Met Office forecasters say a rare weather phenomenon could descend over the UK on Christmas Day, although temperatures are not expected to be ‘particularly cold’
A “rare” weather phenomenon could impact Brits on Christmas Day, according to the latest Met Office forecast.
The Met Office says it expects “unsettled” weather conditions over the Christmas period, with low pressure likely “centred somewhere to the southwest of the UK” and high pressure pushing in “from the east across northern areas”.
Its long-range forecast for December 21 to 30 adds that a “broadly easterly flow” will become established, while rain will be “increasingly confined to southern or southwestern” parts of the country. As we progress through the Christmas period, the Met Office expects high pressure to become more dominant, with more “dry and settled weather” expected.
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This means temperatures will likely be lower compared to recent weeks, “closer to or perhaps a little below average for some”. Although the Met Office says “particularly cold conditions look unlikely at this stage”, the forecast does add that a rare phenomenon – “freezing fog” – could become more widespread.
What is freezing fog?
The Met Office describes freezing fog as a “rare” weather event which tends to impact the UK “during winter when temperatures are around freezing and there are clear skies and calm conditions”.
We get freezing fog in the same way as regular fog when the land cools overnight under clear skies. Clear skies mean heat radiates back into space, which leads to cooling at the earth’s surface. This, in turn, means the air holds less moisture, which allows water vapour to condense into tiny water droplets – eventually leading to fog.
The Met Office explains: “When fog forms in temperatures that are below freezing, the tiny water droplets in the air remain as liquid. They become supercooled water droplets remaining liquid even though they are below freezing temperature.
“This occurs because liquid needs a surface to freeze upon. When droplets from freezing fog freeze onto surfaces, a white deposit of feathery ice crystals is formed. This is referred to as rime; rime is a characteristic of freezing fog and is often seen on vertical surfaces exposed to the wind.” According to the Met Office, it is rare for rime to develop at low levels in the UK, but it is more frequently seen on mountain tops and higher ground.
America’s National Weather Service says freezing fog can pose dangers. It states: “Extreme caution should be taken if travel is necessary. Freezing fog can cause black ice to form on roadways. Black ice is difficult to see and so particularly dangerous.
“Drive more slowly when you suspect icy conditions. For those flying, a thin layer of ice can form on aircraft, making flight very dangerous unless the aircraft is treated or has effective de-icing equipment.”
