Freezing Fog

[Victor Mike]

Sorry to trouble you all, but is there an official definition of freezing fog? Have looked through all my company docs + racal stuff with no luck.
Is it as simple as fog with temp below zero, or does it need to be mentioned on the ATIS as such? Holdover times aren't great...

Regards

Victor Mike

[Few Cloudy]

VM, it used to be defined in a past company in Switzerland (where they get a lot of it) as fog which freezes on to exposed surfaces. The supposition was that the surfaces were at ambient temp. and that the fog was composed of unstable super cooled droplets (ie below 0 deg) ,waiting for a contact to solidify them.

Trouble is, as you know, you can have ice formation from fog well over 0 deg., if the airframe is cold. Cold soaked tankered fuel in wing tanks can attract ice even over 10 deg OAT if the air is damp enough. The Hold Over Table is a pretty good guide of when to expect the traditional stuff but one peek is worth a thousand tables!

[Duke of Burgundy]

Victor Mike - the definition of freezing fog I am familiar with is " a visible aggregate of predominantly supercooled water droplets which do not fall to the ground. The visibility must reduce to 1000 metres or less and the air temperature must be below 0 degrees Celsius."

As I am sure you know the danger of freezing fog is that the supercooled water droplets tend to freeze on impact with surfaces and create a significant risk of airframe icing. The reason it is mentioned as a separately defined element in METARS, ATIS etc is to highlight this danger to aircrew.

[Victor Mike]

Thanks guys.
D of B does that mean any time its below 0 + below 1000m it's freezing fog, or only if it is super-cooled water as well?
I've got a bit of paper that says ATPL met passed, but I think it must have been meant for someone else...
Regards
Victor Mike

[Red Four]

VM
The correct way of coding is that if it is below 0 degrees, and less than 1000m,then it is FZFG. This is simpler for (relatively) untrained Met Observers to cope with and gives element of objective standardisation, rather than the subjective 'are the water dorplets super-cooled or not?'

Having said that, if you examine a lot of METARS in such conditions, many reporters still forget to call it FZFG when it is less than 0 degrees; I would suggest that if it is reported as FG (only) with the temp below 0, that you treat it as freezing fog, for an aircraft operation point of view.