Can Airplane Vapour Trails Cause Climate Problems?

We have a lot of plans, we just can't get on the same page. Bill Gates is putting spokes in the wheels of the old standby pestilence. There's always "the war to end all wars" but we have nuclear accords because no one wants to go first (that water looks cold). And there's the unreliable blunt instrument, famine - we've put some serious effort into that these past couple of years - but, as always, the cannibals will survive it.

I'm putting my money on "Pigs in Space" - space based solar. It has everything going for it, new frontiers, glamour, grandiose benignancy and hubris - Last Days of Atlantis - lots of stage construction going bust. I love it.

Missed this yarn in the thread before, and since Tornado hasn't picked up on it and I need to regain some credibility , I'd just like to point out that it's "must affect the local area" not "effect"

Transitive verbs vs nouns and all that.

Sorry E-man, but I do feel better now!

Ahhh.... I would like to say I put that there just for your satisfaction, but alas I didn't... My mistake

I noticed it, but a full explanation of affect/effect is about three layers deep.

I think there are more errors in typed text than in handwriting. Where/were/we're, that/than/then, from/form, you/your/you're, etc. All are good words, but not always the right words. It seems that the fingers type reflexively, but the mind fails to check. This happens to me, too, and I miss a few.

... and the problem does NOT disappear with age...

I was never a good typist, but I'm definitely getting worse!

So consider what would happen if 500 grams of liquid water were released into the vacume of space.

What would happen? Would it instantly freeze? Where would the thermal energy go? Wouldn't the energy have to radiate out? Or would the vacume instantaneously vaporize the water - but wait - where would the thermal energy come from?

What do you think?

Gavilan

No, it would not be instantaneous. Since by definition it was liquid, it had to be under pressure. When it was released to the vacuum of space, it must be released through some kind of an opening, resulting in a spray of droplets. Much of the molecular velocity would be in the direction dictated by that opening, but the molecules would still be bouncing against each other, and as a result the outer layer of each droplet would expand rapidly. As each molecule on the surface leaves the main body of water, it carries its kinetic energy with it, leaving the remaining water cooler, until the main body freezes.

Once it freezes, the crystals of ice will sublimate, but that is a much slower process, as the Apollo astronauts discovered.

That's what I think!

Dick

Interesting reply. I gave the mass of the water to be 500 grams. I forgot to give an initial temperature of 50 degrees centigrade.

In order to simplify the hypothetical case let us assume that the water is a sphere placed into the ideal vacuum. Since it is an ideal vacuum the only way thermal energy can be dissipated or absorbed is through radiative cooling or heating.

I would look at this as a thermodynamic process using the known values of specific heat, heat of fusion, and heat of vaporization to look at the overall process. What I come to conclude is that there isn't enough thermal energy available to sublimate the entire 500 grams. The total energy available would approximate the (Temperature * specific heat * Mass) + (Heat of fusion X mass). The energy required to sublimate the entire mass would be (Heat of Vaporization X mass). Because the Heat of Vaporization is so much higher than the sum of the available specific heat and heat of fusion only a portion of the water would sublimate without some source of thermal input.

Is this correct?

Gavilan

Does it matter if its 500 grams or 5 grams? 50°C is a pretty generous initial temperature, but I can accept that.

So you have your water in a spherical container that suddenly shatters and disappears... OK...

In any body of matter, there is, I'm led to believe, a Gaussian distribution of molecular velocities, with the mean (or is it median?) molecular velocity corresponding to the temperature of the body. Those molecules nearest the surface and having the greatest velocities will be the first to escape the surface tension of the sphere, either immediately or after bouncing off a neighboring molecule (depending on the initial direction of motion at the instant the containing sphere disappeared).

These escaping molecules carry their kinetic energy with them, in effect providing another means of cooling (of the remaining liquid) besides radiation. I don't know how broad the Gaussian distribution is, so I have no way of calculating the rate of energy loss, but clearly the remaining water will be cooled, just as water is cooled by evaporation here on earth, only faster.

Once the main body of water has cooled enough to freeze (well below zero °C, due to supercooling), the much stronger molecular bonds will slow the escape of molecules, but not stop it. You still have radiation cooling plus the additional cooling due to the loss of higher energy molecules, until all of the ice has sublimated.

There is another way to look at this, to whit*:

Consider temperature as its fundamental (?) definition: the amount of movement/vibration of individual molecules. Now consider the bonds at the molecular level. We consider a substance to be gaseous when its intermolecular bonds are overcome by the molecules' vibrations, liquid when they are able to keep the molecules in a defined volume, but able to move about within that volume and solid when they can hold the relative positions of individual molecules fixed.

Compressiblity is a measure of how closely packed the molecules are, and pressure their "wish" to move further apart.

Anyhoo...in a vacuum there is no "constraining force" (pressure) so the molecules wander off, on their own sweet lonesome, obeying the "urge" to distribute themselves evenly about the volume available. I'm not sure that boiling/vaporising or subliming (which a liquid can not do by definition) is actually a helpful term to use, in that we instinctively associate those with the addition of heat, because that's what we need to do under conditions of (approximately) constant pressure.

A vacuum is cold simply because there are so few atoms/molecules within it to vibrate and thus transfer heat, or allow us to measure temperature. It's not Absolute Zero because there are some, and they are not at rest.

I've probably (definitely) oversimplfied, and there are plenty of areas for refinement in that explanation, however, I think I covered the main thrust of the idea I was trying to convey.

Now is light a ray or a particle?

*_{^{Thanks JDG for the reminder of this most excellent word}}

The correct term is "to wit," not "to whit." "Whit" is only a noun meaning a tiny amount; "wit" can be a verb meaning something like "see" or "know." [Just an Editor Crankshaft thing.]

Ooooh you'll be in trouble now....lecturing ER on correct English usage
...I'm going to hide under the sofa and watch the fight.
A thousand Quatlous on the lady with bow and arrow.
Del

You're gonna lose--maybe.

Just thought you might be interested in this link to see real time flight control over Europe!!!

Yep.... the planes are flying again....

As its Whitsun I'm sure ER is allowed to spell Whit as she has done....

I'm a bit concerned about the one over Leeds without a call sign... ...and the circle with a question mark at Manchester.

Thanks for the backup and sorry Del, Tornado's right...

Good catch. _{^{I'm gonna get that JDG....}}

It seems to me that you have put into words the concepts I took for granted. Well Said!

Now as I think about it, one question comes to mind: Would, as I assumed, the higher energy molecules in fact be lost from the main body faster than the lower energy molecules, or would all surface molecules leave at the same rate regardless of energy?

I believe the fact that the Apollo astronauts observed the crystals of their own ejected urine confirms the idea that at least part of the liquid will indeed freeze, and therefore that the higher energy molecules do in fact leave faster, and the remaining mass does indeed drop in energy (temperature).

Your post does clarify one concept indicated by Gavilan in post #28: The energy required to evaporate/sublimate the water does not have to come from anywhere - it's already there. Energy is required to evaporate/sublimate water here on Earth only because of the atmospheric pressure. Without that constraint, no energy beyond that already held by the molecular vibration/motion of the original liquid is required.

--------"The energy required to evaporate/sublimate the water does not have to come from anywhere - it's already there. Energy is required to evaporate/sublimate water here on Earth only because of the atmospheric pressure. Without that constraint, no energy beyond that already held by the molecular vibration/motion of the original liquid is required."

Are you sure about that?

I did not suggest that that the phase changes in a vacuum were not endothermic; just the opposite. I suggested that the vaporization caused by the vacuum would occur using the energy available from the capacitated energy from its original temperature (50degrees) and the heat of fusion as the water phased from the liquid to solid state. But because the heat of vaporization is so much higher than those combined energy sources that vaporization would stop once that limited energy was used up during the process. That for vaporization to continue some source of energy would be required.

Intuition tells me that using a heat of vaporization of 2255 Joules per gram it would require 1.1275e6 joules to vaporize the 500 grams of water.

The available latent heat from the initial temperature would be 2092 joules.

As the water phased from the liquid to the solid state it would provide about 335 Joules per gram or a total of 1.675e5 joules for the 500 grams.

The total energy made available to support the vaporization would be about 1.69592e5 joules. This would provide enough thermal potential to phase only 75 grams of the water to the vapor state leaving about 425 grams in the solid state to sublimate as energy was made available from whatever radiative source.

Did my intuition fail me here? If so, what basic principle did I miss?

Gavilan

capacitated?

The heat of vaporization is essentially the energy required to overcome atmospheric pressure and allow the molecules to become independent of each other. In the absence of pressure, essentially no energy is required.

Get out of your calculation mode and just think about it!

Dick

The latent heats of fusion and evaporation (and their sum, sublimation) vary slightly with the pressure encountered, but I don't think they approach zero just because the absolute pressure approaches zero. I'm not sure about this, though, so it needs further review.

dkwarner:

Thanks, much as I love using maths to describe things, sometimes you have to go back to "observations" and approximations.

I'm not sure if the higher energy ones would necessarily be lost first - I think the surface ones would be lost first, simply because the escape route, thanks to Brownian Motion, would randomly send them away from the body of the gas/liquid/solid. Of course some would be directed toward the body, but since the escaped molecules would have no way back(nothing with which to collide), the volume would decrease over time.

Drew K's model is excellent. I would add that above any non-gaseous body (but much more so with a liquid rather than a solid) there is a layer of molecules in vapour form...this layer tends to stay close, because of the surrounding air which, by way of diffusion, increases the escape route distance, so more of the molecules tend to be returned to the body during collisions with air molecules.

dkw's comment about freezing is well made - I hadn't fully considered that before. It would be interesting to be able to measure how long these persist and whether there is a minium size. In fact, I now recall that ice crystals have been observed floating in space (possibly only a "few" molecules together) so this must be the case, which means that Gavilan and Tornado (again! ) are also right in that there is a minimum value of latent heat. Which is logical if you look at it using both my "model" and Drew's model, as it's all about the individual molecule's vibrational energy (its temperature) overcoming the attractive bonds between itself and its neighbours. I'm not sure if escaping molecules cause a reduction in the vibrational energy of the remaining molecules...and that is the factor that will determine whether it freezes or not.

Going back to the not quite original question in #28, I think that if the liquid were released at rest and "magically" as dkw suggested in #35, then you probably would be left with quite a long persisting lump of ice...but that this would slowly evaporate / sublime to a minimum. If the liquid were sprayed out into space, any persisting solids would be very fine and probably already at the minimum, because the greater surface area would allow a speedier Brownian Motion dispersion and might even allow complete dissociation to individual molecules through the additional velocity energy.

We have been assuming a perfect vacuum - and space is not that, so the urine freezing observed would also have been caused by molecular vibration energy (temp) being transferred to those few other molecules in the vicinity. This is the factor not included in my "model". This would have halted the evaporation and allowed a greater volume of matter to reach the critical temperature.

Thanks everyone, this is fun and has got the grey matter smoking gently! All the accumulated dust burning off I think...

What a shame this section of the thread is Off Topic!!

Yes, you're right. As ER said, it depends on whether the vibrational velocities are active enough to break the bonds to neighboring molecules. In liquid water, they clearly are. For ice, apparently not always...

As I think about it, there must be a temperature below which water no longer sublimates even in vacuum. Otherwise they would not have found water ice on the moon.

http://www.engineeringtoolbox.com/saturated-steam-properties-d_457.html

This is a link to the steam tables. Could someone explain to me the relationship between pressure and heat of vaporization?

Your patience is appreciated.

Gavilan

You're quite welcome!

Thanks for the link. I found it interesting that they listed a boiling point of somewhat less than 100°C for 1 bar pressure. Perhaps some one else can explain that!

By definition, the bonds between molecules in a liquid are strong enough to keep most individual molecules at a specific average distance from each other (constant volume), and thereby keep them from escaping the liquid volume, while NOT strong enough to keep the molecules in fixed positions (NOT fixed shape). The heat of vaporization is the added energy required to cause a standard amount of liquid to escape from the liquid condition into the vapor condition, where there is sufficient molecular velocity that two colliding molecules bounce off each other rather than sticking together, as they do in the liquid condition.

In a vapor or gas, there is neither a fixed shape nor a fixed volume, except as provided by a container (which may effectively include gravity - as in the Earth's atmosphere). There is a variation in velocity among molecules. If you could somehow follow a single molecule, its velocity would change every time it collided with another molecule, but on average it will have a velocity which corresponds to the temperature of the gas or vapor. The pressure of the gas depends on the velocity of those collisions, and also on the number of collisions. In other words, at a given temperature, the pressure is directly proportional to the number of molecules per unit volume, which is effectively the density of the gas.

When there is a boundary between the liquid and a gas, such as air, the molecules nearest the surface of the liquid have fewer neighbors holding on to them.

If a certain molecule on the surface of the liquid phase happens to bounce against a neighboring molecule in such a way that its velocity is increased by the collision, and that velocity is directed away from the surface, then that molecule is a candidate for escaping the liquid. If there are few gas molecules nearby, then the given molecule may escape the liquid and become a vapor molecule. If there are gas molecules nearby, the escaping molecule may collide with one or more gas molecules. These collisions may reduce the velocity of the escapee, and/or redirect its motion back into the liquid. It should be more or less obvious that a higher gas pressure (more gas molecules per unit volume) increases the probability of the escapee colliding and being redirected back into the liquid, so with higher pressure, the escaping molecules have to have more energy on average than would be the case with fewer gas molecules.

I hope this helps...

Dick

"I found it interesting that they listed a boiling point of somewhat less than 100°C for 1 bar pressure. Perhaps some one else can explain that!"

Perhaps it has something to do with the units used. In the MKS system (The Standard International System) temperature is given in Kelvin and pressure is given in Pascals. In the steam tables the unit for pressure is the bar which is equal to .987 standard atmospheres on which the boiling point is based?

Anyway; as you can see, as the pressure decreases from .987 atmosphere (1 bar)to .01974 atmosphere (.02 bar) the heat of vaporization actually increases by about 8.2 percent.

I still don't understand how this can be given the Kinetic Theory of Gases.

What is meant by Saturation Steam Table? Perhaps this would explain it?

http://www.engineeringtoolbox.com/saturated-steam-properties-d_457.html

Thanks again! Hopefully we are clarifying each others' understanding...

I had forgotten that 1 bar is not quite one atmosphere... That explains the difference from 100°C.

Unless I am mistaken, 'Saturation Steam' is the condition in the space above a quantity of water that has been boiling in a closed (or at least restricted, as in a pressure cooker or steam boiler) container for sufficient time that the number of molecules evaporating from liquid has reached equilibrium with the number of molecules re-entering the liquid from vapor in a given time. Thus the atmosphere above the liquid is saturated with water vapor.

I believe the increase of energy of vaporization with lower pressure is because at lower pressure, the initial liquid temperature (the initial molecular kinetic energy) is lower. It takes more energy to vaporize a 20°C water molecule than it does to vaporize a 100°C molecule