What is freezing fog?

Freezing fog sweeps across UK on Monday

Freezing fog can form in the UK when temperatures are low (Getty Images)

With the UK in the midst of a cold snap, freezing fog has been wreaking havoc and causing dozens of flights to be cancelled at Heathrow.

British Airways, the largest carrier at Heathrow - UK’s busiest airport - has cut around 80 flights due to the difficult weather conditions.

On Monday, during the night, temperatures were the lowest at the airport for more than a decade.

The Met Office released a yellow weather warning for fog and it was in place until 11.00am on Monday for parts of southern and eastern England.

So what is freezing fog?

Freezing fog is a type of fog that, according to the Met Office, “forms in the same way as normal fog when the land cools overnight under clear skies.”

This is because if the skies are clear then heat radiates back into space which means that the surface of the earth cools. In turn, the air is less able to hold water which means that water vapour condenses into little water droplets which form fog.

Freezing fog deposits are known as rime (AFP via Getty Images)

However, as temperatures are below freezing when fog forms, the tiny water droplets in the air remain liquid.

The Met Office writes: “They become supercooled water droplets remaining liquid even though they are below freezing temperature.”

This is due to liquid needing a surface to freeze on and so when droplets from freezing fog freeze onto surfaces, a white deposit of feathery ice crystals are formed.

This deposit is known as rime and can often be seen on vertical surfaces that are exposed to the wind.

When do we see freezing fog in the UK?

In the UK, freezing fog tends to appear during winter when temperatures are around freezing.

Clear skies and calm conditions - as we have currently in the UK - are perfect for the formation of freezing fog.

Flights have been cancelled in the UK due to freezing fog (Getty Images)

Any rime that appears as a result of freezing fog “will be visible on the windward side of the object,” says the Met Office.

It is also “quite rare” for rime to develop at low levels in the UK, the Met Office writes.

Freezing fog is most frequent on mountain tops and higher ground.