The big freeze gripping the UK will get worse this week with temperatures plunging as low as minus 20C, forecasters warned.
The lowest temperature in Northern Ireland was at Lough Fea, in the Sperrin Mountains, Co Londonderry, which dropped to -9.5C (14.9F).
The blanket of snow across Northern Ireland also deepened, with police urging motorists to exercise caution on the roads.
The cold front has seen snow fall right across the UK, with many areas due to remain below zero.
Forecasters have warned that the severe conditions could last well into next week, with rain, sleet and more snow heading our way.
The Arctic conditions have been caused by a combination of light winds, snow cover and clear skies — and could see readings down to -20C in parts of Scotland later this week.
Scotland and the north east have been worst hit, with 40cm of snow.
Yesterday, many Scottish airports were closed as a result of the snow, which caused cancellations at Northern Ireland airports.
The weather caused havoc on some roads, with cars sliding in the icy conditions.
Gritting lorries have come under attack from young people in Londonderry.
A window was smashed on one vehicle and Roads Service have been able to grit a number of roads in the Creggan and Gobnascale area.
SDLP MLA Pat Ramsey says vulnerable people may now be trapped in their homes as a result of the irresponsible behaviour.
“When the gritter arrived in that area in the afternoon, the vehicles were attacked by a group of young people and some of them are badly damaged,” he said.
“Young people should know what’s right from wrong in these situations.”
A number of schools are closed this morning because of bad weather. They include Our Lady of Lourdes in Greencastle, Co Tyrone, New Buildings Primary School outside Londonderry and Jonesborough Primary School in Newry.
The Northern Ireland Ambulance Service responded to three minor traffic accidents — two in Belfast and one in Ballynahinch — but there were no casualties.
The A1 between Dromore and Banbridge was down to one lane in both directions last night.
Conditions on the stretch of road were extremely treacherous and police were reminding people only to make strictly necessary journeys. If motorists do have to venture out, police are advising them to take extra care and reduce their speed.
The Roads Service’s Colin Brown urged drivers and pedestrians to be careful. “Black ice is what we would ask the public to be aware of,” he said.
The early snowfall has left bookmakers in a sweat as they slashed the odds of a white Christmas from 6/1 to 5/2.
Bookmaker Paddy Power decided to follow the advice of Donegal postman, Michael Gallagher, who is known for his accurate predictions of the weather, after he forecast Belfast will have a white Christmas.