Freezing fog has descended on parts of the UK, forcing airports to cancel flights - and more delays are expected.
Heathrow Airport says 70 flights due to take off on Wednesday have been scrapped.
A spokesman said further cancellations were likely as adverse conditions affecting large areas parts of the country worsen.
Flights to Warsaw, Nice, Stockholm, Aberdeen and several German cities are among those cancelled.
"There have been some delays as well as cancellations. There is bound to be a bit of a knock-on effect during the day. Also, some European airports are also affected by the cold weather."
There are also 20 cancellations at London City Airport. Passengers are urged to contact airlines to check details.
Sky News weather presenter Nazaneen Ghaffar said: "The freezing fog patches across central and eastern England, and eastern Wales, will continue to lift from the west this afternoon.
"However, it may linger for most of the day across parts of eastern England which could pose a problem during this evening's rush hour, as well as further flight disruptions.
"Into tonight we'll see the return of fog again, however it should be less extensive than Tuesday night as the breezes pick up in strength."
Monday night was one of the coldest of the year, with temperatures falling as low as minus 10C (14F), with Scotland worst affected.
After another bitterly cold night on Tuesday, much of the country woke up to sub-zero temperatures this morning .
The fog hit areas including central England, Wales and East Anglia.
There was congestion on a number of major road routes, while a section of the A1(M) between junctions 39 and 40 in Yorkshire was closed.
On the railways, overhead wire problems caused 50-minute delays in the rush-hour between Bedford and Luton, while a broken-down train at Watford Junction in Hertfordshire also caused hold-ups.
An electrical supply problem led to delays to train services between Seaford and Lewes in East Sussex, while a signalling problem caused hold-ups to services in and out of another Sussex coastal station - Brighton.
In Scotland, a broken-down train at Bathgate led to delays between Edinburgh and Airdrie.
On the Tube, there were severe delays on the Metropolitan line between Rickmansworth and Amersham and minor delays on the rest of the line due to a lack of trains.
There were also severe delays between High Barnet and Finchley Central on the Northern line due to a signal failure at Woodside Park.
Another signal failure - at Arnos Grove in north London - led to delays on the Tube's Piccadilly line between Arnos Grove and Cockfosters.
Police closed a three-mile stretch of the A24 southbound in the Horsham area of West Sussex after "treacherous" black ice caused 12 vehicles to skid and crash since 9.45am this morning.
Nobody was seriously hurt but a few drivers suffered minor injuries and damage to their cars, Sussex Police said. The northbound lane of the A24 is open but traffic is heavy.
The coldest spot in the UK on Tuesday night was Tulloch Bridge in the Highlands, where the temperature was minus 10.2C (13F).