Mist and Fog

We have just learnt about water up in the air condensing high in the sky to form clouds. However, this moisture can also condense closer to the ground, to form what we know as mist and fog.

But what is the difference between mist and fog? The answer lies in their thickness – and how far we can see through them (ie: the visibility). If we can see less than 1 km through the cloud of water droplets, it is known as fog. If we can see between 1 and 2 km, we call it mist.

Key fact: although mist and fog look like smoke, they are actually tiny drops of water floating in the air. In fact, they are clouds that have formed at ground level.

Mist and fog usually form at night when the air is too cold to hold all its moisture. Clear skies mean that the ground gets cold and it then cools the air close to it. This cool air causes condensation and water droplets form in the air. Fogs are thickest when the air can hold a lot of moisture.

Although mist is not as thick as fog, they are both formed in this same way. Mist, however, usually stays closer to the ground and you can see over the top of it. Mist is often seen on autumn mornings when nights get longer and cooler again. This is particularly true in valleys, because cold air sinks down and collects in the valley during the night.

Mist and fog also form over areas where there is plenty of moisture, such as river valleys, lakes and harbours. If warm air meets cold seas or waters (or any cooler surface) then condensation will once again occur and fog will form. This explains the example of San Francisco, where the Golden Gate Bridge is often surrounded by mist and fog. This is formed as the warm California air from the south is chilled as it blows over the cold sea currents in the Pacific.

Pea-soup

Dust and smoke make fog much worse. In the early twentieth century, London was a very dirty city as a result of heavy industry and millions of coal fires. It was then famous for its fogs, which were "as thick as pea-soup". They were so thick and polluted that visibility (the distance you can see) would drop to 15 metres (50 ft) or less. As a result, the government was forced to try and clean up the city – and coal fires were banned in the 1950s. This dramatically reduced the number of fogs and fortunately pea-soupers are now a thing of the past!

Smog

However some cities still suffer from polluted fogs such as this – what we now call smog. Los Angeles is a particularly good example. Smog forms here not only because the city is situated in a low-lying area close to water. Fog also forms more readily when there are more condensation nuclei in the air to form on. In cities such as LA, these nuclei are provided by the large quantities of particles in the air. These particles come from car exhausts, fires and industry, and as well as helping fog and smog to form, they can also force cyclists and people working outside to wear masks. Have you seen people wearing masks when cycling before?

You have now learnt that clouds, mist and fog are all formed from moisture in the air. Sometimes though, this moisture falls to the ground in the form of rain, or even snow or hail. To find out more about these weather features, let's move on to the precipitation section by clicking on the sunshine icon below.  
Alternatively, you can have a look at some of the Activities related to the What Powers the Weather section (for example playing the cloud tile slider game), or return to the Weather Features homepage to select a different topic to learn about.