Part of a busy motorway has been shut due to wintry conditions, while severe weather warnings for snow and ice have been upgraded across the UK.
Forecasters warn of "heavy squally showers of snow" which may cause power cuts and disruption on the roads, with 15cm-25cm of snow possible on high ground, and 5cm-10cm more typical at lower levels.
Traffic Scotland closed part of the M74 heading southbound in Lanarkshire at around 6pm, due to wintry conditions.
There have also been several minor road collisions in the Highlands and Strathclyde as motorists face treacherous driving conditions, while dozens of schools have been closed.
The Met Office has now issued amber "be prepared" warnings of snow and ice for southern Scotland and Northern Ireland.
A spokesman for the forecaster said: "It's looking pretty unsettled, there's a lot of weather to come.
"Through the night there will be some heavy showers which bring the risk of several centimetres of snow and risk of ice - tomorrow morning people need to be aware."
The warnings, which cover Central Scotland, Tayside, Fife, southern Scotland, Lothian and Borders, Strathclyde and Northern Ireland, are valid from 3pm on Tuesday until 8am on Wednesday.
Yellow "be aware" warnings of snow and ice across much of Scotland and Northern Ireland are in force until 6pm on Wednesday.
Similar warnings are also in place for parts of Wales, the Midlands and southern England through Tuesday evening until 11am on Wednesday.
In areas that saw snow overnight, temperatures dropped to 0C-2C (32F-36F).
Tulloch Bridge in Scotland had the largest snowfall with 8cm.
Gritters were out in force across Scotland, with pictures on social media showing particularly difficult driving conditions on the M8 near Easterhouse.
Drivers were asked to avoid the M8 after long delays were reported during rush hour.
Snow and sleet affected many roads in Scotland including the A9, M9, M80, A720, M77, A7 and M73.
The A9 at Dalwhinnie was closed for two hours after a lorry jack-knifed close to the snow gates.
More than 50 schools and nurseries in the Highlands have been closed due to the weather with over 3,000 children enjoying a snow day.
A number of schools in South Lanarkshire and Dumfries and Galloway have also been affected.
Northern England and Northern Ireland have also seen flurries of snow, with 1cm reported in Lough Fea, Northern Ireland, and further accumulations expected through the day.
Showers are expected to continue into the night with wintry falls on Wednesday, especially across the north.