Norwood Park in south London at 0400 GMT (Photo by Clive, London)
Commuters are battling rush-hour chaos after heavy snowfalls across large swathes of England and Wales.
Luton, Stansted, Cardiff, Bristol and Birmingham airports have closed runways and train services have been hit.
Schools in parts of England and south Wales are shut and drivers are being urged to travel only if necessary.
Up to 5cm (2in) of snow has already fallen in parts of England and more is expected across central and southern England and Wales.
Some hilly areas are likely to receive up to 15cm (6in) of snow during the day, the BBC Weather Centre said.
The runway at Gatwick airport was closed because of the severe weather, but has now reopened. Passengers are being advised to check in as normal.
The runways at Luton, Stansted, Cardiff, Bristol, Birmingham and London City airports have also been temporarily closed.
British Airways warned that many short-haul flights from Heathrow airport would be cancelled after 1000 GMT.
Passengers are being advised to contact their airline before travelling.
The Highways Agency - which sent out gritting lorries overnight - has warned of poor driving conditions, and urged drivers to travel only if necessary.
A spokesman for the AA's Roadwatch service said motoring conditions were worst in the south-east of the country, with Surrey, London, Buckinghamshire, Hertfordshire and Essex all affected by the snow.
All schools in Birmingham, Solihull and Dudley have been closed, as well as some in Gloucestershire, Herefordshire, Worcestershire and East Anglia.
Nearly 100 schools are also closed in Wales but the figure is set to rise.
Much of the snow across central and southern England, including London, has coincided with the morning rush-hour, before moving north-eastwards towards Liverpool, Manchester and Derby.
Residents in Wiltshire, especially around the Devizes, Swindon and Chippenham areas, and Herefordshire have reported several inches in places.
Heavy snow fell in south and mid Wales overnight, with the south Wales valleys the worst hit.
A mixture of sleet and snow is expected in Northern Ireland.
On the railways, Virgin Trains warned passengers it would be running revised services and advised people not to travel unless absolutely necessary.
First Great Western, Midland Mainline and Southern Railways also said services would be disrupted.
On the Tube in London, services have already been disrupted on the Central, Bakerloo, Jubilee, Metropolitan, Northern and Piccadilly lines.
The Transport for London spokeswoman said staff were working to clear platforms which had been closed because of the weather.
In the evening, black ice and freezing fog could also become a hazard, especially across many parts of England.
The Highways Agency is advising drivers to check weather forecasts and road conditions before they set out.
"If the weather is severe, don't travel unless your journey is essential," it said.
People who must travel are advised to take warm clothes, food, water, a torch and a spade.
If the weather is severe, don't travel unless your journey is essential
Highways Agency spokesman