Freezing fog

(Section 3.2.1.1.1)

“Freezing fog” occurs at temperatures below 0 °C when supercooled fog droplets freeze on impact with the ground or other objects to deposit rime. In aviation observing and forecasting, any fog consisting predominantly of water droplets at temperatures below 0 °C is reported as freezing fog whether it is depositing rime ice or not.

At temperatures typically below –10 °C, fog composed of ice crystals may form, usually from freezing of droplets. Like diamond dust, this gives rise to optical phenomena.

Fog may form in a variety of meteorological situations, but in all cases, its formation is as a result of the air becoming saturated. The mechanism of formation can be indicated with the following terms.