Blizzards and heavy snow have hit parts of Scotland and Northern Ireland, causing travel chaos and forcing schools to close.
A penguin at a Scottish safari park enjoys the cold snap
The cold weather has also caused frost and some snow in northern England.
Fife and central Scotland saw up to 25cm (10ins) fall in some places and a number of roads were closed, including sections of the A93, A81 and A85.
Dozens of schools closed in Northern Ireland and on Monday evening four men died in separate road accidents.
Drivers in many affected parts of the UK have been warned to leave extra time for their journeys.
The Road Service in Northern Ireland said it was some of the worst weather in recent years and driving conditions across the region were hazardous.
The north and west part of Northern Ireland were worst hit, with reports between 10 and 15cm (4 - 6in) of snow fell overnight.
Driving conditions are icy in Scotland and Northern Ireland
Driving was also made difficult and dangerous in Scotland by strong winds and heavy snow, which fell overnight.
Snow and icy patches caused the closure of A93 between Blairgowrie and Braemar, the A81 between Callander and Aberfoyle and the A85 between Killin and Crianlarich, while the A83 is closed five miles north of Furnace.
The cold weather also caused problems in the north of England.
Major routes remained open across the North East and Cumbria after the first significant snowfall of 2005 hit the region.
The M6 was passable, albeit with care, at its high point at Shap in Cumbria, as was the A66 near Bassenthwaite.
Elsewhere, snow on higher ground created slippery driving conditions.
But at lower levels the only problems reported were on untreated estate roads and hills.
Conditions to the south on Teesside and in Yorkshire were more manageable than other parts of northern England, with only the A59 proving difficult.
A number of ScotRail train services have been disrupted by the weather, as have several flights to and from Scotland's airports.
All schools in the Western Isles, where a weather alert was issued on Monday, will be closed on Tuesday.
A Met Office spokesman said: "A vigorous depression has led to this severe weather with cold showers bringing the snow."
The snow is expected to be replaced by rain later in the day, but forecasters have warned of winds gusting up to 70mph.
In Northern Ireland, which was hit by hail and snow showers on Tuesday morning, conditions are expected to become milder in the afternoon.