There are dramatic variations in visibility around the UK
Thousands of passengers have seen their flights cancelled as dense fog has blanketed much of England over the past three days.
BBC weather presenter Chris Fawkes explains why the fog has lingered and gives some advice for Christmas travellers.
Why is there so much fog at the moment and how unusual is such severe fog?
This persistent fog has been with us recently because we've had a large area of high pressure with us, and this has given ideal conditions for fog to develop and thicken during the long nights.
It's not unusual at all to get fog during autumn and winter months when conditions are favourable, but the widespread disruption caused by the fog has been quite unusual. It is the result of the fact that the densest fog is sitting over the airport runways at one of the very busiest times of the year for people trying to travel.
Why is it remaining over such a wide area ?
The same "air mass" covers quite a large area of the United Kingdom, but yet there are quite dramatic variations in visibility in a very short distance.
For example Maidenhead has stayed sunny and clear today, yet, just 16 miles down the road, Heathrow has kept the fog with poor visibility.
There is little wind to shift the fog at the moment, and the sun above the fog has been too weak to dry the air out - so the fog has lingered.
When is it likely to clear up?
Fog will affect parts of the UK right up until Christmas, but the areas affected will change subtly from day to day.
The current thoughts of the Met Office are that the visibility at Heathrow, for example, may not be quite as poor on Saturday as it has been on Friday.
How does fog form and what is making it so dense?
Fog develops when a thin film of water forms around minute dust particles present in the air - the greater the number of these fog particles there are, the poorer the visibility will be.
An area of high pressure can be important for fog to develop for a number of reasons.
The high pressure can trap moist air near to the surface with very little wind to mix or dry the air. The high pressure can also bring cloud free air which helps the air nearest the ground cool rapidly during long nights.
As the air cools down, it can hold less water vapour, and so the vapour condenses which will then form a fog particle.
Are other countries affected by this?
With the high pressure centred over England and Wales, the fog has been more of a problem on this side of the English Channel, although there has been some morning fog in other parts of western Europe too.
What is your advice to Christmas travellers?
Keep up to date with the weather forecast as the foggy conditions can change quite quickly.
If you're planning to travel by plane over the next few days it will be worth checking with your operator before you travel as, even if the fog clears, it may take some time before operations get back to normal.