Freezing Fog

[stuharri2002]

Is there an international definiton for freezing fog. In the UK it is 1000m but i think that the US may use a statute mile?

thanks

stu

[BOAC]

Stu - it is 'FOG' that in the UK is less than 1000m vis due to water droplets. The 'freezing' bit just means that the droplets are 'super-cooled' below zero C. Don't know about the USA.

[mcdude]

Here's one;

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.

mcdude

[Omark44]

I believe it is correct to say that subject to satisfying all other minima, you may land in freezing fog but may not take off in freezing fog, in the UK, that is?.:confused:

[CJ Driver]

Omark44, can you point to a regulation or limitation that supports that view? I'm not aware of anything that prohibits it.

I'm specifically talking about regulatory limits, not aerodynamic ones. If there's fog freezing onto the wings, I'm not going flying. But if it isn't (perhaps because the airframe is above freezing, or is adequately de-iced, or the air isn't quite as cold as the met man thinks it is) then can I go flying, and if not, why not?

[BOAC]

CJ - I am too lazy (or too busy) to check but I have a feeilng that hold-over times for FF are so tight that it is difficult to go flying!

[CJ Driver]

BOAC - obviously I have too much time on my hands, because I went and looked it up. According to my Ops Manual, type I fluid can give as little as 6 minutes of protection, so there would need to be a de-ice bay near the departure runway. Type II fluid however, in a typical 50:50 fluid/water mix, is good for 20 to 45 minutes of protection against freezing fog. Ample to get from the parking stand to the runway, I would have thought.

But in any case, I don't dispute that it is difficult, but the original question (particularly interesting given that my Ops Manual apparently tells me HOW to depart in freezing fog) was - is there anything that prohibits trying it?

[80/20]

Another serious problem is ice on jet engine fan blades.
Freezing fog can cause very serious and expensive damage and there is no protection on common engines used in modern airliners.

[BOAC]

There may be specific, individual airline restrictions on it, but "20 to 45 minutes" could make life very difficult at major airports, bearing in mind when the 'clock' starts running and typical taxi to airborne times.

[idg]

BOAC,
Do you use Type IV fluid in europe now? Seems to have even better holdover times than Type II.

80/20,
You are correct, but when operating in such conditions we have to accelerate the engines to a mid-power setting to centrifuge the ice off the fan every few minutes....the RR Trent has a more complex procedure but the outcome is the same. Do you have a more limiting operating procedure?

idg

[BOAC]

Yes, idg - all types 1-IV

OK I've been forced to dig out the Ops Manual - probably a good time for a bit of 'revision' anyway!

Firstly, for 'Stu', here is one (UK) company's definition
"A suspension of numerous minute water droplets which freezes
upon impact with ground or other exposed objects, generally
reducing the horizontal visibility at the earth's surface to less
than 1 km."

Secondly, FF hold-over times vary from 'difficult to get airborne' to 'easy', depending on the temperature/mix/de-icing facility. I guess you have to play it as you see it (as usual) :D

[FullWings]

Looking at the holdover tables, I would tend to concur that freezing FOG is not that problematical (usually). After all, it's just like flying through cloud above the freezing level and we do that all the time. There are set/timed procedures for shedding ice build-up on fan blades and the holdover times using the better class of fluids are not too bad compared with other forms of precipitation.

The real 'nasty' is freezing RAIN and you wouldn't catch me even attempting to taxi in that...

[80/20]

80/20,
You are correct, but when operating in such conditions we have to accelerate the engines to a mid-power setting to centrifuge the ice off the fan every few minutes....the RR Trent has a more complex procedure but the outcome is the same. Do you have a more limiting operating procedure?
idg


Not really - idg, Boeing introduced a pre-takeoff 70%N1 static run-up procedure after a freezing fog situation where most airlines cancelled all flights but a few decided to depart. Many of the departing carriers experienced severe vibrations which resulted in 12 CFM engines with serious blade damages.

The static run-up might help, but there continues to be cases where the fan icing is too severe to be shedded. Some airports, experienced with this problem, offers a procedure where engines are shut down, front covered and heated by hot air and inspected just prior to takeoff.

I once observed thick clear ice on each fan blade and an “icicle-like" spear of clear ice the size of a small coke bottle sticking straight out of the spinner after only 5 minutes of taxiing in freezing fog. Freezing fog got my respect since it can cause rapid fan icing too severd to be sheeded without engine damage.
It can even be just fog which becomes freezing fog as pressure drops (expansion cooling) in the engine inlet.

BOAC,
Do you use Type IV fluid in Europe now? Seems to have even better holdover times than Type II.


Both are used in Europe, IV will give better protection around freezing, but both II & IV can give you as little as 15 min freezing fog HOT (hold-over-time) at colder temps. The freezing fog that I have encountered has always been below -5°C with very strong inversions.

The real 'nasty' is freezing RAIN and you wouldn't catch me even attempting to taxi in that...
FullWings... I agree 100%

[stuharri2002]

is this 1000m a definition/requirtement for just the UK? Does Amercia have a different definition, e.g 1 statute mile?

[TheOddOne]

Stu,

I appreciate that your original question hasn't been answered...

I had a trawl through the FAA sites and cannot find a proper quantifiable definition of 'fog' - just the usual 'water droplets, reduced surface visibility, cloud within 50 ft of the surface' stuff. It's almost as if they don't want to join in with the rest of us and admit to a definition based on metric measurements.

HOWEVER, do go to

http://www.faa.gov/avr/AFS/afs400/ac00-45e.pdf

and look at their METAR constructions for RVR reporting in 'statute' measurements.

If someone else can point to an FAA definition, I'd be pleased to see it!!!!

The Odd One

[80/20]

Stu it’s an interesting question.. Could you please expand a bit - are you asking for general definition, when it should be included in surface observation SA/METAR, forecast FT/TAF, ATIS and so on..
Where is the UK definition from (source)?

[stuharri2002]

I was looking to see if there was an international definition that everyone knew/used. The ICAO state that it is horizontal vis less than 1000m but that the temp doesnt nessesarily have to be below 0 degrees. The UK Met Office agrrees with this definition and the 1000m or less. However i know that Boeing use 1 statute mile. I presumed that evrybody used the same definition but it does not appear to be the case. Are there any other variations?


cheers

stu

[604guy]

In Canadian METARS if there is an obscuration to visibilty of the "fog" variety less than 5/8 sm it's listed as FG (fog). More than or equal to 5/8 sm it's listed as BR (mist). The addition of the descriptor FZ (freezing) requires a temp of less than 0'C.

[PAXboy]

non-pilot asking the obvious question:

What makes freezing rain nastier than freezing fog?

--------------------
"I tell you, we are here on Earth to fart around, and don't let anybody tell you any different." Kurt Vonnegut, Jr.

[BOAC]

Paxboy - in simple terms, fr fog is composed of small suspended 'supercoolled' water droplets which freeze instantly on meeting any (hard) surface. This makes a significant icing accretion rate. Freezing rain is larger, falling drops of same which do the same and REALLY accrete! Aircraft can very rapidly acquire a heavy coating of ice in fr rain with sometimes dire consequences. The rate of accretion can often defeat any deicing system on the aircraft and obscure windows. Best avoided if possible.

[B Fraser]

The definition of fog is when visual range is reduced to less than 1000 meters by the presence of airborne suspended water droplets in the direction of lowest visibility as viewed by the observer. If the lowest visible range is greater than 1000 meters due to etc. etc. then it is termed mist.

The droplets can be any temperature however if the droplets are observed to freeze onto a structure then the current weather is freezing fog. A dry bulb temperature below zero deg C is usually sufficient grounds to declare freezing fog. I could make a bad joke about hoar frost but that's for another day.

Freezing rain is extremely nasty as water droplets (either supercooled or > zero deg C) coming into contact with the ground or a structure at a temperature below zero will freeze on impact quickly covering the runway / taxiway / road home with an instant sheet of ice.

One surprising aspect of overnight radiation fog is it often thickens shortly after sunrise as a bit of thermal mixing stirs up the cold and moist layers.

Lovely stuff meteorology but the pay is absolutely :yuk:

[Astra driver]

The US defintion of fog is visibility below 5/8 statute mile, above that it is defined as mist.

[BOAC]

If it helps anyone else, a mile is 1,609.344 mtrs (roughly), so I guess 5/8 is ..............................around 1000m.

'Two countries separated by a common language' as someone famous once said:D

[stuharri2002]

thanks for the replies.

Astra Driver:
is this an official definition, what is the source, FAA definition?

thanks

stu

[Ice-bore]

"Freezing Fog

A suspension of numerous minute water droplets which freezes upon impact with ground or other exposed objects, generally reducing the horizontal visibility at the earth's surface to less than 1 km (5/8th mile)."

Published in: AEA 'Recommendations for De-icing / Anti-icing of Aircraft on the Ground' (19th Edition September 04), JAA ACJ OPS 1.345, CAA AIC 105/2003 and SAE ARP4737 Revision D (this is not the latest revision of the ARP).