Why is it so foggy and how long will it last?

Rain last week and now a high pressure system over Britain mean conditions have been perfect for fog to form and linger

The fog blanketing parts of the UK is known to meteorologists as radiation fog. It has nothing to do with radioactivity, though.

Common in the winter, radiation fog builds when the land cools under clear night skies by thermal radiation. As the ground chills, so does the air directly above. And since cooler air holds less moisture, the further the temperature drops, the more moisture condenses to form layers of fog.