Why is it that when the temperature is at 0c, it says fog instead of
freezing fog?

I guess if it’s +0 (i.e. between 0.0 and +0.4C) it would be fog. But
if -0C (i.e. between -0.1 and -0.5C it would be freezing fog.

Metars always get confusing around zero but the difference between +0
and -0 is significant.

Jack

"Joe Egginton" wrote in message
...
Why is it that when the temperature is at 0c, it says fog instead of
freezing fog?

Just because the temperature falls below the melting point of water it does
not mean that fog magically turns into freezing fog. Freezing fog is *NOT*
fog with a temperature below zero, despite what some say. For fog to be
freezing fog it has to be depositing rime. I have had this argument before,
as a professional observer many years ago in still air we had a fog with a
temperature just below zero, I was told off for not calling it freezing fog,
but I stood my ground as all surfaces were wet despite the temperature, in
no way was it rime icing.

Will
--

"Will Hand" wrote in message
...

"Joe Egginton" wrote in message
...
Why is it that when the temperature is at 0c, it says fog instead
of freezing fog?

Just because the temperature falls below the melting point of water
it does not mean that fog magically turns into freezing fog.
Freezing fog is *NOT* fog with a temperature below zero, despite
what some say. For fog to be freezing fog it has to be depositing
rime. I have had this argument before, as a professional observer
many years ago in still air we had a fog with a temperature just
below zero, I was told off for not calling it freezing fog, but I
stood my ground as all surfaces were wet despite the temperature, in
no way was it rime icing.


.... I agree with you Will, but unfortunately, according to ICAO, if
the air temperature is *below* 0.0°C, then it *has* to be coded (in
the METAR) as FZFG. I believe that for the SYNOP, then the 'old' rules
still apply, i.e., 'fog depositing rime' etc. However, with more AWS
coming on stream, I suspect the ICAO definition will win out.

Martin.


--
Martin Rowley
West Moors, East Dorset (UK): 17m (56ft) amsl
Lat: 50.82N Long: 01.88W
NGR: SU 082 023

"Martin Rowley" wrote in message
. ..
"Will Hand" wrote in message
...

"Joe Egginton" wrote in message
...
Why is it that when the temperature is at 0c, it says fog instead of
freezing fog?

Just because the temperature falls below the melting point of water it
does not mean that fog magically turns into freezing fog. Freezing fog is
*NOT* fog with a temperature below zero, despite what some say. For fog
to be freezing fog it has to be depositing rime. I have had this argument
before, as a professional observer many years ago in still air we had a
fog with a temperature just below zero, I was told off for not calling it
freezing fog, but I stood my ground as all surfaces were wet despite the
temperature, in no way was it rime icing.


... I agree with you Will, but unfortunately, according to ICAO, if the
air temperature is *below* 0.0°C, then it *has* to be coded (in the METAR)
as FZFG. I believe that for the SYNOP, then the 'old' rules still apply,
i.e., 'fog depositing rime' etc. However, with more AWS coming on stream,
I suspect the ICAO definition will win out.

Martin.

I fear you are right Martin. It's all very very sloppy. AWS are so annoying
sometimes as they don't do cloud type and when you are forecasting
convection there is no way to distinguish cumulus types.
Cameras and satpics have their place but you cannot beat the MK1 eyeball for
a lot of things still.
Very sad.

Will
--

Will Hand wrote:

"Joe Egginton" wrote in message
...
Why is it that when the temperature is at 0c, it says fog instead of
freezing fog?

Just because the temperature falls below the melting point of water it
does not mean that fog magically turns into freezing fog. Freezing fog
is *NOT* fog with a temperature below zero, despite what some say. For
fog to be freezing fog it has to be depositing rime. I have had this
argument before, as a professional observer many years ago in still air
we had a fog with a temperature just below zero, I was told off for not
calling it freezing fog, but I stood my ground as all surfaces were wet
despite the temperature, in no way was it rime icing.

Will


So is freezing fog super cooled water droplets that turned into ice upon
contact of an object in the sub zero air?

Joe Egginton
Wolverhampton

On Nov 9, 9:31*pm, Joe Egginton wrote:
snip
So is freezing fog super cooled water droplets that turned into ice upon
contact of an object in the sub zero air?

.... yes, that's the basic physics of what is going on. The final
product is rime ice, and in suitable conditions, it can build up
dramatically - essentially the same thing that aircraft encounter when
they fly through clouds of *very small* droplets, with weak updraughts
supporting the cloud development - stronger updraughts tend to larger
droplets and harder, clear or near-clear ice.

However, as Will points out, it is perfectly possible for NO rime to
form even though the air temperature is (usually just) below freezing:
the complex temperature changes / latent heat exchanges may mean that
the temperature of a particular object doesn't reflect the 'screen'
temperature - which is of course a rather artificial environment. I've
experienced several instances of this where surfaces remain wet for
some time.

Rime ice gives rise to some stunning pictures when the sun comes out!

Martin.

--

weatherhobbit wrote:
On Nov 9, 9:31 pm, Joe Egginton wrote:
snip
So is freezing fog super cooled water droplets that turned into ice upon
contact of an object in the sub zero air?

... yes, that's the basic physics of what is going on. The final
product is rime ice, and in suitable conditions, it can build up
dramatically - essentially the same thing that aircraft encounter when
they fly through clouds of *very small* droplets, with weak updraughts
supporting the cloud development - stronger updraughts tend to larger
droplets and harder, clear or near-clear ice.

However, as Will points out, it is perfectly possible for NO rime to
form even though the air temperature is (usually just) below freezing:
the complex temperature changes / latent heat exchanges may mean that
the temperature of a particular object doesn't reflect the 'screen'
temperature - which is of course a rather artificial environment. I've
experienced several instances of this where surfaces remain wet for
some time.

Rime ice gives rise to some stunning pictures when the sun comes out!

Martin.

--

Thanks for your reply Martin.

What would be recorded, if the scenario was:

Air Temp: -1c
Dew Point Temp: -0.5c

There's rime on a metal post, but no rime on a wooden fence. Would that
be recorded as freezing fog?

Joe
Wolverhampton
175m asl

"Joe Egginton" wrote ...
snip
What would be recorded, if the scenario was:

Air Temp: -1c
Dew Point Temp: -0.5c
.... Before recording anything here, I'd be re-checking my calculations
with the slide rule! Dew point temperature should be same as or
_lower_ than air temperature, here you've got the dew point
temperature *higher* :-)


There's rime on a metal post, but no rime on a wooden fence. Would
that be recorded as freezing fog?

.... as long as the fog was depositing rime-ice, then it should be
entered as freezing fog (for SYNOP & other records); When I was
observing (professionally), the best object to keep an eye on (and one
of the easiest to monitor as it was usually close to the office) was
the cloud-searchlight alidade: this is the metal plate with a movable
pointer which you used to assess the angle of elevation of the cloud
'spot', then the angle is converted to cloud height. Sometimes (as at
Northolt for example) the office was close to the flight-line and you
could watch for riming on the starter trucks or extinguisher trolleys
etc.

Martin.

--
Martin Rowley
West Moors, East Dorset (UK): 17m (56ft) amsl
Lat: 50.82N Long: 01.88W
NGR: SU 082 023