase—the sudden stratospheric warming.

The temperature can surge by up to 50C in just a couple of days.

Think of it a bit like when you swirl a cup of tea - the tea tends to move rapidly, high around the periphery of the cup. If the swirling slows or stops, the tea sinks back downwards into the centre of the cup.

Over a period of days and weeks this sinking, warming air can work its way down through the atmosphere influencing our jet stream, causing it to weaken, buckle, or reverse.

A weaker, meandering jet stream can deliver colder northerly or easterly winds which may bring us snow and a drop in temperature.

However, SSWs don't always translate into changes in weather patterns at the Earth's surface, but the dice becomes loaded for a highly chance of colder wintry weather in the UK.

Equally, periods of a meandering jet stream and cold winter weather often occur without significant changes in the polar vortex.