The temperature has plummeted overnight with some parts of Scotland seeing what could be the coldest January night in 14 years.
The freezing temperatures and snow are expected to continue for much of Britain this week because of cold Arctic air.
And this is expected to be replaced by "potentially disruptive" stormy weather which lands over the weekend.
A "cold plunge of Arctic air" has moved south across the whole country over the past few days, making it 5C to 6C lower than usual for this time of year, the Met Office said.
A Met Office spokeswoman said: "It's staying cold until Friday, and then looking further ahead into the weekend we've got some deep areas of low pressure pushing in, so a big change in weather type, and we could see some stormy conditions by the end of the week.
"The cold isn't lasting right to the end of the week, but we have a very different type of potentially-disruptive weather arriving."
More than 100 schools were closed in Scotland on Tuesday, while drivers faced difficult conditions thanks to the wintry weather across north-west England, including in Merseyside, Cheshire and Cumbria.
There are three yellow weather warnings in place for Wednesday.
One for snow and ice covers northern England and parts of Wales and comes into place from midnight for 24 hours.
Another ice warning covering parts of Scotland around Glasgow and Edinburgh came into force from midnight for 24 hours.
The third snow and ice warning covers the northern section of Scotland and Northern Ireland and is in place until 11.59pm on Wednesday.
More than 100 schools in Aberdeenshire will delay opening this morning, with a handful closed for the day, while in the Shetland Isles all schools and early learning centres will be shut.
More than 40cm of snow could be seen on high ground in northwest Scotland by the end of Friday as it continues to build up over the coming days, the Met Office added.
Meanwhile, lower ground in northwest Scotland could see between five and 10cm of snow by the end of the working week.
The Met Office is reviewing the situation and any new warnings could be issued at short notice, it said.
The UK Health Security Agency has issued a cold-health alert, which warns of possible impacts for the health and social care sector.
National Rail has warned the wintry weather could affect train journeys all week.