Freezing Fog is a weather phenomenon that may seem rare, but is actually rather normal around these parts.
In typical fog-fashion, freezing fog is a direct result of water vapor condensing out of the air and forming a cloud at the surface level. This low-lying cloud, or fog, can remain in it's gaseous/liquid state for longer than pure water.
The freezing point of water is 32°, but a fog cloud, made up of particulate matter and water vapor molecules-- among other things-- can withstand the cold down to temperatures in the teens. This is mainly because the water vapor still needs something to freeze TO. On it's own, the fog can hover, but once the air reaches an environmental temperature below 14°, freezing fog becomes a concern as ice slowly adheres to local surfaces.
|Bowling Green State University Students walk to class in a post freezing fog environment|
The true nature of the phenom occurs when the water vapor clings, or freezes directly, to any and all surfaces. Roads, trees, power lines, etc. Anything in the environment with a cold enough surface temperature can accumulate ice that is freezing on contact with the fog clouds.
The deposition of ice on trees and other surfaces is known as hoar frost.
|Hoar Frost in Clyde, OH by @timafranks|
|Hoar frost pic by @NUTSHELMARSHALL|
Another phenom known as ice fog, may also occur, but only once the fog cloud is composed of tiny ice crystals, not water vapor droplets. This is typically in an environment that does not support supercooled water formation. This may also be referred to as 'Diamond Dust'.