Why the weather has turned cold again - and could stay that way through March

Temperatures dropped back near freezing overnight on Monday, with some rural areas of the country going into minus figures, according to the Met Office.

"This week is going to be feeling a lot more like winter, all thanks to an area of high pressure dragging in a more northerly flow across the country," Met Office meteorologist Honor Criswick said.

Sleet and snow could be on the cards for the north east of England, parts of Scotland and the North York Moors, she added, with temperatures across the country feeling closer to freezing due to brisk winds.

Although temperatures are not out of the ordinary for March, the UK could be about to experience a cold snap later in the month than is usual.

That's thanks to a phenomenon known as sudden stratospheric warming (SSW).

Despite the name, it's often responsible for colder weather events - including 2018's Beast from the East.

The Met Office said there is more than an 80% chance of SSW happening by mid-March.

What is a sudden stratospheric warming?

SSW is used to describe a sudden change in temperatures in the stratosphere - the second layer of Earth's atmosphere.

The rapid warming happens so high above the Earth that it is not felt directly on its surface. But a few weeks later, knock-on effects can be seen on the jet stream - a strong and narrow band of wind that is a key driver of weather patterns across the globe.

After sudden warming, the jet stream can "snake" more, which creates a large area of high pressure that becomes stubborn to shift.

When this happens, northern Europe, including the UK, is likely to get a long spell of dry, cold weather, whereas southern Europe will tend to be more mild, wet and windy.

In some cases, the drop in temperatures leads to snow, which is what happened in early 2018.

What is the polar vortex?

One of the defining characteristics of SSW is the reversal of winds in the stratosphere, known as the stratosphere polar vortex.

These winds circulate air high over the Arctic and any sudden temperature change can lead to their flow reversing or weakening.

When the polar vortex weakens or breaks apart, the jet stream becomes more wavy and cold air can spill into regions that usually experience milder winters, causing extreme cold events or winter storms.

The Met Office said recently the winds are currently "weakening rapidly", a good indicator that SSW will occur.

The stratosphere polar vortex collapses roughly every couple of years in mid-winter.

How likely is an extreme weather event?

It is important to note that SSW events are not always followed by severe weather.

A Met Office spokesperson told Sky News around 70% of SSW events are followed by a change to cold conditions with easterly winds across northern Europe and the UK.

They added that there are "signs of this in the forecast, and the risk of a cold snap later in March is higher than normal".

Sky News meteorologist Jo Robinson added it is not clear at the moment how the current SSW will affect the troposphere and in turn the weather at the surface, but said "any impacts will be in a few weeks' time".

She said: "The SSW event will increase the chance of high pressure and a blocked weather pattern for the latter part of March, but computer models are currently favouring something milder and more unsettled."

She said according to current forecasts, the rest of the week looks rather cold, with overnight frosts bringing the risk of icy stretches.

Into early April, the Met Office predicts a transition to more frequent drier and more settled spells, with temperatures above average.

Will there be a repeat of the Beast from the East?

Forecasters do not yet know if a Beast from the East weather event will be triggered by the current SSW, but it seems unlikely.

Jo Robinson said the Beast from the East of 2018 was linked with SSW, but the event can also cause record warming too - as it did in 2019.

In 2018, 17 people died in the two weeks from 22 February to 5 March due to below freezing temperatures and heavy snow that swept across the nation.

The cold snap was swiftly followed by Storm Emma and the mini Beast from the East, which brought more snow.

-SKY NEWS