Much of Britain and Ireland was blanketed in snow on Wednesday as freezing Siberian weather dubbed "the Beast from the East", disrupted the travel plans of thousands.
Hundreds of schools were closed and there were delays on roads, railways and at airports on one of the coldest days Britain has experienced at this time of the year for almost three decades.
Temperatures fell to minus 12 Celsius (10 degrees Farenheit) in some rural areas while Britain's weather service warned of up to 40 cm of snow in higher areas of Scotland. Ireland could see its worst snow since 1982.
"It is the coldest air we have seen over the UK at this time of the year since around 1991," Becky Mitchell, a meteorologist at Britain's Met Office, said by telephone. "It is all tied in with the Beast from the East."
The cold spell has been caused by a jump in temperatures high over the Arctic, known by meteorologists as sudden stratospheric warming, which has weakened the jet stream that brings warm air in from the Atlantic to Ireland and Britain.
"The Jet Stream weakens and so we tend to get a strong easterly flow of air and that air is really originating right from over in Siberia - so very dry, cold continental air," Mitchell said.
The unusual weather could continue as Storm Emma, packing more snow and ice, approaches western England from Portugal and France.
Across London, there were severe delays on public transport. Ireland's weather service issued a status red warning - the highest level of alert - for five countries in the east, including the capital Dublin after heavy overnight snowfall led to accumulations of 5 to 10 cm.
Total snowfall may reach 25 cm by Thursday, it added, prompting the closure of many schools and universities. Business advised workers to stay at home with some roads inaccessible and public transport operating a limited service.
Runway operations at Dublin airport were suspended early on Wednesday with a number of flights cancelled and others diverted before the snow was cleared shortly after 0730 GMT to allow services to resume.
Photographs on social media showed bare supermarket shelves on Tuesday as people prepared for the cold snap that the weather service had warned could bring the heaviest snowfall since 1982.
The chief executive of Irish insurer FBD said on Tuesday she expected the snowfall would be an event for the industry, leading to frozen pipes and possible accidents, though not as severe as Tropical Storm Ophelia which battered Ireland in October.