Forecasters are warning people across the UK to prepare for snow this weekend - after a rare phenomenon sparks freezing conditions.
Temperatures could drop to -5C at night, while snow could fall as early as Saturday.
A change in the upper atmosphere called a Sudden Stratospheric Warming (SSW) is responsible for the onset of the white stuff.
Dan Williams, spokesman for the Met Office, told the Daily Mail: "It would be fair to say that nobody should be surprised to see sleet or snow over the weekend and into the next week.
"We have got warm air trying to push in from the west and colder air pushing from the east and north east and there is a bit of a war into which will gain supremacy over the UK.
"With that combination there is the possibility that we could see some snow across any part of the country on Saturday and Sunday."
He added: "I think eastern areas will see sleet and snow at times over the weekend and it will certainly be colder across most places."
The news comes as widespread fog caused disruption at airports in the north west on Wednesday morning, particularly at Manchester, where a flight was cancelled, and Liverpool, where two flights due to land were diverted.
At weatheronline.com, duty forecaster Simon Keeling runs us over the the weekend and early next week, writing: "We have been warning for some time about the possibility of the weather turning colder through the second half of January. This does seem to be the case with winter arriving and bringing the threat of sleet and snow as well as severe frosts.
Saturday's outlook: "Fronts are wrapped around the low bringing cloud and periods of rain to most of England, Wales and Ireland. As the rain comes against colder air it may turn to snow over the hills of Wales and northern England, and perhaps even to lower levels. Highs at 1 to 3C for most, 8C in the southwest."
Sunday's outlook: "Wintry showers affecting eastern coasts, with some outbreaks of sleet and snow clearing from southern coasts of England. A cold day with highs at 1C to 5C."
Monday's outlook: "A cold day with periods of rain, sleet and snow in the west and an early severe frost. However, an area of low pressure may sink south through the UK and Ireland during the day bringing a much increased risk of snow later. Highs at 0C to 4C.
Tuesday's outlook: "Low pressure to the south brings a threat of snow here, with snow showers affecting eastern coasts. Brighter skies to the west but very cold. Highs of -1C to 2C."
And there are warnings the cold snap could last well into February.
An SSW always attracts interest in the UK as, more often than not, a cold period follows.
Find out exactly what it is here:
Freezing temps set to return after mild start to January
Freezing temps could last until February
Fancy a quick getaway? We don't blame you...