How does fog form?

Fog is particularly common at this time of year, but do you know how it's formed?

Fog is a type of stratus cloud, often low-lying and therefore it can affect visibility.

Fog is distinguished from mist by its density and the extent to which it can decrease visibility. Fog reduces visibility to less than 1000 metres whilst mist reduces visibility to no less than 1000 metres.

Louise Lear has more.