Supercooled Water - More on Icing

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?

"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

"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.

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

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

"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.

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.

"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

"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.

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.