Supercooled Water - More on Icing

[O. Sami Saydjari]

This question is a question on physcial phenomena, NOT on regulation (so
I am starting a new thread).

As I understand it, icing happens between +2C and -10C. Assuming this
is correct, I have three questions.

(a) Does that mean one is relatively safe if the surface temperature is
below -10C (and there is no temperature inversion, meaning that the
temperature is known to decrease as one goes up in altitude)?

(b) Is icing from 0C to +2C a possibility only when your aircraft skin
is colder than 0C (probably because you are descending from altitude)?
Or is there some other condition where this is possible?

(c) I have been told that icing is possible from -10C to 0C because
water sometimes get "super-cooled" (which I assume means that water gets
below freezing, but does not form ice for some reason). Is that right?
If so, under what atmospheric conditions does water get super-cooled?

[Mike Rapoport]

"O. Sami Saydjari" > wrote in message
...
> This question is a question on physcial phenomena, NOT on regulation (so
> I am starting a new thread).
>
> As I understand it, icing happens between +2C and -10C. Assuming this
> is correct, I have three questions.
>

MOST icing occurs between 0C and -10C but icing has been shown to occur
at -40C. Icing in cumulus clouds virtually always extends below -10C.and
often to below -20C


> (a) Does that mean one is relatively safe if the surface temperature is
> below -10C (and there is no temperature inversion, meaning that the
> temperature is known to decrease as one goes up in altitude)?
>

Ice is less likely to be a problem than if it was 0C at the surface but,
again, icing can occur at temperatures much lower than -10C particularly in
clouds with vertical movement (cumulus).
..
> (b) Is icing from 0C to +2C a possibility only when your aircraft skin
> is colder than 0C (probably because you are descending from altitude)?
> Or is there some other condition where this is possible?
>
This was the conventional thiinking buy a recent NASA study was unable to
document ANY ice formation above 0C.

> (c) I have been told that icing is possible from -10C to 0C because
> water sometimes get "super-cooled" (which I assume means that water gets
> below freezing, but does not form ice for some reason). Is that right?
> If so, under what atmospheric conditions does water get super-cooled?
>
If you do a search on supercooling you will find a lot of info. Basically
water almost NEVER cools to 0C and then starts crystalizing. It usually
cools to -7C before crystalizing starts. Once the crystalization starts the
temperature increases as the latent heat is released and when the last water
freeezes the temp is at 0C again. You can verify this with a glass of
water.and a thermometer. This assumes that the water is still. It the
water is turbulent then the temperature will go even lower before
crystalization starts.

Mike
MU-2

[Roy Smith]

"O. Sami Saydjari" > wrote:

> This question is a question on physcial phenomena, NOT on regulation (so
> I am starting a new thread).
>
> As I understand it, icing happens between +2C and -10C.

There was just a posting from Mike Rapoport in which he said he's seen
icing as cold as -20C.

> (b) Is icing from 0C to +2C a possibility only when your aircraft skin
> is colder than 0C (probably because you are descending from altitude)?

I don't really know the answer to this one, but I will point out that
just because your OAT gauge reads +2C, doesn't mean it is. If you've
got the standard "meat thermometer" type, it's likely that the last time
it was calibrated was 25 years ago when it left the factory (if then).
I wouldn't trust it to be accurate to +/- 2C.

> (c) I have been told that icing is possible from -10C to 0C because
> water sometimes get "super-cooled" (which I assume means that water gets
> below freezing, but does not form ice for some reason). Is that right?
> If so, under what atmospheric conditions does water get super-cooled?

For water vapor to freeze, you need three things. First, you
(obviously) need water. Second, the temperature has to be below the
freezing point. And third (this is the one most people don't realize),
you need what's called a "condensation nucleus". This is some piece of
solid matter providing a surface on which the phase change (i.e. liquid
to water) can occur. It could be a tiny dust spec, or in a marine
environment, tiny salt crystals in the air serve the same purpose.
Think of it like a catalytic converter.

When water freezes, it releases a lot of energy (called the heat of
fusion). If memory serves, it takes 1 calorie per gram per degree to
cool liquid water, and 80 calories per gram to go from liquid at 0C to
solid at 0C. That energy has to go somewhere. I believe what the
condensation nucleus does is provide a heat sink for that energy.
There's also an energy barrier tied up in surface tension, as you go
from a spherical droplet of liquid to an ice crystal.

If you want to see a good demonstration of how much heat it takes to
effect a phase change, go to the drugstore and buy a bottle of rubbing
alcohol. Wipe some on your arm and feel how cold your arm gets. What's
going on is the alcohol is changing phase from liquid to vapor and the
energy (heat) to do that is coming from your arm.

It's been a long time since I took physical chemistry, so I'm afraid I
can't give a better explanation than that. If you want to persue the
topic further, I would suggest googling for "condensation nucleus" or
perhaps consulting an advanced meteorology textbook.

Anyway, what's going on with supercooled water is that there's no
condensation nuclii available for the droplets to freeze onto. Along
comes the leading edge of your wing and the droplets go SPLAT! As far
as the droplet is concerned, your leading edge is just the mother of all
condensation nuclii and it instantly freezes.

[Julian Scarfe]

Adding to Roy's and Mike's excellent responses...

"O. Sami Saydjari" > wrote in message
...
> This question is a question on physcial phenomena, NOT on regulation (so
> I am starting a new thread).
>
> As I understand it, icing happens between +2C and -10C. Assuming this
> is correct, I have three questions.

Bear in mind that the upper and lower limits are for different reasons.
Above the upper limit, no part of your airframe should be cold enough to
freeze the water. Below the lower limit, the water has already frozen, so
it doesn't form ice on you. In cumuliform cloud, supercooled water persists
well below -10 degC.

> (a) Does that mean one is relatively safe if the surface temperature is
> below -10C (and there is no temperature inversion, meaning that the
> temperature is known to decrease as one goes up in altitude)?

Such surface temps often happen because of radiative cooling of the surface.
It's likely that there *will* be a temperature inversion as the air mass
doesn't usually cool that much, except in places that stay cold for very
long periods. It's always worth checking the temps aloft too.

> (b) Is icing from 0C to +2C a possibility only when your aircraft skin
> is colder than 0C (probably because you are descending from altitude)?
> Or is there some other condition where this is possible?

In principle, even in equilibrium, parts of your wing where there's low
pressure can be below zero while your OAT probe is above zero. The
magnitude of this effect depends on wing loading, so it tends to be much
greater for big jets than GA aircraft.

Julian Scarfe

[Matthew S. Whiting]

Mike Rapoport wrote:
> "O. Sami Saydjari" > wrote in message
> ...
>
>>This question is a question on physcial phenomena, NOT on regulation (so
>> I am starting a new thread).
>>
>>As I understand it, icing happens between +2C and -10C. Assuming this
>>is correct, I have three questions.
>>
>
>
> MOST icing occurs between 0C and -10C but icing has been shown to occur
> at -40C. Icing in cumulus clouds virtually always extends below -10C.and
> often to below -20C
>
>
>
>>(a) Does that mean one is relatively safe if the surface temperature is
>>below -10C (and there is no temperature inversion, meaning that the
>>temperature is known to decrease as one goes up in altitude)?
>>
>
>
> Ice is less likely to be a problem than if it was 0C at the surface but,
> again, icing can occur at temperatures much lower than -10C particularly in
> clouds with vertical movement (cumulus).
> .
>
>>(b) Is icing from 0C to +2C a possibility only when your aircraft skin
>>is colder than 0C (probably because you are descending from altitude)?
>>Or is there some other condition where this is possible?
>>
>
> This was the conventional thiinking buy a recent NASA study was unable to
> document ANY ice formation above 0C.


However, keep in mind that most thermometers have some error in them.
So, even though ice doesn't form above 0C, it may form above 0C as
indicated on your airplane thermometer.


Matt

[Tarver Engineering]

"Roy Smith" > wrote in message
...
> "O. Sami Saydjari" > wrote:
>
> > This question is a question on physcial phenomena, NOT on regulation (so
> > I am starting a new thread).
> >
> > As I understand it, icing happens between +2C and -10C.
>
> There was just a posting from Mike Rapoport in which he said he's seen
> icing as cold as -20C.

Icing is statistical in nature and contrary to pilot observations to the
contrary, is more a function of the size of the droplets, than some exacting
temperature range. FAA studied icing under a program tittled to reflect
"large droplet" icing, but the results of the study were the opposite of the
observed information. (large vs small droplets) You are in more danger of
an icing event where the size of the droplets is small and the airplane is
small.

[Mike Rapoport]

You still have it partly backwards on droplet size. In general, large
droplets are worse, primarily because they take more time to freeze and end
up freezing behind the protected surfaces leaving a "ridge" of ice on the
top and bottom of the wing. The ridge then functions as a spoiler. I do
recall reading that when the drops get really large (huge), like in
freezing rain they do not form the high drag shapes of drizzle size drops.
So basically the *effects* of icing get worse as droplet size increases to
some maximium size and then diminishes.

Droplet size is related in some way (I forget exactly what way) to
temperature with large drops being unlikely at really cold temperatures, so
the 0C to -10C caution range is useful since you will only find moderate or
greater icing at temps below -10C where there is a lot of vertical movement
like in cumulus, cumulonimbus and (occasionally) wave clouds.

Small radius surfaces indeed collect more ice than large ones because the
"preasure wave" they form as they advance through the air does not extend as
far forward.

Mike
MU-2


"Tarver Engineering" > wrote in message
...
>
> "Roy Smith" > wrote in message
> ...
> > "O. Sami Saydjari" > wrote:
> >
> > > This question is a question on physcial phenomena, NOT on regulation
(so
> > > I am starting a new thread).
> > >
> > > As I understand it, icing happens between +2C and -10C.
> >
> > There was just a posting from Mike Rapoport in which he said he's seen
> > icing as cold as -20C.
>
> Icing is statistical in nature and contrary to pilot observations to the
> contrary, is more a function of the size of the droplets, than some
exacting
> temperature range. FAA studied icing under a program tittled to reflect
> "large droplet" icing, but the results of the study were the opposite of
the
> observed information. (large vs small droplets) You are in more danger of
> an icing event where the size of the droplets is small and the airplane is
> small.
>
>

[Mike Rapoport]

"Matthew S. Whiting" > wrote in message
...
> However, keep in mind that most thermometers have some error in them.
> So, even though ice doesn't form above 0C, it may form above 0C as
> indicated on your airplane thermometer.
>
>
> Matt
>
That is true and in addition there is some aero heating of the airplane as
it travels through the air and different parts of the airplane will
experience different amounts of heating. Most turbine airplanes have a
chart that shows OAT guage error at different altitudes and airspeeds.

Mike
MU-2

[Tarver Engineering]

"Mike Rapoport" > wrote in message
nk.net...
> You still have it partly backwards on droplet size.

Let me provide you with a more probabilistic view of the data you are
relying on, being "observed". Now in fact FAA funded a study of icing from
large droplets, as that was the "observed phenomena". Consider an
experiment where 50 pilots encounter large droplet icing and 50 pilots
encounter small droplet icing. At the end of the experiment, the group with
"observed" large droplet reports 49 incidents, while the small droplet
"observed" group has only 5 advocates.

[Mike Rapoport]

I have no idea of what you are trying to say. Aircraft performance in icing
is measured in both wind tunnels and in flight. It doesn't matter what a
bunch of pilots think, the data is quantified and it shows that the
performance degradation is highest with large droplets which form ice in
ridges aft of the leading edges.

Mike
MU-2


"Tarver Engineering" > wrote in message
...
>
> "Mike Rapoport" > wrote in message
> nk.net...
> > You still have it partly backwards on droplet size.
>
> Let me provide you with a more probabilistic view of the data you are
> relying on, being "observed". Now in fact FAA funded a study of icing
from
> large droplets, as that was the "observed phenomena". Consider an
> experiment where 50 pilots encounter large droplet icing and 50 pilots
> encounter small droplet icing. At the end of the experiment, the group
with
> "observed" large droplet reports 49 incidents, while the small droplet
> "observed" group has only 5 advocates.
>
>

[Bob Gardner]

Gotta wonder why the AWC icing page in ADDS offers the user to choose
between a graphic display of "all icing" and of "SLD icing" if Supercooled
Liquid Droplets are not a hazard. The Roselawn accident (ATR-72, if I recall
correctly), was attributed, in part, to SLD icing.

Bob Gardner

"Tarver Engineering" > wrote in message
...
>
> "Mike Rapoport" > wrote in message
> nk.net...
> > You still have it partly backwards on droplet size.
>
> Let me provide you with a more probabilistic view of the data you are
> relying on, being "observed". Now in fact FAA funded a study of icing
from
> large droplets, as that was the "observed phenomena". Consider an
> experiment where 50 pilots encounter large droplet icing and 50 pilots
> encounter small droplet icing. At the end of the experiment, the group
with
> "observed" large droplet reports 49 incidents, while the small droplet
> "observed" group has only 5 advocates.
>
>

[Icebound]

O. Sami Saydjari wrote:
> This question is a question on physcial phenomena, NOT on regulation (so
> I am starting a new thread).
>
> As I understand it, icing happens between +2C and -10C. Assuming this
> is correct, I have three questions.
>
> (a) Does that mean one is relatively safe if the surface temperature is
> below -10C (and there is no temperature inversion, meaning that the
> temperature is known to decrease as one goes up in altitude)?
>


1. How the cloud got formed is as important as the current temperature
of the cloud.

As many have pointed out, cumuloform clouds often retain supercooled
water well below -10.

But not only that, the original air that formed the cumuluus cloud may
have come from low, warm, humid levels. If the air in the cloud started
out at, say, 20 degreec C with a 15 C dewpoint, it will have condensed
out about 11 grams of liquid water per cubic meter by the time it cooled
to -10. (This is also why the worst icing in cumuloform clouds is often
near the top.)

If the air started with a dewpoint of -5, it will have condensed about 1
gram per cubic meter... only about a tenth of the above scenario.

If the air in the cloud STARTED with a dewpoint of -10 or less, the
cloud will have condensed less than 2 grams of liquid water per cubic
meter NO MATTER HOW COLD IT GOT.

2. As Roy mentioned, water condenses only onto a nucleus of some type.
In nature these nuclei are MOSTLY the wrong shape for ice crystals to
form, so it is natural for the condensation to stay liquid as long as
possible, and -10 or so appears to be "common". (By the way, Roy, the
latent heat release when it DOES freeze just goes to warm up the air a
tiny bit). Typically, once a few drops DO freeze, or when snowflakes
are fall into the liquid layer from above, then the liquid will migrate
to the ice fairly quickly...

Below -10, it is common that at least SOME ice crystals will form, and
this usually erodes the water content quickly. However, in vertically
developing cloud (Cumulus, etc.) the updrafts may prevent the
introduction of ice crystals for some substantial period of time.

[Tarver Engineering]

"Mike Rapoport" > wrote in message
ink.net...
> I have no idea of what you are trying to say.

OK.

> Aircraft performance in icing
> is measured in both wind tunnels and in flight. It doesn't matter what a
> bunch of pilots think,

There you go.

> the data is quantified

Yes.

> and it shows that the
> performance degradation is highest with large droplets which form ice in
> ridges aft of the leading edges.

But icing events that lead to catastrophic failure are more likely to occur
in small droplets forming rapid acretion on the tail surface. (rudder
reversal)

[Tarver Engineering]

"Bob Gardner" > wrote in message
news:xkRAb.329519$9E1.1623342@attbi_s52...
> Gotta wonder why the AWC icing page in ADDS offers the user to choose
> between a graphic display of "all icing" and of "SLD icing" if Supercooled
> Liquid Droplets are not a hazard. The Roselawn accident (ATR-72, if I
recall
> correctly), was attributed, in part, to SLD icing.

Management of acretion may have been a factor in the ATR-72 AP problems, but
we know now that f the operator disables the autopilot in such conditions
that the airplane remains in controlled flight. There is a second issue
with certain German Litton gyros and their switcher power supplies passing
through 0 C, but that is something clearly referenced on the equipment's
boiler plate.

[Julian Scarfe]

"Matthew S. Whiting" > wrote in message
...

> However, keep in mind that most thermometers have some error in them.
> So, even though ice doesn't form above 0C, it may form above 0C as
> indicated on your airplane thermometer.

Absolutely true, but remember that your thermometer is one of the easiest
instruments in the aircraft to calibrate. The ATIS gives the temperature on
the ground before flight -- it's well worth a check.

Julian Scarfe

[Nathan Young]

"O. Sami Saydjari" > wrote in message >...
> This question is a question on physcial phenomena, NOT on regulation (so
> I am starting a new thread).
>
> As I understand it, icing happens between +2C and -10C. Assuming this
> is correct, I have three questions.
>
> (a) Does that mean one is relatively safe if the surface temperature is
> below -10C (and there is no temperature inversion, meaning that the
> temperature is known to decrease as one goes up in altitude)?

Sami, I believe you are in Wisconsin - in the Midwest this is
generally the case, but as you know, with icing there are no
guarantees.

-Nathan

[Dave Butler]

O. Sami Saydjari wrote:
> This question is a question on physcial phenomena, NOT on regulation (so
> I am starting a new thread).
>
> As I understand it, icing happens between +2C and -10C. Assuming this
> is correct, I have three questions.
>
> (a) Does that mean one is relatively safe if the surface temperature is
> below -10C (and there is no temperature inversion, meaning that the
> temperature is known to decrease as one goes up in altitude)?

If there is any chance you will pick up ice at some altitude, you'll want to be
sure there is some altitude you can get to where you can shed the ice before
landing, unless you can land with an iced-up windscreen.

For me, that means somewhere within range where the temperatures are above
freezing at the surface.

This is probably not possible in Wisconsin for a big part of the year.

I've never had the experience of accumulating ice and then not being able to
descend to above-freezing temperatures, but I'd think it would be a pretty
serious situation without a heated windscreen. When I've had an iced-over
windscreen, the defroster wouldn't touch it. It was only descent into warmer air
that allowed me to see again.

Maybe someone who has landed with ice when the surface temps are below freezing
can commment on the windscreen issue.

Dave
Remove SHIRT to reply directly.

[Doug]

It is not just a calibration problem (accuracy of themometer), nor is
it just the problem of getting an accurate static temperature (due to
moving air). There are low pressure areas on the wing surfaces, and
lower pressure causes the air temperature to go down, in these places.
So these are all reasons why we don't use zero degrees (which IS
required to freeze water), but use some temperature a little higher.

I use the airmets as my guide. If there is an airmet for icing, I
don't go. Even then, it is possible to pick up ice. I have also broken
this rule when I talked to an incoming pilot who flew the other way
through the clouds, and he informed me that not only did he not pick
up ice, he didn't think there was any ice in those clouds. It was a
bit risky, but he was right, no ice. It was the tail end of the front
(I have found less ice in the tail end of the fronts than the leading
part), and the icing airmet hadn't gone away, yet, but the clouds were
"dry". None of the PIREPS indicated icing either. It think I was legal
as I had more complete information than the airmet (debatable). I
agree with MU-2 Mike in that we do get ice below -10 degrees Celcius.
It is also possible to get ice when there is no airmet, so always have
an out.

One big reason the airliners can deal with ice is they have the climb
ability to outclimb the ice. A turbine or at least a turbo charged
aircraft (with either oxygen or pressurization), has protection at
least as good as having deice system. Also keep in mind, that if you
go fast enough (300 knots or so), you wont get ice due to friction
warming of the wings (specific info needed on the type from the mfg
for this, no general rule).

One final piece of information. The smaller diameter of the surface
area, the more ice will accumulate. This is why antennaes and struts
get more ice. This is due to the physics of the air in front of the
surface. Large diameter surfaces deflect the air coming toward the
surface more thand smaller diameter surfaces.



"Julian Scarfe" > wrote in message >...
> "Matthew S. Whiting" > wrote in message
> ...
>
> > However, keep in mind that most thermometers have some error in them.
> > So, even though ice doesn't form above 0C, it may form above 0C as
> > indicated on your airplane thermometer.
>
> Absolutely true, but remember that your thermometer is one of the easiest
> instruments in the aircraft to calibrate. The ATIS gives the temperature on
> the ground before flight -- it's well worth a check.
>
> Julian Scarfe

[Peter R.]

Doug ) wrote:

<snip>
> I
> agree with MU-2 Mike in that we do get ice below -10 degrees Celcius.
> It is also possible to get ice when there is no airmet, so always have
> an out.

During this season (my first full winter winter since receiving my IFR
rating last March), I have picked up ice at +2 C, -12c, and in areas that
were outside of icing Airmets by hundreds of miles.

In my limited experience, one fact seems apparent to me: There are no
reliable rules pertaining to ice except, perhaps, plan for possible icing
from October to March (at least downwind of the US Great Lakes).

--
Peter















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[Andrew Gideon]

Mike Rapoport wrote:

[...]
> Ice is less likely to be a problem than if it was 0C at the surface but,
> again, icing can occur at temperatures much lower than -10C particularly
> in clouds with vertical movement (cumulus).

I've been wondering why the vertical movement makes a difference.

> This assumes that the water is still. It the
> water is turbulent then the temperature will go even lower before
> crystalization starts.

Is that the answer? The vertical movement counts as "turbulence" in this
context?

Why does the "activity" of the water alter the temperature at which freezing
starts? The kinetic energy of such movement? The friction which results
from such movement? But wouldn't the friction merely raise (or slow the
decrease) of the temperature?

- Andrew

[Andrew Gideon]

Icebound wrote:

> But not only that, the original air that formed the cumuluus cloud may
> have come from low, warm, humid levels. If the air in the cloud started
> out at, say, 20 degreec C with a 15 C dewpoint, it will have condensed
> out about 11 grams of liquid water per cubic meter by the time it cooled
> to -10. (This is also why the worst icing in cumuloform clouds is often
> near the top.)

I'm questioning how this leads to "worse icing...near the top". I don't see
how you got there from the above explanation. I'd guess I'm missing a step
or two in the reasoning that you're assuming to be understood.

The condensation would be even across all altitudes of the cloud, right?
That is, a meter at the bottom and a meter at the top should be equally
wet, as the temperature change - and therefore the water condensing out -
would be the same.

So...what am I missing?

- Andrew

[Tarver Engineering]

"Andrew Gideon" > wrote in message
online.com...
> Mike Rapoport wrote:
>
> [...]
> > Ice is less likely to be a problem than if it was 0C at the surface but,
> > again, icing can occur at temperatures much lower than -10C particularly
> > in clouds with vertical movement (cumulus).
>
> I've been wondering why the vertical movement makes a difference.
>
> > This assumes that the water is still. It the
> > water is turbulent then the temperature will go even lower before
> > crystalization starts.
>
> Is that the answer? The vertical movement counts as "turbulence" in this
> context?
>
> Why does the "activity" of the water alter the temperature at which
freezing
> starts?

It is because icing is a statistic phenomenon and water may be neither
solid, or liquid, but some inter quantum state. It is your presence that
collapses water into either state. (wave partical duality)

> The kinetic energy of such movement?

It is any energy, until some group energy matches the energy well's volume.

> The friction which results
> from such movement?

Think billiard balls bouncing into one another.

> But wouldn't the friction merely raise (or slow the
> decrease) of the temperature?

Clouds do some interesting things we do not fully understnd.

[Tarver Engineering]

"Andrew Gideon" > wrote in message
online.com...
> Icebound wrote:

> So...what am I missing?

The physics education.

Is there an alt.powerpoint group for Andrew. :)

[Doug]

I landed my Husky with a windshield covered with prop grease (grease
fitting) and I was able to do it fine. I looked out the side. Landing
with the windsheild iced would not be too big of a concern for me, in
my airplane, unless it was limited visibility due to fog or ground
haze also. If it was clear, I could do it. YMMV. I would be more
concerned about the ice on the wings. Even a little slows me down, a
LOT could cause me to stall. Avoid ice.

Dave Butler > wrote in message >...
> O. Sami Saydjari wrote:
> > This question is a question on physcial phenomena, NOT on regulation (so
> > I am starting a new thread).
> >
> > As I understand it, icing happens between +2C and -10C. Assuming this
> > is correct, I have three questions.
> >
> > (a) Does that mean one is relatively safe if the surface temperature is
> > below -10C (and there is no temperature inversion, meaning that the
> > temperature is known to decrease as one goes up in altitude)?
>
> If there is any chance you will pick up ice at some altitude, you'll want to be
> sure there is some altitude you can get to where you can shed the ice before
> landing, unless you can land with an iced-up windscreen.
>
> For me, that means somewhere within range where the temperatures are above
> freezing at the surface.
>
> This is probably not possible in Wisconsin for a big part of the year.
>
> I've never had the experience of accumulating ice and then not being able to
> descend to above-freezing temperatures, but I'd think it would be a pretty
> serious situation without a heated windscreen. When I've had an iced-over
> windscreen, the defroster wouldn't touch it. It was only descent into warmer air
> that allowed me to see again.
>
> Maybe someone who has landed with ice when the surface temps are below freezing
> can commment on the windscreen issue.
>
> Dave
> Remove SHIRT to reply directly.

[Icebound]

Andrew Gideon wrote:
>
> The condensation would be even across all altitudes of the cloud, right?
> That is, a meter at the bottom and a meter at the top should be equally
> wet, as the temperature change - and therefore the water condensing out -
> would be the same.
>
> So...what am I missing?
>
> - Andrew
>

Typically, in clouds of vertical development, the amound of liquid water
is NOT even across all altitudes. The air in the "cloud" at the
"bottom" is still updrafting. It just hasn't reached the "top" yet.

When it DOES reach the "top" it will be cooled some more by the physics
of expansion, and hence will have to lose MORE of the invisible water
vapor which it "contains". It is already at 100% relative humidity, so
the moisture loss (from vapour to liquid) will be the maximum that it
can be at its coldest point... the top of the cloud.


In the typically sunny Sunday afternoon, most of the air in a cumulus
cloud actually started near the earths surface. You can predict its
starting dewpoint fairly well... in fact observers use the dewpoint,
plus the known rate of cooling as air travels upward, in order to
determine the probable base of the CU. At any given level of the CU,
you can pretty well predict how much moisture the air will have given up
into liquid, using the same information.

In frontal, or in embedded, cumuloform cloud, it is not so easy to
determine the starting dewpoint of the air, but once the air is in
updraft mode, it has to keep losing more water into liquid, the higher
it travels.

This effect is never "perfect", but the principles apply.

In stratiform cloud, the lift is usually much more widely spread
horizonatally than cumuloform cloud, may affect a thick layer more or
less uniformally, and might not cool any particular part of the air as
much... (although never say never, as some long-lasting systems may
continue to lift and cool the layer for days.)

It becomes extremely difficult to predict what the dewpoint was at the
start of the lift, and how much the air has cooled, so it is more
difficult to predict how much moisture has had to be condensed. Also, if
the layer was lifted equally, then at first the greatest icing may
actually be near the middle or lower levels of the cloud, because the
starting dewpoints were probably higher where the warmer air was (in the
lower levels).

But once more, if the lift continues for a long-enough time, that air
from the bottom of the cloud will eventually reach the colder higher
levels where it will have to lose a lot more of its original moisture,
and you may again get the most liquid near the top of the layer, eventually.

[Bob Gardner]

That is the icing rule all pilots should remember: There are no
hard-and-fast rules with regard to airframe icing.

Bob Gardner

"Peter R." > wrote in message
...
> Doug ) wrote:
>
> <snip>
> > I
> > agree with MU-2 Mike in that we do get ice below -10 degrees Celcius.
> > It is also possible to get ice when there is no airmet, so always have
> > an out.
>
> During this season (my first full winter winter since receiving my IFR
> rating last March), I have picked up ice at +2 C, -12c, and in areas that
> were outside of icing Airmets by hundreds of miles.
>
> In my limited experience, one fact seems apparent to me: There are no
> reliable rules pertaining to ice except, perhaps, plan for possible icing
> from October to March (at least downwind of the US Great Lakes).
>
> --
> Peter
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
> ----== Posted via Newsfeed.Com - Unlimited-Uncensored-Secure Usenet
News==----
> http://www.newsfeed.com The #1 Newsgroup Service in the World! >100,000
Newsgroups
> ---= 19 East/West-Coast Specialized Servers - Total Privacy via Encryption
=---

[Bob Gardner]

I'm hardly an expert, just an interested observer and student of airframe
icing, and this is what I have picked up over the years: Water droplets are
lifted by convective currents, which lose force as the temp within the
column of rising air nears the temp of the surrounding air. Thus, some
droplets have reached this stable state while others are playing
catch-up....thus the increase in numbers of droplets at the top. And they
bump into one another, becoming bigger droplets....but you know what happens
after that.

Bob Gardner

"Andrew Gideon" > wrote in message
online.com...
> Icebound wrote:
>
> > But not only that, the original air that formed the cumuluus cloud may
> > have come from low, warm, humid levels. If the air in the cloud started
> > out at, say, 20 degreec C with a 15 C dewpoint, it will have condensed
> > out about 11 grams of liquid water per cubic meter by the time it cooled
> > to -10. (This is also why the worst icing in cumuloform clouds is often
> > near the top.)
>
> I'm questioning how this leads to "worse icing...near the top". I don't
see
> how you got there from the above explanation. I'd guess I'm missing a
step
> or two in the reasoning that you're assuming to be understood.
>
> The condensation would be even across all altitudes of the cloud, right?
> That is, a meter at the bottom and a meter at the top should be equally
> wet, as the temperature change - and therefore the water condensing out -
> would be the same.
>
> So...what am I missing?
>
> - Andrew
>

[Mike Rapoport]

Vertical movement does not alter the temperature that freezing starts, but
cooling and freezing take. So the AIR temperature will be colder before
freezing starts, the water temperature will be the same.

Mike
MU-2

"Andrew Gideon" > wrote in message
online.com...
> Mike Rapoport wrote:
>
> [...]
> > Ice is less likely to be a problem than if it was 0C at the surface but,
> > again, icing can occur at temperatures much lower than -10C particularly
> > in clouds with vertical movement (cumulus).
>
> I've been wondering why the vertical movement makes a difference.
>
> > This assumes that the water is still. It the
> > water is turbulent then the temperature will go even lower before
> > crystalization starts.
>
> Is that the answer? The vertical movement counts as "turbulence" in this
> context?
>
> Why does the "activity" of the water alter the temperature at which
freezing
> starts? The kinetic energy of such movement? The friction which results
> from such movement? But wouldn't the friction merely raise (or slow the
> decrease) of the temperature?
>
> - Andrew
>

[Mike Rapoport]

There has never been a case of rudder reversal caused by icing. Aileron
reversal has been observed but only with large droplet icing.

Mike
MU-2


"Tarver Engineering" > wrote in message
...
>
> "Mike Rapoport" > wrote in message
> ink.net...
> > I have no idea of what you are trying to say.
>
> OK.
>
> > Aircraft performance in icing
> > is measured in both wind tunnels and in flight. It doesn't matter what
a
> > bunch of pilots think,
>
> There you go.
>
> > the data is quantified
>
> Yes.
>
> > and it shows that the
> > performance degradation is highest with large droplets which form ice in
> > ridges aft of the leading edges.
>
> But icing events that lead to catastrophic failure are more likely to
occur
> in small droplets forming rapid acretion on the tail surface. (rudder
> reversal)
>
>
>

[Ash Wyllie]

Andrew Gideon opined

>Mike Rapoport wrote:

>[...]
>> Ice is less likely to be a problem than if it was 0C at the surface but,
>> again, icing can occur at temperatures much lower than -10C particularly
>> in clouds with vertical movement (cumulus).

>I've been wondering why the vertical movement makes a difference.

Over time supercooled water will freeze. The top of a cumulus cloud has just
arrived, so it has a lot of water just waiting for an airplane to come by...

It is the same reason that the north and east sides of a low pressure area are
the worst for ice. The water hasn't had a chnce to freeze.

Remember, Murphy rules. Icing is most likely when you are least able to do
something about it. And the Feds are watching.


-ash
for assistance dial MYCROFTXXX

[Tarver Engineering]

"Mike Rapoport" > wrote in message
ink.net...
> There has never been a case of rudder reversal caused by icing.

Wrong. Flow seperation due to icing is normal.

[Matthew S. Whiting]

Julian Scarfe wrote:
> "Matthew S. Whiting" > wrote in message
> ...
>
>
>>However, keep in mind that most thermometers have some error in them.
>>So, even though ice doesn't form above 0C, it may form above 0C as
>>indicated on your airplane thermometer.
>
>
> Absolutely true, but remember that your thermometer is one of the easiest
> instruments in the aircraft to calibrate. The ATIS gives the temperature on
> the ground before flight -- it's well worth a check.
>
> Julian Scarfe
>
>

Yes, but where is their thermometer? On the ground near the level of
your plane, or on top of the tower? Makes a lot of difference. Also,
the error may be a slope error, not just a offset. It may be off by 2
degrees at 70 degrees, but less than that at 32 (or more, but typically
error gets less at lower temps).

I prefer to watch things very carefully when the temp is between about 0
and 35F. Most ice I've picked up was between 26 and 32 as indicated by
my Skylane thermometer. No idea how accurate it was, but I think it was
within a couple of degrees which is close enough for most purposes. I
make decisions based on what is happening on the wing, not what the
thermometer says, generally anyway.


Matt

[Matthew S. Whiting]

Bob Gardner wrote:
> That is the icing rule all pilots should remember: There are no
> hard-and-fast rules with regard to airframe icing.

Well, I think there is one ... get out of the condition causing the
airframe icing ASAP!


Matt

[Mike Rapoport]

Please cite a single reference of rudder reversal caused by icing.

Mike
MU-2

"Tarver Engineering" > wrote in message
...
>
> "Mike Rapoport" > wrote in message
> ink.net...
> > There has never been a case of rudder reversal caused by icing.
>
> Wrong. Flow seperation due to icing is normal.
>
>

[Hilton]

Julian Scarfe

> Absolutely true, but remember that your thermometer is one of the easiest
> instruments in the aircraft to calibrate. The ATIS gives the temperature
on
> the ground before flight -- it's well worth a check.

Although the temperature at the ATIS station and the temperature where you
thermometer is located may quite a few degrees off. Kinda like setting your
Heading Indicator when lined up on the runway.

Hilton

[Roy Smith]

In article et>,
"Hilton" > wrote:

> Julian Scarfe
>
> > Absolutely true, but remember that your thermometer is one of the easiest
> > instruments in the aircraft to calibrate. The ATIS gives the temperature
> on
> > the ground before flight -- it's well worth a check.
>
> Although the temperature at the ATIS station and the temperature where you
> thermometer is located may quite a few degrees off. Kinda like setting your
> Heading Indicator when lined up on the runway.
>
> Hilton
>
>

Not to mention that:

1) The ATIS could be up to an hour old

2) The ATIS temperature was taken with a thermometer in a carefully
designed enclosure which keeps it out of direct sun and wind. Your OAT
probe has none of those protections.

On the other hand, a slurry of crushed ice and water makes a pretty good
home-made 0C temperature reference. If you wanted to, I suppose you
could use that to calibrate your OAT-o-meter.

[Tarver Engineering]

"Mike Rapoport" > wrote in message
ink.net...
> Please cite a single reference of rudder reversal caused by icing.

It is all part of the FAA's results from their icing study. Brownlee
expressed FAA's surprise that the results of their experiment were the
opposite of what their expectations had been. If you mean that you want me
to do a google search for you, I think you know better than that.

[Julian Scarfe]

"Matthew S. Whiting" > wrote in message
...

> Yes, but where is their thermometer? On the ground near the level of
> your plane, or on top of the tower? Makes a lot of difference.

Fair point. At least in my part of the world, reported temps are supposed
to be 2 metre temps (measured about 6 ft above the ground). But as others
have pointed out there may be differences because of the local measurement
environment.

I still think it's well worth making a comparison between your OAT and the
reported temperature. It's not difficult to spot consistent deviations of a
couple of degrees.

Julian Scarfe

[Dan Luke]

"Tarver Engineering" wrote:
> > Please cite a single reference of rudder reversal caused by icing.
>
> It is all part of the FAA's results from their icing study.

LOL

[Mike Rapoport]

No, I want you to provice a single reference of rudder reversal caused by
icing which you are contending is a major problem. Not only is it not a
major problem, it has never occured

Mike
MU-2

"Tarver Engineering" > wrote in message
...
>
> "Mike Rapoport" > wrote in message
> ink.net...
> > Please cite a single reference of rudder reversal caused by icing.
>
> It is all part of the FAA's results from their icing study. Brownlee
> expressed FAA's surprise that the results of their experiment were the
> opposite of what their expectations had been. If you mean that you want
me
> to do a google search for you, I think you know better than that.
>
>

[Matthew S. Whiting]

Julian Scarfe wrote:
> "Matthew S. Whiting" > wrote in message
> ...
>
>
>>Yes, but where is their thermometer? On the ground near the level of
>>your plane, or on top of the tower? Makes a lot of difference.
>
>
> Fair point. At least in my part of the world, reported temps are supposed
> to be 2 metre temps (measured about 6 ft above the ground). But as others
> have pointed out there may be differences because of the local measurement
> environment.
>
> I still think it's well worth making a comparison between your OAT and the
> reported temperature. It's not difficult to spot consistent deviations of a
> couple of degrees.

And I don't disagree, I just don't think we should rely too much on this
form of calibration.


Matt

[Andrew Gideon]

Icebound wrote:

> Typically, in clouds of vertical development, the amound of liquid water
> is NOT even across all altitudes. The air in the "cloud" at the
> "bottom" is still updrafting. It just hasn't reached the "top" yet.

And the flow of air is (effectively) stopped at the top, right? That is,
the temperature of a rising air parcel has matched that of the local air,
thus eliminating the "upgraft" (assuming no external factors like drafts
off of mountains or such)?

> When it DOES reach the "top" it will be cooled some more by the physics
> of expansion, and hence will have to lose MORE of the invisible water
> vapor which it "contains".

That's precisely what I'm missing: why? The air has been expanding all the
way up. It stops expanding at the top.

> It is already at 100% relative humidity, so
> the moisture loss (from vapour to liquid) will be the maximum that it
> can be at its coldest point... the top of the cloud.

But the air is at 100% for the entire cloud, no? That's why moisture has
been condensing out as the parcel rises.

I do see that the cloud is coldest at its highest point. But couldn't that
mean that the top consists of ice, which isn't going to be an airframe
icing concern?

[...]
> In frontal, or in embedded, cumuloform cloud, it is not so easy to
> determine the starting dewpoint of the air, but once the air is in
> updraft mode, it has to keep losing more water into liquid, the higher
> it travels.

It starts losing more water once it reaches saturation, right? And that's
at the bottom of a cloud, yes?

[...]
> It becomes extremely difficult to predict what the dewpoint was at the
> start of the lift, and how much the air has cooled, so it is more
> difficult to predict how much moisture has had to be condensed. Also, if
> the layer was lifted equally, then at first the greatest icing may
> actually be near the middle or lower levels of the cloud, because the
> starting dewpoints were probably higher where the warmer air was (in the
> lower levels).

I'm not following this paragraph either. A higher starting dewpoint just
means that the air *could* hold more water. But it cools as it exands, and
starts condensing out moisture at the saturation point independent of what
the dewpoint was.

At least, that's how I see it. Obviously, I'm missing something.

- Andrew

[Andrew Gideon]

Ash Wyllie wrote:

> Over time supercooled water will freeze. The top of a cumulus cloud has
> just arrived, so it has a lot of water just waiting for an airplane to
> come by...

This is what I'm missing: why is the top special? There's water at every
level of the cloud. Is it the lower temperature? The fact that the air is
no longer rising? Something else?

- Andrew

[Tarver Engineering]

"Mike Rapoport" > wrote in message
ink.net...
> No, I want you to provice a single reference of rudder reversal caused by
> icing which you are contending is a major problem. Not only is it not a
> major problem, it has never occured

Now Mike, as you must already know, NASA failed to accrete any ice on the
ATR at any flap setting recorded in the Roselawn DFDR. Only when the flaps
were fully extended, with three times the Manufacturer's recomendation, was
there accretion. Earlier, you presented wind tunnel data, as though it
somehow trumps flight test. Let me help you out on that wind tunnel icing
datum, Mike, it is a collapsed wave and particle only environment which has
no relationship to a cloud in free space. I know you want your "everybody
knows" to be true, but it is not. Just as the "law of the wall" is no
longer regulatory, icing has made a scientific change.

[Julian Scarfe]

"Andrew Gideon" > wrote in message
gonline.com...

> > It becomes extremely difficult to predict what the dewpoint was at the
> > start of the lift, and how much the air has cooled, so it is more
> > difficult to predict how much moisture has had to be condensed. Also, if
> > the layer was lifted equally, then at first the greatest icing may
> > actually be near the middle or lower levels of the cloud, because the
> > starting dewpoints were probably higher where the warmer air was (in the
> > lower levels).
>
> I'm not following this paragraph either. A higher starting dewpoint just
> means that the air *could* hold more water. But it cools as it exands,
and
> starts condensing out moisture at the saturation point independent of what
> the dewpoint was.

It does, but what's important for ice formation is the amount of water that
has condensed out.

Imagine taking a "box" of air at 25 degC, dewpoint 20 degC and cooling it
to - 10 degC. Water vapor starts condensing out at 20 degC when the
relative humidity reaches 100%, but continues condensing out all the way
down to -10 degC. The water vapor content of the air started at about 17
g/m^3. At -10 degC, the releative humidity is still 100%, but the water
vapor content is now only 2 g/m^3. 15 g/m^3 is condensed out as supercooled
droplets.

Repeat the experiment with a box of air starting at 10 degC, dewpoint 5
degC. That starts at about 7 g/m^3. At -10 degC the water vapor content is
also 2 g/m^3 but only 5 g/m^3 is condensed out as supercooled droplets.

The difference in a cloud is the vertical motion. Instead of being
contained in a box which preserves the water, the parcel of air at low level
is propelled upwards. What happens to the water that condenses out as it
rises? It would be reasonable to think that *some* of it gets left behind
to form the lower part of the cloud. But the upward air current is like a
conveyor belt, pulling high moisture content air in from below to replace
what's rising. So a substantial part of that liquid phase water is going to
get dragged up with the air parcel to the upper part of the cloud. The
water droplet concentration will depend on the dewpoint of the air at the
base of the cloud where it all started.

You also wrote:
> I do see that the cloud is coldest at its highest point. But couldn't
that
> mean that the top consists of ice, which isn't going to be an airframe
> icing concern?

Indeed it could. If the cloud is going to turn to ice, a process called
glaciation, I think it will tend to do so from the top. But until that
happens, the top of the cloud will tend to be the place with the highest
liquid droplet concentration, and therefore the worst place for ice.

Julian Scarfe

[Andrew Gideon]

Julian Scarfe wrote:

> Imagine taking a "box" of air at 25 degC, dewpoint 20 degC and cooling it
> to - 10 degC. Water vapor starts condensing out at 20 degC when the
> relative humidity reaches 100%, but continues condensing out all the way
> down to -10 degC. The water vapor content of the air started at about 17
> g/m^3. At -10 degC, the releative humidity is still 100%, but the water
> vapor content is now only 2 g/m^3. 15 g/m^3 is condensed out as
> supercooled droplets.
>
> Repeat the experiment with a box of air starting at 10 degC, dewpoint 5
> degC. That starts at about 7 g/m^3. At -10 degC the water vapor content
> is also 2 g/m^3 but only 5 g/m^3 is condensed out as supercooled droplets.

Okay, I see this. But, if we were to do the cooling by elevating that box
of air, wouldn't the difference appear in the form of the depth of the
cloud? That is, the cloud would have a higher bottom in the second
example.

But between the altitudes of 5 degrees and -10 degrees, the clouds in the
two above examples would be effectively identical, right?

> The difference in a cloud is the vertical motion. Instead of being
> contained in a box which preserves the water, the parcel of air at low
> level
> is propelled upwards. What happens to the water that condenses out as it
> rises? It would be reasonable to think that *some* of it gets left behind
> to form the lower part of the cloud. But the upward air current is like a
> conveyor belt, pulling high moisture content air in from below to replace
> what's rising. So a substantial part of that liquid phase water is going
> to
> get dragged up with the air parcel to the upper part of the cloud.

Oh! So water doesn't stay where it condensed out. I see.

> The
> water droplet concentration will depend on the dewpoint of the air at the
> base of the cloud where it all started.

I get, from the "water doesn't stay where it condensed out" idea, that water
will tend to accumulate at the top of the cloud. But I'm still not seeing
how the concentration varies with the air's starting dewpoint. Is it that
a deeper cloud will accumulate more water at the top because the
condensation has been occurring over more altitude, and that water has been
rising?

- Andrew

[Julian Scarfe]

> > Imagine taking a "box" of air at 25 degC, dewpoint 20 degC and cooling
it
> > to - 10 degC. Water vapor starts condensing out at 20 degC when the
> > relative humidity reaches 100%, but continues condensing out all the way
> > down to -10 degC. The water vapor content of the air started at about
17
> > g/m^3. At -10 degC, the releative humidity is still 100%, but the water
> > vapor content is now only 2 g/m^3. 15 g/m^3 is condensed out as
> > supercooled droplets.
> >
> > Repeat the experiment with a box of air starting at 10 degC, dewpoint 5
> > degC. That starts at about 7 g/m^3. At -10 degC the water vapor
content
> > is also 2 g/m^3 but only 5 g/m^3 is condensed out as supercooled
droplets.

"Andrew Gideon" > wrote in message
online.com...
>
> Okay, I see this. But, if we were to do the cooling by elevating that box
> of air, wouldn't the difference appear in the form of the depth of the
> cloud? That is, the cloud would have a higher bottom in the second
> example.

I think I might be missing the point of your question but as posed, the
scenarios (25/20 at the surface and 10/5 at the surface) would lead to
similar cloud bases of 2000 ft. The depth of the cloud would depend on the
environmental temperature vs height relationship.

> But between the altitudes of 5 degrees and -10 degrees, the clouds in the
> two above examples would be effectively identical, right?

Not if you believe my model of the parcel carrying the condensed out water
with it as it ascends, see below.

> > The difference in a cloud is the vertical motion. Instead of being
> > contained in a box which preserves the water, the parcel of air at low
> > level
> > is propelled upwards. What happens to the water that condenses out as
it
> > rises? It would be reasonable to think that *some* of it gets left
behind
> > to form the lower part of the cloud. But the upward air current is like
a
> > conveyor belt, pulling high moisture content air in from below to
replace
> > what's rising. So a substantial part of that liquid phase water is
going
> > to
> > get dragged up with the air parcel to the upper part of the cloud.
>
> Oh! So water doesn't stay where it condensed out. I see.
>
> > The
> > water droplet concentration will depend on the dewpoint of the air at
the
> > base of the cloud where it all started.
>
> I get, from the "water doesn't stay where it condensed out" idea, that
water
> will tend to accumulate at the top of the cloud. But I'm still not seeing
> how the concentration varies with the air's starting dewpoint. Is it that
> a deeper cloud will accumulate more water at the top because the
> condensation has been occurring over more altitude, and that water has
been
> rising?

I think you may be reading too much into it. I'm just using the starting
dewpoint as a measure of the starting total water content of the air. At a
particular level and temperature, the air can hold only so much water. The
rest of the water content that started in the air parcel must have condensed
out.

Julian

[Icebound]

Andrew Gideon wrote:

>
> This is what I'm missing: why is the top special? There's water at every
> level of the cloud. Is it the lower temperature? The fact that the air is
> no longer rising? Something else?
>
> - Andrew
>


Okay, lets try just once more. The air at the top has been lifted the
greatest amount. In cumulus clouds especially, it probably started its
lift from near the surface. It had the near-surface dewpoint (which is
a direct measure of how much invisible water vapor it "contains" in each
unit of volume) .

During its lift to, say 18,000 feet, it cools at a known predetermined
rate governed by the physics of expansion. At 18,000 feet it has COOLED
MORE than the air that has so far been lifted to only, say, 10,000 feet.

Since it is colder than the air which has only been lifted to 10,000
feet, it can hold less moisture as invisible vapor than the air lifted
to 10,000.

Its original moisture had to go somewhere, and it condensed into liquid.
The air lifted only to 10,000 feet hasn't cooled as much yet, so a
correspondingly lesser amount of its original moisture was forced to
condense into liquid.

I say again:... a correspondingly LESSER amount of its original moisture
was forced to condense into LIQUID (for the air at 10,000 compared to
18,000).

Assuming the liquid has not frozen (common to at least -10C and often
lower), and has not fallen out as precipitation... then you can expect
more liquid in a given volume of air at 18,000 feet when compared to
10,000 feet in the same cloud.

In real clouds, not all air starts its lift exactly from the same level
with exactly the same dewpoint, but cumulus clouds is one area where
this principle can come close to reality. It can also apply in other
clouds that have been lifting for a very long period of time, as in SOME
warm-frontal situations.

[Icebound]

Andrew Gideon wrote:

>
> But I'm still not seeing
> how the concentration varies with the air's starting dewpoint. Is it that
> a deeper cloud will accumulate more water at the top because the
> condensation has been occurring over more altitude, and that water has been
> rising?
>


The dewpoint is a (reasonably) exact measure of the invisible moisture
in the air.

If you start with a parcel at temperature 30, dewpoint 20, it contains
about 15 grams of water vapor for each kilogram of air. If lifted, it
will start to condense at about 4,000 feet.

If lifted all the way to 18,000 feet, it will have to lose about 10.5
grams into liquid per kilogram of air. And yes, as it continues upward,
the already-condensed water follows in the updraft.

The air following it, which has only been lifted to 10,000 feet, will
only have to lose about 6 grams into liquid for each kilogram of air.

So there is about 4.5 more grams of liquid water at 18,000 feet for
every kilogram of air.

[Teacherjh]

The short answer is that air from far below has more water in it. When the air
is lifted, the water doesn't "go away". The package of air still has just as
much water in it, only now it's in the form of water droplets. Since air from
far below had more water to begin with, it will have more water droplets to end
with.

More cold water droplets -> more ice.

When I say "air from far below" I"m simplifiying - I mean air from the base of
the clouds... where the air is completely saturated. The lower the base, the
warmer the air (at normal lapse rates), and the more water it can hold (warm
saturated air holds more water than cold saturated air).

You may be getting lost in figuring out where the water condenses, how much
condenses, stuff like that. IT doesn't matter. The droplets go with the air -
consider the whole package. To first order, it stays the same.

Jose

--
(for Email, make the obvious changes in my address)

[Viperdoc]

Try landing a Pitts, Eagle, Mustang or Extra. Forward visibility is
non-existent in the landing attitude. However, even a little ice on the
airframe could or will significantly increase the stall speed. Aerodynamics
are probably more a factor with icing than decreased visibility. Running off
the side or end of the runway is probably a lot more survivable than doing a
stall/spin in the pattern.