Fog is typically defined as an area of high moisture that is close to the ground. One can basically think of this phenomenon as a low-lying cloud. Areas that frequently experience fog are generally very high in relative humidity. Visibility in thick fog is severely reduced, which is why this otherwise harmless weather pattern can prove quite troublesome in some situations (i.e., driving). It is also somewhat common in colloquial speech to define a sufficient mass of solid particles as fog. For example, it is not considered erroneous to describe a cloud of smoke as a thick fog. Akin to its ability to mask objects and lower visibility, one can also use the term in a metaphorical state. To say that one's mind is in a fog means that they are in a state of mental confusion.