What is freezing fog and why is it dangerous?

Freezing fog? Yes it's a thing and something you should take extra caution in.

Winter is well underway here in the UK and it's a season that certainly divides opinion, with some wishing for a sprinkling of snow in the UK (opens in new tab) and others longing for it to get warmer (opens in new tab) and start getting lighter (opens in new tab) instead. Temperatures have dropped to freezing in some parts across the country during January 2023 which has meant plenty of frost and ice to contend with. But there's another weather condition that some aren't as familiar with: Freezing fog.

We Brits are well accustomed to a bit of regular fog here and there, but freezing fog is another variety that only crops up during very cold conditions. We've explained what it is and why you should take care when forecasters warn of freezing fog incoming. 

What is freezing fog?

Put simply, freezing fog forms when temperatures are below freezing. Water droplets in the air freeze on contact with solid objects like roads, footpaths, cars and trees. This quickly turns into ice, making for slippery and dangerous conditions.

The Met Office (opens in new tab) 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."

When does freezing fog occur?

"In the UK, we tend to get freezing fog during winter when temperatures are around freezing and there are clear skies and calm conditions," states the Met Office. 

The forecast for late January 2023 has seen warnings of freezing fog. Those in the East and Southeast of England are currently being warned of freezing fog conditions by the Met Office.

Why is freezing fog dangerous?

Freezing fog is dangerous because of the poor visibility and ice that comes with it. These conditions can make it difficult to fly, drive or walk outside.

Weather forecasters have urged people to avoid travelling in the fog if possible. Those who have to travel should drive very slowly with headlights dipped, the Met Office said. This is because full-beam lights reflect off the fog causing a "white wall" effect.

As parts of the UK currently grapple with freezing fog, airlines have been forced to cancel a number of flights due to poor visibility.

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