Is Freezing Fog Considered ‘Known Icing Conditions’?

What freezing fog is all about and does it constitute “known icing conditions”?

Wednesday, November 08, 2023 at 08:06 PM ET

Verified

Fog or freezing fog is an obstruction to visibility simply based on the measured surface visibility and temperature. [Shutterstock]

Key Takeaways:

Freezing fog (FZFG) in weather reports signifies an obstruction to ground visibility (0.5 statute miles or less) with temperatures at or below 0°C; however, it is not officially designated as "known icing conditions" for flight by National Weather Service directives.
While FZFG itself is not typically an in-flight icing hazard, it can lead to ice accretion on aircraft surfaces while taxiing, especially when static air temperatures are between 0°C and -10°C, particularly near large bodies of water or after precipitation.
At temperatures below -10°C, the likelihood of rime icing from FZFG is minimal, as the fog primarily consists of ice crystals rather than supercooled water droplets.

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Question: Is freezing fog considered “known icing conditions”?

Answer: As we start to move into the cold season over the next couple of months, you may begin to see freezing fog (FZFG) appear in a surface observation (METAR) or terminal aerodrome forecast (TAF). So, let’s get ahead of the game and discuss some high-level details of what freezing fog is all about and whether or not it constitutes known icing conditions.