Fog and mist are both made of tiny water droplets suspended in air. The difference between them is the density. Fog is denser so contains more water droplets than mist. For pilots, if you cannot see further than 1000 meters, then it is called fog. For most people including motorists though, visibility has to be less than 200 metres before it is classed as fog.

Fog forms in many different ways, but generally the air near the ground can hold its moisture for longer, so water droplets condense to form fog. The amount of moisture that air can hold depends, amongst other things, on temperature. The cooler the air is, the less moisture it can hold, so fog can form. Fog usually clears soon after dawn as the sun heats up the air again, allowing it to hold more moisture.

At night, the ground loses or 'radiates' heat. In turn the air near the ground cools too and cannot hold as much moisture. Any excess moisture condenses into 'radiation' fog, the UK's most common fog. The longer the night and the clearer the skies, the more the temperature can drop, so the denser the fog can become. However, a wind can prevent fog from forming, because it 'mixes' the cool air by the surface with the warmer air above, preventing the air from getting cool enough for its water to condense.

Another way fog forms is for air to travel over a colder area of land, or sea. This will cool the air and its moisture will condense into fog. This 'Advection' fog often forms by the coast, especially in spring and summer when there is a greater difference between the temperatures of the sea and land.

'Steam fog' is sometimes seen rising from the ground after a shower. If the ground is warm, the water from the shower may evaporate. If the air above it is saturated and cannot hold any more water, the excess moisture condenses and looks like steam. This 'steaming' can also be seen at the Poles, when cold air sweeps over a slightly warmer sea. The sea warms the air a little and some water evaporates. The warmer damper air then rises, cools again and the moisture condenses. The sea seems to be steaming hot. Be warned though, it may look like a sauna, but it will feel more like an ice bucket!

Other features in the Weather Basics series:

- Clouds
- Day and Night
- Fog and Mist
- Fronts
- Frost
- High Pressure
- Humidity
- Jet Streams
- Lightning
- Low Pressure
- Mirages
- Northern Lights
- Rain or Showers?
- Thunder
- Watercycle