Warnings of widespread travel disruption and tricky driving conditions have been issued as snow, sleet and rain swept across large parts of the UK on Thursday.
The warnings came as a big chill enveloped all four nations. A temperature of -8C was recorded on Thursday morning in the village of Topcliffe, North Yorkshire, making it the coldest night in England of the winter so far, the Met Office said.
Richard Miles, a Met Office spokesperson, said wintry snow showers were possible everywhere from mid-Wales upwards, with northern England, Scotland and Northern Ireland bearing the brunt.
Blustery conditions would make the temperatures feel colder for many throughout the day and night as air from the Canadian Arctic swept across the UK, he said.
Four Met Office yellow warnings for the west of Northern Ireland, northern England and much of Scotland were in place on Thursday, all of them cautioning about snow, sleet, ice and difficult travel conditions which were likely to continue into Friday morning.
The alerts included warnings of thundersnow in Scotland, a relatively uncommon phenomenon.
“It is just thunder when it’s snowing,” Miles said. “It happens for the same reason that thunder and lightning happens in the summer, it’s to do with the temperature gradient. It is the same mechanisms but it sounds and looks slightly different in snowy conditions.”
The Met Office says the phenomenon is meteorologically identical to the thunder and lightning storms that take place in summer, just with snow instead of rain or hail.
This means it can look and sound different. Lightning reflecting off snowflakes appears brighter, and the thunder is muffled by snow drifts. During a summer storm, the thunder can be heard many miles away, but during a thundersnow event it will only be heard if you are within 2-3 miles of it.
Thundersnow is rarer than regular thunderstorms, since elevated convection – moist air rising into the troposphere to produce thunderstorms – rarely happens at temperatures that support snowfall.
Thunderstorms develop when a front of warm air collides with colder air making the atmosphere unstable. As warm air rises it cools and condenses, forming small droplets of water to create a cumulonimbus cloud in less than an hour. As the warm air continues to rise, the water droplets form ice crystals, and circulating air in the clouds causes water to freeze on the surface of the droplet or crystal. Eventually, the droplets become too heavy to be supported by the updraughts of air and they fall as hail.
Lightning is formed when the negative charge of the hail rubs against smaller, positively charged ice crystals and they discharge in a flash of lightning. The rapid expansion and heating of air caused by the lightning produces the accompanying loud clap of thunder.
Miles said there was a small risk of power cuts or mobile phone coverage issues because of the weather conditions.
The weather was affecting some ferry services in Scotland, including one of the busiest routes, between Ardrossan, in north Ayrshire, and Brodick, on the Isle of Arran. They were cancelled on Thursday and are expected to resume at 8.20am on Friday.
Sailings to the islands of Coll, Tiree, Iona and to Armadale on Skye were also cancelled due to strong winds and swell conditions.
Traffic Scotland tweeted safety tips for drivers.
The wintry conditions follow an unseasonably mild new year, with record temperatures as high as 16.5C in Bala, Wales, on New Year’s Eve.
“We are back to what you would expect in the winter,” Miles said. “It is more like average temperatures.”
The yellow warnings are in place until late morning on Friday, with the weekend weather across the UK expected to be unsettled and changeable.
