Supercooled Water - More on Icing

In article . net,
"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.

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" 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.

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

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

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

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.

"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.

"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.