Sublimation vs transport

Guys, the question isn't where you took physics, its where did you
take Chem 201 - Physical Chemistry. That's where we usually introduce
students to the basic science that is the foundation for avi work.

At a given temperature, sublimation goes on at exactly the same rate
no matter if there is a temperature gradient present or not. In fact,
the reverse process, the deposition of molecules that are in the gas
phase onto solid surfaces, is ALSO going on whether there is a
temperature gradient or not.

When there is no temperature gradient, the above two processes are
constantly shuffling water molecules back and forth between one
snow/ice crystal and others that are nearby. This continual process
of sorption/desorption changes the crystal structure of the snow/ice
without any net macroscopic movement of water into or out of a
particular layer of the snowpack. If the rate is high enough (higher
temps), if it goes on for a long enough duration without disturbance
(eg, compaction, freeze-melt cycles, etc.), and if other processes
don't overwhelm it, the change in crystal structure will be observable
macroscopically because of changes in the mechanical properties,
texture, etc. of the layer.

On the other hand, when there is a temperature difference between two
nearby layers, the rate of the sublimation process is higher in the
warmer layer, and the probability of gas phase molecules sticking when
they hit (ie, the reverse process) is higher in the cooler layer.
There is still the back-and-forth shuffling of water molecules that I
described above, but now, the net effect is that there will be a net
movement of water molecules from the warmer layers to the cooler
layers. This is called "mass transport". If the temp gradient is
large enough, molecules will tend to move to the cooler adjacent layer
before they do much back-and-forth shuttling, so relatively speaking,
recrystallization will be a less important process in the evolution of
the snowpack.

In the previous string of messages on this topic, I didn't see any
place in it where anyone distinguished between sublimation and
transport. Maybe I missed it. I didn't wade through every message
in that string because of all the name-calling / name-dropping,
posturing and other non-technical BS present.

Dr. P. Chem wrotw:



Guys, the question isn't where you took physics, its where did you
take Chem 201 - Physical Chemistry. That's where we usually introduce
students to the basic science that is the foundation for avi work.

At a given temperature, sublimation goes on at exactly the same rate
no matter if there is a temperature gradient present or not. In fact,
the reverse process, the deposition of molecules that are in the gas
phase onto solid surfaces, is ALSO going on whether there is a
temperature gradient or not.

When there is no temperature gradient, the above two processes are
constantly shuffling water molecules back and forth between one
snow/ice crystal and others that are nearby. This continual process
of sorption/desorption changes the crystal structure of the snow/ice
without any net macroscopic movement of water into or out of a
particular layer of the snowpack. If the rate is high enough (higher
temps), if it goes on for a long enough duration without disturbance
(eg, compaction, freeze-melt cycles, etc.), and if other processes
don't overwhelm it, the change in crystal structure will be observable
macroscopically because of changes in the mechanical properties,
texture, etc. of the layer.

On the other hand, when there is a temperature difference between two
nearby layers, the rate of the sublimation process is higher in the
warmer layer, and the probability of gas phase molecules sticking when
they hit (ie, the reverse process) is higher in the cooler layer.
There is still the back-and-forth shuffling of water molecules that I
described above, but now, the net effect is that there will be a net
movement of water molecules from the warmer layers to the cooler
layers. This is called "mass transport". If the temp gradient is
large enough, molecules will tend to move to the cooler adjacent layer
before they do much back-and-forth shuttling, so relatively speaking,
recrystallization will be a less important process in the evolution of
the snowpack.

In the previous string of messages on this topic, I didn't see any
place in it where anyone distinguished between sublimation and
transport. Maybe I missed it. I didn't wade through every message
in that string because of all the name-calling / name-dropping,
posturing and other non-technical BS present.

I was going to say that.

Fred

FHemmer209 wrote:
Dr. P. Chem wrotw:




Guys, the question isn't where you took physics, its where did you
take Chem 201 - Physical Chemistry. That's where we usually introduce
students to the basic science that is the foundation for avi work.

At a given temperature, sublimation goes on at exactly the same rate
no matter if there is a temperature gradient present or not. In fact,
the reverse process, the deposition of molecules that are in the gas
phase onto solid surfaces, is ALSO going on whether there is a
temperature gradient or not.

When there is no temperature gradient, the above two processes are
constantly shuffling water molecules back and forth between one
snow/ice crystal and others that are nearby. This continual process
of sorption/desorption changes the crystal structure of the snow/ice
without any net macroscopic movement of water into or out of a
particular layer of the snowpack. If the rate is high enough (higher
temps), if it goes on for a long enough duration without disturbance
(eg, compaction, freeze-melt cycles, etc.), and if other processes
don't overwhelm it, the change in crystal structure will be observable
macroscopically because of changes in the mechanical properties,
texture, etc. of the layer.

On the other hand, when there is a temperature difference between two
nearby layers, the rate of the sublimation process is higher in the
warmer layer, and the probability of gas phase molecules sticking when
they hit (ie, the reverse process) is higher in the cooler layer.
There is still the back-and-forth shuffling of water molecules that I
described above, but now, the net effect is that there will be a net
movement of water molecules from the warmer layers to the cooler
layers. This is called "mass transport". If the temp gradient is
large enough, molecules will tend to move to the cooler adjacent layer
before they do much back-and-forth shuttling, so relatively speaking,
recrystallization will be a less important process in the evolution of
the snowpack.

In the previous string of messages on this topic, I didn't see any
place in it where anyone distinguished between sublimation and
transport. Maybe I missed it. I didn't wade through every message
in that string because of all the name-calling / name-dropping,
posturing and other non-technical BS present.


I was going to say that.

I said it before, but the post got lost.

ObSki - I've been working straight through and haven't been out in
several - how's the snow?

Dr. P. Chem wrote:
When there is no temperature gradient, the above two processes are
constantly shuffling water molecules back and forth between one
snow/ice crystal and others that are nearby. This continual process
of sorption/desorption changes the crystal structure of the snow/ice
without any net macroscopic movement of water into or out of a
particular layer of the snowpack. If the rate is high enough (higher
temps), if it goes on for a long enough duration without disturbance
(eg, compaction, freeze-melt cycles, etc.), and if other processes
don't overwhelm it, the change in crystal structure will be observable
macroscopically because of changes in the mechanical properties,
texture, etc. of the layer.

Cool.. Chemistry was always my toughest subject. Makes a lot of
sense.. so my question would be, in the real world and a real
snowpack, say at -3 to -6 C, which is what we see arond here, which
phenomenon is the one that matters, the change in crystal structure,
or the "other processes". I would suspect, in a "high" (several skiers
every two to three days) traffic area, with even 15-30 cm. new snow
per week, that the, help me out here, the sorption/desorption process
is pretty much negligible in the stabilization of a buried surface
hoar layer. The buried surface hoar is not going anywhere at that
temperature *before* it is bridged or collapses, or goes through
melt/freeze. I would suspect, that this is also the case in lower
traffic areas... judging from how long buried sirace hoar sticks
around.

On the other hand, when there is a temperature difference between two
nearby layers, the rate of the sublimation process is higher in the
warmer layer, and the probability of gas phase molecules sticking when
they hit (ie, the reverse process)

What is the name of this reverse process.. i.e. gas to solid? Is there
one. There must be?

solid to is higher in the cooler layer.
There is still the back-and-forth shuffling of water molecules that I
described above, but now, the net effect is that there will be a net
movement of water molecules from the warmer layers to the cooler
layers. This is called "mass transport". If the temp gradient is
large enough, molecules will tend to move to the cooler adjacent layer
before they do much back-and-forth shuttling, so relatively speaking,
recrystallization will be a less important process in the evolution of
the snowpack.

Ok.. I'm confused.. when there is a temperature gradient,
recrystalization is *less* important? Please explain. I would be under
the impression that, and again, help me on the terminology here, that
with a temperature gradient, when there is a cold layer such as
surface hoar, that is buried, that the "mass transport" from the
warmer layer to the colder layer, that the "mass transport" and
accompanying energy transport, (what is it, 2KCal/gr, it's been a
while) , from the phase change, would cause *more* change in the
crystal structure, than in an isothermal snowpack? I.e .. with a
temperatire gradient, you have a greater likelyhood of the buried
surface hoar recrystalizing. I.e. the net *mass* transport will be
far greater in a TG snowpack. But maybe net is not the most important
factor? just the "flux"? Which would seem to be tied absolute temp?

Next question.. in an isothermal snowpack, where does the energy for
the mass transport and phase change come from, and in an insulator
such as snow, why doesn't this affect the temperature? For example,
you have an insulator, and isotherm, you get mass transport and phase
change that sucks up and gives up energy, but the isotherm doesn't get
a gradient? Even at micro scales? Quasistatic equilibrium?

In the previous string of messages on this topic, I didn't see any
place in it where anyone distinguished between sublimation and
transport. Maybe I missed it. I didn't wade through every message
in that string because of all the name-calling / name-dropping,
posturing and other non-technical BS present.

Yah.. welcome to USENET...

Also,, I see why you went into physical chemistry.. I mean.. with a
name like that and all..

This beats the **** out of the Superbowl.

-klaus

On Mon, 2 Feb 2004 03:19:57 +0000 (UTC), klaus
wrote:

...so my question would be, in the real world and a real
snowpack, say at -3 to -6 C, which is what we see arond here, which
phenomenon is the one that matters, the change in crystal structure,
or the "other processes". I would suspect, in a "high" (several skiers
every two to three days) traffic area, with even 15-30 cm. new snow
per week, that the, help me out here, the sorption/desorption process
is pretty much negligible in the stabilization of a buried surface
hoar layer. The buried surface hoar is not going anywhere at that
temperature *before* it is bridged or collapses, or goes through
melt/freeze. I would suspect, that this is also the case in lower
traffic areas... judging from how long buried sirace hoar sticks
around.

I'm no avi expert, but from my experience in the lab, I would agree.
Think about putting a piece of food in the freezer in an airtight,
transparent plastic bag. It usually takes many weeks before water
vapor starts moving out of the food and crystallizing on the inside
surface of the plastic bag. The rate of this surface-to-surface
process is almost independent of the nature of the surfaces, and only
dependent on temperature, so a many week time scale is entirely
reasonable, and as you point out, other things can happen during this
period.
---------

What is the name of this reverse process.. i.e. gas to solid? Is there
one. There must be?

When you are looking at events at the level of individual molecules,
the processes are simply called molecular desorption and molecular
adsorption. When you are looking at the processes on a macroscopic
scale, sublimation and condensation (ie, onto a solid surface) would
probably be the best terms.
---------

Ok.. I'm confused.. when there is a temperature gradient,
recrystalization is *less* important? Please explain. I would be under
the impression that, and again, help me on the terminology here, that
with a temperature gradient, when there is a cold layer such as
surface hoar, that is buried, that the "mass transport" from the
warmer layer to the colder layer, that the "mass transport" and
accompanying energy transport, (what is it, 2KCal/gr, it's been a
while) , from the phase change, would cause *more* change in the
crystal structure, than in an isothermal snowpack? I.e .. with a
temperatire gradient, you have a greater likelyhood of the buried
surface hoar recrystalizing. I.e. the net *mass* transport will be
far greater in a TG snowpack. But maybe net is not the most important
factor? just the "flux"? Which would seem to be tied absolute temp?

I wasn't precise in the statement you are referring to and hence it
obviously was confusing. I should have said:

"If the temp gradient is large enough, molecules will tend to move to
the cooler adjacent layer before they do much back-and-forth
shuttling, so relatively speaking, recrystallization FROM SUCH
INTRA-LAYER MOLECULAR EXCHANGE will be a less important process in the
evolution of the snowpack."

As you point out, a thermal gradient can quickly thin out one layer
(and make it structurally weaker). I re-read this string of related
messages and this does seem to be one of the key points you and the
other fellow were discussing.

Next question.. in an isothermal snowpack, where does the energy for
the mass transport and phase change come from, and in an insulator
such as snow, why doesn't this affect the temperature? For example,
you have an insulator, and isotherm, you get mass transport and phase
change that sucks up and gives up energy, but the isotherm doesn't get
a gradient? Even at micro scales? Quasistatic equilibrium?

Excellent question. Lets first consider the center of a deep
isothermal snowpack (ie, negligible temperature gradient, can neglect
mass transport to and from the upper and lower surfaces, etc.). In
this region, there will be no net mass transport between layers, just
the back-and-forth shuttling described above. Molecular desorption is
an endothermic event. It requires energy input to get a molecule out
of the liquid/solid phase and into the gas phase. This energy comes
from the molecular motion of the surrounding molecules. In other
words each desorption event cools the surrounding material. OTOH,
each adsorption event (ie, when one of the gas phase molecules hits
and sticks to a solid surface) is exothermic by exactly the same
amount (on average). Thus, since every molecule that leaves the
solid phase returns to the solid phase at some slightly different
location, there is no net gain or loss of energy needed to keep this
redistribution process going. It is indeed a quasi-equilibrium
process.

Now, we can deal with the question you actually asked, namely, "where
does the energy for mass transport come from in an isothermal
snowpack?" If a snowpack is open to the air at its upper surface, net
evaporation/sublimation can occur at that surface. As in the previous
paragraph, this indeed tends to cool that surface. However, there are
other sources of energy input which can dominate this loss,
particularly, solar heating and convective/conductive heat transport
to/from the air. With the assumption that the snowpack is isothermal,
there obviously will be no thermal gradient driven mass transport
within the snowpack, but there can still be diffusion of water
molecules to the upper surface and then loss from that surface since
diffusion is essentially an iso-ergic process.
---------

Yah.. welcome to USENET...

Heh. Thanks.


OBSKI: I'll be attending a PSIA event next weekend and about the only
mass transport processes I'll be thinking of is making sure that I
huck my carcass diagonally forward and down the hill at every
transition.

Dr. P. Chem wrote:

snippage

Dr. Chem:

I hope you stick around... You obviously have a lot to offer and I
still (obviously) have much to learn about this stuff. You've helped
greatly in that regard... Thanks. Like I said, chemistry has always
been tough for me.. judging from my grades. Did great in thermo and
physics, sucked in chem.

What is the name of this reverse process.. i.e. gas to solid? Is there
one. There must be?

When you are looking at events at the level of individual molecules,
the processes are simply called molecular desorption and molecular
adsorption. When you are looking at the processes on a macroscopic
scale, sublimation and condensation (ie, onto a solid surface) would
probably be the best terms.
---------

Maybe this is where some of the confusion comes from. My guess is that
the avalanche literature is more concerned with net effects, hence the
emphasis on gradients w.r.t. sublimation, and less emphasis is placed
on the quasi-static case since in practice, the quasi static effect is
less pronounced.

I wasn't precise in the statement you are referring to and hence it
obviously was confusing. I should have said:

"If the temp gradient is large enough, molecules will tend to move to
the cooler adjacent layer before they do much back-and-forth
shuttling, so relatively speaking, recrystallization FROM SUCH
INTRA-LAYER MOLECULAR EXCHANGE will be a less important process in the
evolution of the snowpack."

As you point out, a thermal gradient can quickly thin out one layer

Ok... that makes sense.

It is indeed a quasi-equilibrium process.

heh.. as most of life is.

Now, we can deal with the question you actually asked, namely, "where
does the energy for mass transport come from in an isothermal
snowpack?" If a snowpack is open to the air at its upper surface, net
evaporation/sublimation can occur at that surface. As in the previous
paragraph, this indeed tends to cool that surface. However, there are
other sources of energy input which can dominate this loss,

Don't forget the effects at the ground which also add energy/heat to
the whole equation. And in a thin snowpack, it can be very pronounced
and wicked... Just ask the Colorad(o)ans. ;

particularly, solar heating and convective/conductive heat transport
to/from the air. With the assumption that the snowpack is isothermal,
there obviously will be no thermal gradient driven mass transport
within the snowpack, but there can still be diffusion of water
molecules to the upper surface and then loss from that surface since
diffusion is essentially an iso-ergic process.

Again thanks. The whole mass transport part of the equation was very
fuzzy to me, but is clearer now. I need to do more research on
that. It seems to be overlooked in the literature.

OBSKI: I'll be attending a PSIA event next weekend and about the only
mass transport processes I'll be thinking of is making sure that I
huck my carcass diagonally forward and down the hill at every
transition.

ObGeekHumor: Ohh. I dunno.. maybe you should think about the mass
transport of malted/hopped beverages. after you're done skiing of
course.... wouldn't want the potential to kinetic energy exchange to
get too out of hand ... And for heaven's sake, don't trade that
kinetic off for heat in an impulse..

Wanna give us a clue where you're headed?

I really do hope you'll check in every now and then.. you know.. just
to keep us honest.

Take care,
-klaus

On Mon, 2 Feb 2004 23:02:48 +0000 (UTC), klaus
wrote:

Don't forget the effects at the ground which also add energy/heat to
the whole equation. And in a thin snowpack, it can be very pronounced
and wicked... Just ask the Colorad(o)ans. ;

Or undercut snowfields/glaciers which now have runoff or strong winds
transporting heat in/out of the underside of the snow. I
inadevertently got personal and very scary experience with one of
these on Ranier many years ago.

ObGeekHumor: Ohh. I dunno.. maybe you should think about the mass
transport of malted/hopped beverages. after you're done skiing of
course.... wouldn't want the potential to kinetic energy exchange to
get too out of hand ... And for heaven's sake, don't trade that
kinetic off for heat in an impulse..

Ah, fluid flow, a topic of much personal interest .

Wanna give us a clue where you're headed?

Nah, sorry. It's good internet policy to keep personal info private,
especially when one of the best known internet bullies in the world
consistently threatens to slander every newbie to the group.

I really do hope you'll check in every now and then.. you know.. just
to keep us honest.

Thanks for the very kind words. As I said, I'm no avi expert, but
I'll try to check back in and say something coherent.


Take care,
-klaus

You too. Many good turns this season.

In article ,
Dr. P. Chem wrote:

On the other hand, when there is a temperature difference between two
nearby layers, the rate of the sublimation process is higher in the
warmer layer, and the probability of gas phase molecules sticking when
they hit (ie, the reverse process) is higher in the cooler layer.
There is still the back-and-forth shuffling of water molecules that I
described above, but now, the net effect is that there will be a net
movement of water molecules from the warmer layers to the cooler
layers. This is called "mass transport". If the temp gradient is
large enough, molecules will tend to move to the cooler adjacent layer
before they do much back-and-forth shuttling, so relatively speaking,
recrystallization will be a less important process in the evolution of
the snowpack.

Sorry I'm late. I'm the dumbass skid that was watching the Super Bowl.
Vapor pressure driven bonding forms in dry snow *in part* because the
vapor pressure on concave surfaces is less than convex surfaces. Bonds
form between ice crystals in a process called sintering. There are some
pretty pictures of this to accompany the text explaination on page 58
of McClung and Schaerer's _The_Avalanche_Handbook_.

Now that we're all settled down now, the crux of the issue for me is this.
Will compression of the *buried surface hoar* accompanied by mass transport,
sublimation, and sintering, change it to the degree that the snow above it
will be skiable on a couple of steep north aspects I've got my eye on? I
am not willing to bet my ass with out direct observation of the snow pack.
I will include snow pit observation on site before I ski any questionable
terrain in the near future.

Rod Newcomb, Founder and Director of the American Avalanche Institute, told
me this morning that he expects pockets of weak snow to persist. That means
I'm going to keep my head in the game and make objective evaluations of snow
conditions before I ski any radical terrain.

And I'm goddam glad I can pick up the phone and call people like Rod and
Ratty Kanzler when my thinking and descision making processes have been
questioned. It will be interesting to compare differences in the snow pack
watching this marker layer between the latitudes as well.
--
According to John Perry Barlow, "Jeff Davis is a truly gifted trouble-maker."

In article ,
klaus wrote:

This beats the **** out of the Superbowl.

Well, they had free chili and Sprite down at the Log Cabin Saloon. I
haven't worked a lick since I quit my off season job the 1st of December.
I had my nose stuck in some reading at half time though and missed the
big show. Oh well.
--
According to John Perry Barlow, "Jeff Davis is a truly gifted trouble-maker."

Jeff Davis wrote:
In article ,
klaus wrote:

This beats the **** out of the Superbowl.

Well, they had free chili and Sprite down at the Log Cabin Saloon.

Wow. Free sprite? But I hope it wasn't vegetarian chili??

I
haven't worked a lick since I quit my off season job the 1st of December.

September 12 for me. Maybe that's the problem. Idle hands.

I had my nose stuck in some reading at half time though and missed the
big show. Oh well.

You didn't miss much. One breast... You get 32.365 times that in a half
hour of the Playboy Channel.

Why do I get this feeling I'm on teletips.

Group hug anyone?




Anyone?


-klaus