Freezing Fog

The Bergeron theory tells us that, at temperatures below 0°C, the air becomes saturated for the formation of ice before it becomes saturated for the formation of water. Hence water vapour will go directly to the solid-state at these temperatures. However, the rarity of freezing nuclei in the atmosphere means that when the dew point is below 0°C condensation will take place producing supercooled water droplets. These droplets will then freeze on contact with a solid object giving hoar frost (or rime ice). 

Freezing fog will also occur when the dew point is above 0°C forming fog but the air then cools to a temperature less than 0°C. 

Note: if water vapour is in contact with a solid object at temperatures below 0°C then it will immediately form ice (hoar frost) missing out the liquid state. 

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