An "exceptional" record-breaking 16.3C was recorded in Scotland at 3am due to a weather phenomenon.
The temperature - a UK record for so late in December - was picked up by a Met Office observation post in Cassley, Sutherland, on Sunday morning.
The average temperature for that area in the Scottish Highlands in December is zero degrees at night.
Describing it as "pretty exceptional", Met Office meteorologist Alex Burkill said: "We have never recorded a temperature that high this late in the month.
"What makes it more unusual was the temperature came at 3am."
It was still short of the record for December, however, which stands at 18.3C recorded on December 2 1948 in Achnashellach, in the Highlands of Scotland.
Meteorologists put the record down to a weather process called the Foehn effect.
In the UK, the most notable Foehn events tend to occur across the Scottish Highlands where the moist prevailing westerly winds encounter high ground along Scotland's west coast.
This results in a marked contrast in weather conditions across the country with the west being subjected to wet weather, while the lower lying east enjoys the warmth and sunshine of the Foehn effect.
Overnight on Saturday into Sunday, mild air from Africa pushed across the UK.
As the air travelled from the south west over the UK, it rose up and lost its moisture. This meant that the other side of the hills ended up with drier air.
The Met Office tweeted: "Cassley, Sutherland reached a remarkable 16.8 °C at 03:00 this morning.
"This is the highest UK temperature recorded this late in the year."
But Mr Burkill warned it would be a chilly end to 2019, despite the recent mild temperatures.
"There will be a bit of a north-south split to the weather," he said.
"It will stay largely clear in the north, and the cloud and rain in the south should clear up.
"You will need a coat and take precautions because of possible fog in the early hours."
