With a large area of high pressure across the UK, conditions are ideal for the formation of dense fog.
Temperatures overnight into Monday morning dropped to around 3 to 5C across England and Wales which meant moisture at ground level condensed into water droplets, creating cloud – which is essentially fog.
Light winds then meant fog was able to get denser through the night, leading to visibilities less than 100 meters on Monday morning.
Fog has been particularly dense and widespread on Monday morning because there has also been a lot of fireworks and bonfires over the last few days.
Tiny smoke particules act as extra cloud condensation nuclei which allows more water vapour being able to turn into droplets.
Patchy fog could also develop outside the warning area in northern parts of the UK wherever skies remain clear for long enough.
Mist forms in the same way but is less dense, with visibility generally greater than 1,000 metres (1km).