© LatitudeStock/AlamySuilven mountain in Sutherland, where the Foehn Effect is contributing to warm temperatures.
North-eastern England and northern Wales were also unusually warm, with 13.3C recorded in Chillingham Barns, Northumberland, and 11.5C in Rhyl.
The Scottish figure, recorded by the Met Office at 3am on Sunday in Cassley in Sutherland, is the highest on record for 29, 30 or 31 December.
Forecasters have attributed the unseasonably warm weather to a meteorological pattern called the Foehn Effect. It occurs in mountainous areas, creating wet and cold conditions on one side of a mountain and warm and dry conditions on the other.
The phenomenon occurs when humid air is pushed over high ground by strong winds. As the moisture-filled air rises, it cools and condenses, resulting in clouds and rain. This then releases dry air, which moves down the mountain's other side, heating up and raising ground level temperatures as it travels.
The effect coincided with gusts of warm air arriving from the southern Atlantic, which have resulted in Britain being warmer than Athens and Rome.
This pattern is usually seen in eastern Scotland, north-eastern Wales and north-eastern England.
Alex Burkill, a meteorologist for the Met Office, said it was extremely rare to see such high temperatures overnight this late in the year.
"Getting temperatures of 16 or 17 degrees in December isn't all that unusual but it's remarkable that this was during the night," he told the BBC.
The rest of the UK also experienced mild overnight conditions, but temperatures are expected to drop in the new year.
The current record for the highest daytime maximum temperature in December in the UK is 18.3C, registered in Achnashellach in the Scottish Highlands on 2 December 1948.
Polar vortex, Foehn Effect, etc...things that historically we've never used or heard / cared about.