What is freezing fog? Met Office issues yellow warning

Weather warnings for freezing fog are currently in place (Damien Storan / PA Wire)
Weather warnings for freezing fog are currently in place (Damien Storan / PA Wire)

The Met Office has issued a yellow warning for parts of England, including west London, warning Brits of freezing fog that will lead to difficult driving conditions and travel delays in some areas.

Thousands of passengers at Heathrow have already seen their flights cancelled after the airport recorded its coldest night since December 2010.

As a result of the freezing fog, London has also been issued a “moderate” air-pollution alert, with foggy conditions making it harder for toxic fumes to dissipate.

Mayor of London Sadiq Khan commented on the weather conditions, saying: “We all need to be careful over the next few days.

“I’m urging Londoners to look after each other by choosing to walk, cycle, or take public transport, avoiding unnecessary car journeys, stopping engine idling, and not burning wood or garden waste, all of which contribute to high levels of pollution.

“This is particularly important in order to protect those who are more vulnerable to high pollution.”

What is freezing fog?

Freezing fog forms through exactly the same process that creates a normal fog, which sees land become colder overnight, radiating heat back into the atmosphere if there are clear skies.

This causes the air’s ability to hold moisture to plummet, allowing water vapour to become tiny droplets that eventually lead to fog.

However, when fog firms in temperatures that are below freezing level, water droplets stay liquid and become immensely cooled.

The Met Office explains that this happens because liquid can only freeze if it’s on a surface. When the droplets eventually make contact with a surface, they create white, feathery crystals known as “rime”.

The UK experiences freezing fog during wintertime, when the temperatures are at freezing levels, the sky is clear, and weather conditions are calm.