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) is reported in weather observations based solely on low visibility (0.5 sm or less) and temperatures at or below 0°C, a determination made independently of actual ice accretion.
According to National Weather Service directives, FZFG itself, as presented in METARs or TAFs, is not classified as "known icing conditions."
While primarily a ground icing concern, ice accretion from freezing fog is most likely between 0°C and -10°C, particularly near moisture sources, but is minimal below -10°C; regardless, aircraft should never take off with contaminated 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.