The Met Office has issued a yellow weather warning for fog that could freeze in some parts of the UK, with potential to cause transport disruption.The warning covers most of England and the eastern edge of Wales, with temperatures set to dip to lows of 1 degree celsius in London and zero in Cardiff overnight.
The national forecaster says the thick fog expected to blanket much of the UK overnight and beyond rush hour on Tuesday morning could freeze in some parts.
The warning covers a vast area south of Scarborough and east of Manchester in the north, and east of Exeter in the south, and is in place from 7pm on Monday until midday on Tuesday.
Temperatures are set to dip to near 0C overnight for many areas, meaning the fog could freeze on some roads, the forecaster has warned.
In recent days, social media users have snapped photos of thick fog descending on the capital as January's wintry conditions usher in a spell of frosty nights.
Met Office forecaster Craig Snell said motorists in parts of England and Wales affected by the yellow weather warning should allow extra time for their journeys.
He said: “The fog is going to be most widespread in southern, eastern England, into the Midlands and the east side of Wales too.
“Not everywhere within that warning will be foggy, but the main message to drivers is to allow some extra time for your journey because there could be fog around.
“It could have a knock-on effect on public transport, so there could be delays to bus journeys and train journeys.
“The fog will tend to clear as the morning goes on.
“It may linger on in a few spots, but towards lunchtime conditions should be much better.”
Mr Snell added that a combination of long winter nights, clear skies and still air can cause fog to settle.
“When the air is quite still, there’s nothing to move the fog around so that’s why it becomes dense,” he said.
“This time of year, our nights are long so we don’t have the heat from the sun, and the ground starts to radiate heat from the surface.
“This creates a long period of cooling.
“It’s a mixture of clear skies, light winds and long nights.”
Meanwhile, patchy sunshine and cloud is expected in Scotland and Northern Ireland.
The Met Office has said temperatures are around average for the time of year, with overnight temperatures of 3C (37.4F) predicted in Edinburgh, 2C (35.6F) in Belfast, 1C (33.8F) in London and 0C (32F) in Cardiff.
On Tuesday, the mercury is set to rise to around 7C (44.6F) in London, 8C (46.4F) in Edinburgh and Cardiff, and 9C (48.2F) in Belfast.
What is a 'freezing fog?'
The Met Office explains the frosty winter condition occurs when fog forms in temperatures that are below freezing.
The conditions are more often experienced at high ground, such as on mountain-tops and it is rare for it to take place at sea level.
In a freezing fog event, water droplets in the air remain as liquid and become "supercooled", remaining wet even though they are below freezing temperature.
The national forecaster explains the droplets remain wet as the liquid needs a surface to freeze upon.
"When droplets from freezing fog freeze onto surfaces, a white deposit of feathery ice crystals is formed," the Met Office's website explains.
"This is referred to as rime; rime is a characteristic of freezing fog and is often seen on vertical surfaces exposed to the wind."