There is something magical about an autumn morning when you look outside and everything seems to have disappeared behind a blanket of white.
But where does fog come from? Why do we see it more often at certain times of year? And is mist actually the same thing?
There is some fascinating science happening right outside our windows.
What is fog?
Fog is essentially a cloud at ground level. Just like the clouds we see in the sky, it is made up of tiny water droplets suspended in the air.
Those tiny droplets scatter light and make it more difficult for us to see what is ahead of us. When visibility is reduced to less than 1,000 metres, it is officially classed as fog.
How does fog form?
One way fog forms is when the ground cools overnight. This cools the air close to the surface too.
As the air becomes cooler, it is less able to hold moisture. Eventually, the water vapour in the air condenses into tiny liquid water droplets. When enough of these droplets collect in the air close to the ground, we can see them as fog.
Clear skies, gentle winds and plenty of moisture can provide good conditions for this type of fog to develop.
Why do we see foggy mornings in autumn?
Fog is particularly common in the UK during autumn and winter.
The longer nights give the ground more time to cool. If the sky is clear, heat can escape from the Earth's surface more readily and temperatures near the ground can fall.
If there is enough moisture in the air and the conditions are right, fog can form.
Why does fog sometimes disappear when the sun comes out?
You might wake up to a very foggy morning only to find that it has disappeared later.
Radiation fog will usually disperse after sunrise as the ground begins to warm. This is why a morning can start looking grey and mysterious before becoming much clearer as the day progresses.
What is the difference between fog and mist?
Fog and mist are both created by tiny water droplets suspended in the air. The important difference is how much they affect how far we can see.
If visibility is less than 1,000 metres, it is classed as fog. If visibility is 1,000 metres or more, it is mist.
Mist is less dense than fog and usually disappears more quickly. Even a slight wind can help mist disperse.
Is fog really a cloud on the ground?
Yes. Fog really is a cloud at ground level.
Clouds become visible when water vapour condenses into tiny liquid water droplets. These droplets are small enough to remain suspended in the air.
So next time you and your child step outside on a foggy autumn morning, you can tell them that they are actually walking through a cloud.
What can we discover on the next foggy morning?
A foggy morning is a great opportunity to encourage your little scientist to look closely at the weather around them.
Ask them what they can see in the distance. Can they see as far as they normally can? What happens to the fog as the morning gets warmer? Does it stay all day or gradually disappear?
Sometimes the best science experiments are already happening right outside our front door.
