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 METARs or TAFs is an obstruction to visibility based on surface visibility and temperature, and according to NWS directives, it is *not* considered a forecast for known icing conditions for in-flight operations.
While not considered "known icing conditions" for flight, freezing fog can lead to ground icing (rime) on aircraft surfaces, especially in dense radiation fog events with temperatures between 0°C and -10°C, or near large bodies of water.
Freezing fog is primarily a ground icing concern rather than an in-flight hazard, particularly in very cold temperatures where it consists predominantly of ice crystals; the paramount rule is never to take off with contaminated aircraft surfaces.

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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.