How is fog formed?

You may have noticed mention of fog in the forecast recently, but what exactly is fog and how is it 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.

Phil Avery has more.