The London Weather Station

Fog is basically a cloud close to the ground. So for example if you are looking at a mountain and you see that it's peak is covered with clouds, then you would call it "clouds". But when you climb that mountain and you are standing on the peak, you would call the same clouds "fog". So it all depends on your point of view, literally.

Fog consists of millions of tiny droplets

A fog, or a cloud, could be described as very wet air. The water in the air is formed as tiny drops that are not (yet) heavy enough to fall down as rain. As soon as the droplets form larger drops, a drizzle will start. But as long as the droplets are tiny they will hover in the air very close together. They are so close, or dense, that light has difficulty to shine through. This is why cars have extra strong fog lights. This back light shines brighter so that the car can be seen from other cars coming up from behind.
Fog is the same as mist, but it is a bit denser, which means that there is more water in it. When you are in fog, you can only see things which are at closer than 1000 meters to you (3,380 ft), but when you are in a mist you can see further.

How fog starts

Fog will start if for example warm and moist land comes into sudden contact with much cooler air. When it gets colder, the water in the air cannot stay like an invisible gas, it if forced to condensate. This means that it forms little droplets that hover in the air and form the mist.
There are more fogs in autumn because the ground and the plants still have lots of water in them that constantly evaporates, especially if the days are sunny and quite warm, as so often in autumn. But the nights are longer and colder than in summer, and the cold air of the night turns the evaporated water into fog. On the next day, the sun may be able to heat up the air again so that the fog evaporates again into clear air, but if there is no or only little sunshine, the fog will only slowly dissolve.

Create your own fog

You can create your own fog when it is quite cold outside and you breathe into the air. Then you can see a little cloud coming from your mouth. This is because the air coming from your mouth is warm and moist, unlike the air surrounding you.

Check up-to-date weather information for London