Fog and Mist

Fog and mist are both forms of condensation that occur near the ground, but they are not the same thing. The key to understanding them lies in what happens when a large mass of moist air cools suddenly.

When an air mass holding a great deal of water vapour experiences a sudden drop in temperature, condensation takes place directly within the air itself. This happens on fine dust particles floating in the air, which act as nuclei for the water vapour to cling to. The result is a cloud whose base sits at or very close to the ground — that is fog.

The practical difference between fog and mist is measured by visibility. Fog reduces horizontal visibility on the Earth's surface to less than 1 km. Mist, on the other hand, limits visibility to a range between 1 km and 2 km. So if you can see further than a kilometre but less than two, you are in mist; if you can see less than a kilometre, you are in fog.

In urban and industrial centres, smoke provides an abundant supply of nuclei, which actively helps the formation of both fog and mist. When fog gets mixed with smoke, the resulting condition is called smog — a term you will often hear in discussions of city air pollution.

The only difference between mist and fog, in terms of their composition, is that mist contains more moisture than fog. In mist, each nucleus carries a thicker layer of moisture around it. This is why mist feels wetter and denser with water than fog does.

Where each forms is also telling. Mists are frequent over mountains, because rising warm air moving up the slopes meets a cold surface and condenses. Fogs, being drier than mists, are prevalent where warm currents of air come into contact with cold currents. Fogs can be thought of as mini clouds in which condensation takes place around nuclei provided by dust, smoke, and salt particles.

| Feature | Fog | Mist |

|---|---|---| …