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Is freezing fog (FZFG) a forecast for airframe ice?

As we start to move into the cold season over the next couple of months you may begin see a forecast for freezing fog appear in a 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. A forecast for freezing fog (FZFG) is an obstruction to visibility much like mist or haze and is issued in a TAF when the forecaster believes...

  • The surface visibility is expected to be less than 1/2SM

  • The the surface temperature is expected to be at or below 0°C

  • The fog consists predominantly of water droplets

This is all true even if rime ice is not expected to be deposited on the airframe. Therefore, FZFG is not a forecast for airframe ice. But to know if freezing fog is an icing hazard is a bit more complex and depends on many factors (not all of them are specifically discussed here).

Freezing fog is primarily a ground icing issue, not an inflight icing concern. The two environments have a great deal of differences and may depend where you are departing. For example, you are flying out of Grand Forks, North Dakota which is landlocked, your chances of getting any significant liquid water content is probably pretty small especially for radiation-type fog events that occur at warmer subfreezing temperatures. In fact, the forecasters at the Grand Forks weather forecast office don't issue many TAFs with FZFG for that reason. The meteorologist-in-charge at the NWS Grand Forks Weather Forecast Office has mentioned to me,