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,
"...in our temp regimes and winter airmass scenarios we typically don't have that high of true liquid water droplet concentrations, but probably more suspended ice crystals...so we may get a bit of glaze but not a true rime icing scenario."
Many years ago I asked Dr. Marcia Politovich who is an aviation icing expert and she said,
"It's not likely that a surface temperature of -3°C or above could support mostly or only ice crystals, but history matters. Where the crystals came from is important (maybe they formed aloft in very cold air and are falling slowly through a warmer layer near the ground). The local ice nucleus source might not be as important as what would be in the air, carried from some other location. Also cloud nuclei (for water drops) would also be sourced locally so if the air is very clean it'd be hard to produce anything very near the surface."
Therefore, in a much colder temperature regime it's unlikely you'll experience much in the way of rime since the fog will consist predominately of ice crystals. If you are departing out of an airport near a large body of water or when the soil has been moistened after a precipitation event (other than snow) it is quite common to have some ice rime deposit onto the prop and other surfaces of the aircraft when the static air temperature is between 0°C and -10°C.
However, if there’s no ice accreted after you finish your preflight and/or if the static air temperature is below -20°C, then the likelihood of ice riming onto the airframe is minimal while the aircraft is waiting on the ramp or taxiing. Lastly, the most important rule is to never take off if the surfaces of your aircraft are contaminated with ice.
Most pilots are weatherwise, but some are otherwise™
Dr. Scott Dennstaedt
Weather Systems Engineer
CFI & former NWS meteorologist