When water vapour in the air condenses into tiny droplets of liquid water, they stay suspended in the air and fog is formed. The fog currently blanketing the UK is known as radiation fog, which forms under clear winter skies. This causes heat to leave the ground quickly and moisture in the air suddenly condenses into water droplets creating a sudden and thick fog to form. Radiation fogs occur at night, and usually do not last long after sunrise, but they can persist all day in the winter months especially in areas bounded by high ground.

Key Takeaways:

• Fog is kind of low level cloud that originates from a local body of water – such as a lake, marsh or the sea.
• The area forms the meeting place of the cold Labrador Current from the north and the much warmer Gulf Stream from the south.
• his causes heat to leave the ground quickly and moisture in the air suddenly condenses into water droplets creating a sudden and thick fog to form.

“More than half of flights from London City Airport were cancelled after the capital was blanketed by fog.”

https://www.thesun.co.uk/news/2339888/fog-uk-foggy-at-the-moment/