Supercooled Water - More on Icing

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

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

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

"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.

"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

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

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

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.

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.

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

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