Heavy snow is sweeping eastwards across the UK, disrupting travel and leaving around 100 drivers stuck on a major trunk road in Cornwall.
The South West is worst affected - with teenage students at one college in Bodmin having to stay there overnight.
The freezing weather is moving across southern England and Wales, affecting Wiltshire, Hampshire, Sussex and Kent.
There are warnings of rail delays and flights cancellations into Friday morning.
The Met Office has warned of "treacherous driving conditions" in some southern areas.
There are two yellow warnings in place for snow and ice for large parts of the UK, lasting until Friday afternoon.
Snow depths of 12cm (5in) have been recorded in Bodmin, Cornwall, the Met Office said. Parts of Cumbria have seen 8cm of snow, while there was 7cm recorded in Inverness-shire and 5cm in Powys.
Disruption also includes:
- Dozens of schools in the county will be closed on Friday, while students at Callywith College have been forced to stay overnight
- Transport for London had advised commuters to try to finish their journeys by 21:00 GMT due to warnings of ice, sleet, snow and windy conditions
- British Airways has warned that some short-haul flights to and from Heathrow Airport on Friday morning have been cancelled or merged
- Southeastern Trains has introduced a winter timetable for Friday morning and South West Railway has warned of cancellations as tracks are cleared
- Newquay Airport was closed due to the snowstorm but has now reopened
- Some schools in Wales closed early and others have announced they will be shut on Friday
- Chepstow Racecourse has also cancelled Friday's meet
An "intensive effort" is taking place to rescue motorists stuck on the A30 at Bodmin Moor, local police said.
Highways England said traffic was moving "very slowly forward".
Paula Martin, chief executive of Cornwall Air Ambulance, was in one of the cars which got stranded.
"I got within two miles of home and the snow arrived very quickly and very suddenly.
"Within minutes the A30 very quickly became impassable, especially for the lorries - and it went from bad to worse", she said.
@HighwaysSWEST @DC_Police @BBCCornwall more people with small children giving up leaving the car and walking :( more than 3 hours now, how are the rescue heroes getting on? #A30 #Temple pic.twitter.com/od86hsGijV— Paula Martin (@PaulabridgetM) January 31, 2019
Cornwall Council has warned that abandoned vehicles on the A30 and other roads are delaying efforts to free trapped motorists.
Matt Argyle, 46, got stuck on the A30 at around 16:00 but was managing to stay warm after packing a sleeping bag and some extra clothes.
"A police search and rescue vehicle has just driven past but that's the first service vehicle I've seen," he said.
"There's no cars on the other side of the road so it must be completely blocked somewhere."
Journalism student Georgette Beacham, 28, got caught in the snow while returning from Plymouth to Falmouth.
She said: "Everyone was driving slowly but particularly on the hills, cars were jack-knifing and people were getting stuck going up hill and skidding going down hill and having to pull over."
British Airways advised customers to check their flight status for cancellations and said people due to travel on short-haul flights were being offered the option to postpone their journey.
"Safety is always our priority, and we're working hard to keep our operation moving. We're sorry that some of our services are being delayed by the weather conditions," a spokeswoman said.
Southeastern Trains said 21 trains were cancelled or altered to minimise the impact of ice forming on the rails.
It will run its "winter weather timetable" on Friday - with passengers warned of peak services being busier than normal because of changes to some train times.
South Western Railway has warned passengers that trains may be cancelled on Friday morning while the tracks are checked and cleared.
What's the forecast?
Up to 10cm of snow is expected in parts of the UK with snow depths of 5cm (2in) already recorded in Cornwall and Mid Wales, the Met Office says.
Further snow is forecast overnight into Friday, with 5-10cm (2-4in) expected in Wales and south west England.
In other parts of southern England, there could be 1-7cm (up to 3in) of snow.
BBC Weather presenter Stav Danaos said very heavy snow caused disruption for Thursday evening's rush hour across the south west of England.
Mid and south Wales, central and southern parts of England, and south east England are seeing the worst of the weather.
The north east of England and north Scotland will also see snow showers and cold temperatures on Thursday night.
Temperatures fell to their lowest level this winter, with Braemar, Aberdeenshire, dropping to -14.5C (6F) on Thursday evening.
This is the lowest in the UK since 2012 - when temperatures fell to -15.6C in Holbeach, Lincolnshire.
What warnings are in place?
There are yellow warnings for snow and ice until lunchtime on Friday in large parts of Wales and England, including London, and northern and eastern Scotland.
They warn of some snow, but not prolonged falls, and say some stretches of road will be icy.
An amber warning for heavy snow has now passed - it covered an area covering south Wales, south and south west England.
Much of Scotland and the north east of England will see snow showers continuing into Friday.
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