Last December was the warmest since records began in 1910 – but the UK's run of mild winters could be about to end.
An icy polar vortex could descend from the Arctic in the coming months and cause temperatures to plummet, the Met Office has warned.
There is an "increased risk of cold snaps between now and Christmas,” the forecaster said in a recent blog post.
Several factors could make the possibility of a cold start to winter more likely, from tropical rainfall conditions to disruption to air flow over the equator, it said.
But there is a higher chance this year that "weak stratospheric circulation" caused by a polar vortex could bring icy winds and freezing weather to the country.
What is a polar vortex?
A polar vortex is a mass of very cold air which sits above the Earth’s north and south poles.
This dense, cold air is controlled by a large pocket of low pressure, which rotates in an anti-clockwise direction at the north pole and clockwise at the south pole.
Why does it move south from the poles?
The strength of a polar vortex varies from year to year. When it is strong, the vortex is concentrated over the Arctic or Antarctic area.
But when it is weak – which is more frequent – it can split into two or more freezing vortices.
These cover a larger area and can move south to Canada, the USA and Europe, increasing the risk of air temperatures decreasing to potentially dangerous levels.
When is it coming to the UK?
The polar vortex in the northern hemisphere is weaker than usual this year.
“There’s a higher chance of cold being able to sink southwards, whereas if it was fast, it would stay on its normal track around the poles,” Met Office forecaster Emma Sharples told The Independent.
Although polar vortex winds are many kilometers above the ground, they can “influence the strength and position of the jet stream,” according to the Met Office. “This is helping to increase the risk of cold snaps in the UK.”
Ms Sharples said the arrival of a polar vortex could help the jet stream become “more north-south orientated rather than west-east like last winter."
“When there are more kinks in [the jet stream], that means more areas of cold air coming south across the UK,” she said. “But that’s only one thing of many that can affect what kind of weather we can have”.
When is the last time this happened?
A polar vortex made headlines in 2014 when much of the United States was hit by an extended period of cold weather, causing transport chaos and bringing some parts of the country to a halt.
The weather conditions sparked by the vortex were so severe that even the polar bears at Lincoln Park zoo in Chicago had to be brought inside.
In pictures: 'Polar vortex' hits US Midwest and Northeast
In the UK, extreme conditions were experienced in the winter of 2010, when the normal pattern of Arctic winds broke down causing a weakened polar vortex to allow a frigid body of air to move south.
This brought record snow conditions to northern Europe, eastern Asia and eastern North America.
How long could the cold spell last?
Met Office scientist Jeff Knight told The Independent the polar vortex effect on the UK's atmosphere tends to affect the weather "with a few weeks' lag".
Dr Knight emphasised that the vortex was an "external factor" that could influence the weather, rather than being "intimitely linked" with weather conditions.
"You can view the weather as rolling a dice. But something like a weak stratosphere is like loading the dice," he said.
"Because it’s weak, there’s a higher chance we’ll see colder weather with less of the Atlantic storms that bring moisture and warmth to these shores."
He said the weak polar vortex could affect the whole North Atlantic region, meaning the risk of cold snaps is greater for North America as well as Europe.
"It’s important to say that it’s only an increased risk," he said. "There’s still a good chance we’ll have an ordinary winter. We don’t tend to get cold and snowy winters often in the UK, but it’s slightly more likely than usual."
What are the risks?
According to the Office for National Statistics, there were nearly 44,000 extra winter deaths in 2014-15. A particularly cold winter puts elderly and vulnerable people at risk.
The government is encouraging those over 65 and people with health problems to eat hot meals and keep active as part of their ‘Stay Well This Winter’ campaign.