A vortex of artic weather is slowly inching its way towards the UK as forecasters predict snow and freezing temperatures over the coming weeks.
The polar vortex, which dominates the weather cycles around the arctic during winter, has been moving towards the UK and Europe since the start of January.
A series of images published by the US government climate agency shows how the vortex has moved from the arctic and Greenland from early January to hovering near the UK and Scandinavia on 15 February.
In mid-February, the agency warned that "interesting atmospheric happenings are occurring high above the Arctic". It added: "Over the next several days, the stratospheric polar vortex—a band of strong westerly winds that forms each winter in the stratosphere between 10 and 30 miles above the North Pole—will be knocked clear off its lofty throne at the top of the world.
"What the disruption will mean for weather down here ... is still uncertain, but sometimes these events lead to extreme cold air outbreaks in the mid-latitudes of the United States.
It added that the disruption has caused the vortex to shift southward from the pole toward Europe.
What's going on in the UK?
In short, it's going to get cold. The Met Office warned on Thursday that freezing temperatures were due to hit the UK on Sunday and could remain for sometime.
Snow and ice warnings for northern England and Scotland are in place for Monday and Tuesday.
Read more: How cold does it have to be to snow?
Deputy chief meteorologist Chris Almond said: "Although we’ve moved into meteorological spring there will be a distinctly wintry feel to our weather next week."
"Very cold air will spread across the UK bringing snow showers even to sea level in the north on Monday and these snow showers could spread further south on Tuesday.
"With freezing overnight temperatures and the risk of ice it is likely weather warnings will be issued for Monday and Tuesday once the detail of potential impacts becomes clearer, so keep an eye on the Met Office forecast."
The UK Health Security Agency has issued a Level 2 Cold Weather Alert for the whole of England and is likely to be reviewed and extended in the coming days.
What is the polar vortex?
The vortex is a mass of extremely cold air that forms over winter above the North and South Poles.
The tropospheric polar vortex is the technical name for the one that sits above the Arctic and can impact the UK.
It is normally restricted to the Arctic, where a huge area of low pressure keeps it locked in place.
Occasionally it will move south and can contribute to some of the major snowstorms and freezing temperatures that hit Europe, Canada and the US.
This happens when the vortex weakens allowing some artic winds to escape the jet stream, usually around the tail end of winter when it is forming and dissipating.
At its peak the winds over the North Pole can hit 155 miles per hour.
Sometimes, the polar vortex can break down entirely in an event called 'Sudden Stratospheric Warming.'
What is sudden stratospheric warming?
Sudden stratospheric warming (SSW), refers to a rapid warming – up to about 50C in just a couple of days – high up in the Earth's stratosphere causing the polar vortex to break down.
This is so high up that we don’t feel the ‘warming’ ourselves, but it can cause a knock-on effect a few weeks later, affecting the weather we experience lower down in the troposphere.
The rapidly changing temperatures in the vortex can force cooler air to sink closer to earth and cause the jet stream to change shape, dramatically changing temperatures in the northern hemisphere.
A sudden stratospheric warming event was linked to the Beast from the East that swept the UK in 2018.