Foggy weather has been especially persistent this week, not clearing all day in some areas of the UK.

Fog forms when relatively moist and mild air close to the ground cools quickly, causing the moisture in the air to condense (at which point it becomes visible to the human eye). This normally happens in autumn and winter under clear skies, which allows heat from the ground to escape quickly to cause rapid temperature drops.

Over the last few days winds have been light with an area of high pressure sitting rather stubbornly over the UK. This creates the ideal conditions for fog to form. As the fog is so dense in places the temperature has not been warm enough to cause the fog to evaporate, such as in more western parts yesterday.  However in eastern parts both yesterday and today the sun has got to work quite quickly on the fog, either lifting it into low cloud or breaking holes in it and eventually clearing it completely.

The fog is expected to clear today, with more changable weather on the way over the next few days. Keep up to date with your local forecast for the latest.

If you want to find out more about fog, have a look at our fog blog, ‘what is fog?‘ or our water in the atmosphere factsheet.