Why has it been so foggy this week?

Have you found it harder to see on your way to school this week?

Or perhaps your football training or forest school been affected by the poor weather conditions.

Maybe a fireworks display you planned to see was affected by foggy conditions?

Well you are not alone.

Many parts of the UK have been waking up to thick fog over the past few days, with some areas seeing visibility reduced to just a short distance ahead.

In London, a high air pollution warning has been put in place by the city's mayor.

Forecasters at Imperial College London advised Lord Sadiq Khan to trigger the alert after "dense fog and very light winds allowed pollution to build up".

But what exactly is fog, and why has there been so much of it recently?

BBC Weather's Simon King says fog is essentially a cloud that has formed on the ground.

"Fog forms when it starts to get colder when the sun goes down. And because the air is getting colder, water vapour turns into droplets. This then creates a cloud.

"And its cloud on the ground that is what we call mist or fog. If the distance, we can see through it is more than one kilometre then we call that mist. If the cloud is thick enough so we can't see less than one kilometre, then that is fog".

So why has there been so much of it over the last few days?

Well, bonfire events last weekend have added to the poor visibility.

Simon King told Newsround that when wind conditions are calm, fog can get worse.

"The fog has been particularly thick at the moment because over the last few nights we've had lots of fireworks and bonfires for Bonfire night and Diwali.

"These have added a lot more tiny smoke particles into the atmosphere and these act like little seeds that help water vapour turn into droplets.

"The more of these smoke seeds there are, the more water droplets there are, creating a thicker cloud on the ground and meaning we can't see as well through it."

While the fog is thick, it can make some everyday activities a bit more difficult.

Travelling to school by car, bus, bike or on foot may be a little harder as it is trickier to see roads, signs and buildings.

Drivers are being told to take extra care in foggy conditions and may use special fog lights to help them see and be seen.

Pedestrians and cyclists are advised to wear high visibility clothing and take extra care crossing roads in areas of low visibility.

Outdoor activities such as football, rugby, or hockey training can be harder to run safely if coaches or players cannot see clearly across a field.

Have you had any of your sports sessions cancelled or rearranged because of the weather? Let us know using the comments section below.

Fog itself does not normally make healthy people ill, but some people with asthma or other breathing conditions may find symptoms become worse if there are extra particles in the air from smoke or pollution.

Fog is made up of tiny water droplets, while smoke is made up of tiny solid particles.

However, smoke particles can help fog form by giving water vapour something to cling to, which is one reason the fog has been so thick in some places this week.

If you have asthma, it is important to listen to medical advice and keep inhalers close by.

So how long is the fog going to last?

Well, forecasts show the weather is looking changeable into the second half of the week, with winds picking up, meaning thick fog is less likely to form.

However, there could still be some foggy mornings in Northern Ireland and Scotland.

Frosts are looking more likely across the UK with a colder weather front heading from the North.