Water vapor in the air is transparent, but when this water vapor becomes
cooled, a portion of it becomes precipitated, which is no more or less
than drops of water that are extremely small, but yet large enough to
become transparent, and the atmosphere in this state is called fog. In
reality, fogs are nothing more than clouds near the surface of the
earth. When the ground is at a higher temperature than the air, it
produces fogs. They are also produced when a current of moist air and a
current of hot air pass over a body of water at a lower temperature.
Consequently, you can easily see that fog will never form when it is
dry.