We’ve had a few bouts of freezing fog lately (Picture: Getty Images)

After causing havoc earlier this year, freezing fog is once again having an impact on the UK, with flights being cancelled and road conditions becoming treacherous as a result.

As temperatures fall in parts of the country once again, the latest spell of fog led to 23 departing and arriving flights being cancelled at London City Airport, while others, including a flight to Geneva, and those arriving from Frankfurt and Glasgow, have been delayed.

The first fog of 2023 saw thousands of people facing cancellations at Heathrow, as more than 80 British Airways flights were grounded due to the weather.

But what is freezing fog and how does it form?

Here’s what you need to know.

How does freezing fog form?

January has seen London and many parts of the UK hit by freezing fog (Picture: George Cracknell Wright)

According to the UK Met Office, freezing fog forms in the same way as normal fog – when the land cools overnight under clear skies.

As it does so, the heat radiates upwards into space, cooling the earth’s surface.

This means that the air is less able to hold moisture, so the water vapour condenses into tiny droplets which eventually form fog – essentially a cloud at ground level.

But if the temperature is below freezing, these droplets become supercooled water and remain as a liquid.

That’s because to become ice, they need to land on something.

And when they do, you get a pretty deposit of feathery ice crystals known as ‘rime’. It’s usually on higher ground and mountains but we’ve seen it in the city this winter.

When freezing fog lands, it leaves feathery ice crystals known as ‘rime’ (Picture: Geoffrey Swaine/Shutterstock)

When does freezing fog form?

In the UK, it’s usually on winter days when we have clear skies, calm winds and the temperature is below freezing.

Just like we’ve had this January.

If you can see more than 1000m it’s called mist. Anything less than that is fog.

Is freezing fog dangerous?

Heathrow Airport faced major disruption caused by freezing fog (Picture:George Cracknell Wright/ Twitter/pilot_nick)

Not in itself. But it can lead to very dangerous travel conditions.

When visibility falls to below 50m, it’s known as dense fog.

The AA has given its top tips on driving in fog:

Use dipped headlights, plus your wipes and de-misters.

Use fog lights when you need to.

Beware of other drivers not using headlights.

Only drive as fast as conditions allow.

Slow down so you can stop in the distance you can see clearly.

Allow three seconds between you and the car in front instead of two.

Check your mirrors before you slow down.

Open your window to listen for traffic at junctions if you can’t see.

Don’t rely on automatic lights – they might not come on in fog.

Don’t tailgate.

Don’t speed up to get away from a vehicle behind you.

The freezing fog has left some picture-perfect scenes around the UK (Picture: James Linsell-Clark/ SWNS)

How much water is in freezing fog?

If you could somehow fill an Olympic-sized swimming pool with fog and then condense it, you’d be left with about 1.25 litres of water!

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