Supercooled Water - More on Icing

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

"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.

"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.

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.

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.

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















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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

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

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)

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
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