How High can Evaporated H2O go on our planet?

Jaybird180

Jaybird180

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This is a question/ thread that I posted elsewhere and although I've learned a lot in my investigation, I haven't found the answer. I thought I'd post here some of my conversation in the hopes that an answer is derived. I've made a few corrections for spelling and grammar.

Thanks to a few participants who are also members here that helped me get thus far, but no cigar.

Jaybird180 said:
As water evaporates from the surface, it becomes part of the atmosphere and makes weather, the stuff we fly in.
What is the theoretical altitude limit of evaporation and why? Why doesn't water continue into space, escaping our planet?
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Jaybird180 said:
I understand that liquid water cannot remain liquid above a certain pressure but gaseous state H2O? What about a few particles (atoms) escaping our spheres? You're saying they're still too heavy? At what altitude does this occur?
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Jaybird180 said:
Okay, so the water moving at 22,140kph doesn't reach the required escape velocity of 40,270kph to leave. So I guess it just bumps around until it gets heavy enough to return. to the surface. So that answers the question as to why it doesn't leave the Earth but doesn't give us the maximum altitude it can reach. Certainly we know that water can get colder than 0C.
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Since it looks like H2O can exist at extremely low pressures, there is no theoretical floor (max altitude), which brings me back to the question of how high can it go: 5 miles, 6 miles, 7 miles up?
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Jaybird180 said:
Yes, but the answer is more involved. Gravity is M1*M1/D^2, so if gravity were the simple answer it would never leave the surface and we wouldn't have clouds and no IMC and no weather.
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Jaybird180 said:
Hold a second! If the water cannot reach escape velocity there is a point of equilibrium where it will remain in low Earth orbit (?) or at some point in the troposphere. Again bringing us back to the original question: what is the maximum altitude of the evaporated water?
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Jaybird180 said:
I tried to see what the orbital distance would be, but I ran into a problem with one too many unknowns to plug into the formula.
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Jaybird180 said:
Well, we can see on the face of it this can't be true otherwise we'd lose all the water on the planet.
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Jaybird180 said:
In my quest for an answer, I reached out to Scott Dennstaedt and we spoke about this for a few minutes. He says that the answer would involve a specialty that he doesn't get much into but did provide a few nuggets for me to chew on and investigate.

He said that most of the moisture is contained in the troposphere (check!) and that about 99% of it remains below 40,000 feet. He says that convective activity can take water upwards of 65-70,000feet where the moisture in the form of ice crystals can linger for weeks or months until it settles, reaching for hydrostatic balance.

And I learned that Helium is finite on the earth
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I'm still digging
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It may be an interesting theoretical experiment, but in practice it's a non-issue. It rises to the air's dewpoint, condenses, and then comes back down.
 
There is no set answer to that question. The atmosphere gradually peters out. At 100km the atmosphere is about non-existent, but even space has some high energy particles and such in it. I think about 300km is considered the lowest practical orbit so there can't be much atmosphere there at all.

The highest clouds get are about 100km.
 
Stearman said:
It may be an interesting theoretical experiment, but in practice it's a non-issue. It rises to the air's dewpoint, condenses, and then comes back down.
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You're absolutely correct!...I think.
 
Water vapor goes just about as high as every other molecule in the atmosphere.
 
Stearman said:
It may be an interesting theoretical experiment, but in practice it's a non-issue. It rises to the air's dewpoint, condenses, and then comes back down.
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Dew point is the temperature at which water starts to condense at the current absolute humidity. But, only enough condenses to maintain the relative humidity at 100% or so.
 
scottd said:
I thought that my 99% at 40,000 ft answer was a bit high...so I checked. At altitudes of about 10 km (roughly 33,000 ft) the concentration of water vapor is only about 1 percent of that at sea level.
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Very interesting! That works out to about 6-1/4 miles up. Just need to get that last 1%.
 
Capt. Geoffrey Thorpe said:
Water vapor goes just about as high as every other molecule in the atmosphere.
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No, it doesn't.

The reason is transport. The source of the water is at the surface. To get to higher altitude, you need mixing. The troposphere is well mixed, but the stratosphere is stable, and VERY dry. Almost all the water vapor is below the tropopause.

That's the reason in a nutshell why we have an airborne telescope. Water vapor is blinding in some critical parts of the IR spectrum. It's also why we prefer to observe at polar and mid latitudes. The equatorial stratosphere is not accessible by a 747, and the tropical convergence zone throws a whole helluva lot of water vapor into the upper troposphere.
 
scottd said:
I thought that my 99% at 40,000 ft answer was a bit high...so I checked. At altitudes of about 10 km (roughly 33,000 ft) the concentration of water vapor is only about 1 percent of that at sea level.
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When I attempted to calculate precipitable water vapor from stratospheric altitudes, I quickly discovered that both the GFS and Navy models topped out at 1% mixing ratio. I tried to estimate it anyway, to see if it had any predictive value. We found that integrating GFS gave us about a factor of two high compared to our on-board water vapor monitor, but it did show high and low spots, so we could use it roughly for flight planning. But it told us the water vapor was 50 microns when it really was 20-30. The Navy model correlated much worse and had some obvious numerical artifacts at its 200 mb level.

Necessary exposure times depend exponentially on precipitable water vapor, so it is a very useful estimate to have.
 
MAKG1 said:
When I attempted to calculate precipitable water vapor from stratospheric altitudes, I quickly discovered that both the GFS and Navy models topped out at 1% mixing ratio. I tried to estimate it anyway, to see if it had any predictive value. We found that integrating GFS gave us about a factor of two high compared to our on-board water vapor monitor, but it did show high and low spots, so we could use it roughly for flight planning. But it told us the water vapor was 50 microns when it really was 20-30. The Navy model correlated much worse and had some obvious numerical artifacts at its 200 mb level.

Necessary exposure times depend exponentially on precipitable water vapor, so it is a very useful estimate to have.
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Can you share these calculations please?
 
Jaybird180 said:
Can you share these calculations please?
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No, not really. Not my code to share, even if I did write it.

Basically, it integrates mixing ratio roughly across the very few levels distributed with GFS. It was expected to be rough, but not factor-of-two rough. The essential problem is that it uses the model for something it was never intended to do. Weather forecasters generally look at precipitable water vapor from the ground (where it's 10s of mm, not microns), and consider 1% mixing ratio to be unimportant.

We might have been able to do better with ECMWF, which has been used to calculate albedo related to climate change problems, but we don't have daily access, so it doesn't meet our operational needs.
 
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Just to clarify, water molecules do get very high up in the stratosphere, it's just that they are very rare (in proportion to other gases) up there.
However, for those worried we'd lose our water, bear in mind that there is a plentiful supply being delivered to us constantly via comets etc. There are those who believe that that's how water got here in the first place (there is more than could be accounted for from our own mineral origins).
 
Google "Noctilucent cloud wiki" and read about these clouds that are very high clouds (as high as 85km). They are made up of ice crystals (a form of water).
 
The highest documented clouds, "noctilucent clouds", sometimes forms in the mesosphere. The mesosphere starts at 31 miles above Earth's surface and goes up to 53 miles high. Above the mesosphere is the thermosphere. Although the thermosphere is considered part of Earth's atmosphere, the air density is so low in this layer that most of the thermosphere is what we normally think of as outer space. In fact, the most common definition says that space begins at an altitude of 100 km (62 miles), slightly above the mesopause at the bottom of the thermosphere. You call it!
 
Interesting stuff, these noctilucent clouds. Too many unknowns with them to use as a final answer, like where they come from. But I did find it interesting that they too have an altitude cap.
 
Well, above the mesosphere is the thermosphere, where temperatures range from as low as -184°F (-120°C) at the bottom, to as high as 3,600°F (2,000°C) near the top. So, microscopic ice crystals could become vaporous if they can make it there?
 
Not necessarily. Weird things happen at such low pressures. Take a thermos of water and release it in outer space and 1/2 of it will freeze and 1/2 of it will boil based on "chance".
 
I did say "could" ;) Ok, on that note, if you were to place two containers with the same quantity of hot and cold water into a freezer (here on Earth) why does the one with hot water freeze quicker?
 
GlennAB1 said:
Exosphere - the outermost layer of the atmosphere. It extends from the top of the thermosphere to 6,200 miles (10,000 km) above the earth. In this layer, atoms and molecules escape into space. http://www.srh.noaa.gov/jetstream/atmos/layers.htm
Could frozen water molecules escape?
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They wouldn't be frozen since the exosphere is super hot, but yeah a little bit is lost. Water is pretty heavy though, so very little escapes.
 
172andyou said:
They wouldn't be frozen since the exosphere is super hot, but yeah a little bit is lost. Water is pretty heavy though, so very little escapes.
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We stipulated (over on the other forum) that water is in-fact so heavy that it cannot escape the Earth's gravity.
 
Jaybird180 said:
We stipulated (over on the other forum) that water is in-fact so heavy that it cannot escape the Earth's gravity.
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That stipulation is incorrect. :-)

It's a small amount, because of that weight, but it can indeed escape.
 
172andyou said:
That stipulation is incorrect. :)

It's a small amount, because of that weight, but it can indeed escape.
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Through what mechanism can the water escape? I hypothesized that it doesn't because we still have water on the planet and we have a lot of surface area where water can escape into space. The stipulation was that the water cannot reach escape velocity as I discussed in the quoted OP. Please elucidate.
 
I dont think science fully understands it, the atmosphere is difficult to study. But, the atmosphere is constantly impacted by objects traveling thousands of miles an hour.
 
Jaybird180 said:
Through what mechanism can the water escape? I hypothesized that it doesn't because we still have water on the planet and we have a lot of surface area where water can escape into space. The stipulation was that the water cannot reach escape velocity as I discussed in the quoted OP. Please elucidate.
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You agree that hydrogen can escape though, and we still have that too.

Solar wind is probably the best answer. Even with the magnetic field that prevents the earth from turning into mars, there is still some loss of heavier stuff. Especially during a CME.

Really the question is kind of flawed. Does water escape? Yes. It goes high, it gets hit by the solar wind and gets blown away. Or a few molecules get hot enough to have the energy to take off on their own. Or UV splits the molecule into hydrogen and oxygen and then those lighter elements blow away.

Does water escape in any meaningful quantity? Nope.
 
Jaybird180 said:
Through what mechanism can the water escape? I hypothesized that it doesn't because we still have water on the planet and we have a lot of surface area where water can escape into space. The stipulation was that the water cannot reach escape velocity as I discussed in the quoted OP. Please elucidate.
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Through reaching escape velocity. Velocities of molecules in a gas aren't all the same, they're distributed over a broad range. To be more precise, at any given temperature their kinetic *energies* are distributed over a broad range. Because KE = 1/2*mv^2, the lighter molecules of any given energy will be moving faster than the heavier ones. The higher the temperature, the higher the mean energy in the distribution but at any temperature greater than absolute zero, a certain fraction of the molecules, however small, will have sufficient speed to escape Earth's gravity. The lighter ones will escape more easily than the heavier ones (which is why we have essentially no H2 or He in our atmosphere), and the lower the temperature, the fewer of any type will have enough speed to escape.

This is all in general terms; I'm too lazy right now to look up the temperature for the exosphere and do the calculations for water vapor. But *some* fraction of the water molecules that make it up that high will have enough energy to escape.
 
azure said:
Through reaching escape velocity. Velocities of molecules in a gas aren't all the same, they're distributed over a broad range. To be more precise, at any given temperature their kinetic *energies* are distributed over a broad range. Because KE = 1/2*mv^2, the lighter molecules of any given energy will be moving faster than the heavier ones. The higher the temperature, the higher the mean energy in the distribution but at any temperature greater than absolute zero, a certain fraction of the molecules, however small, will have sufficient speed to escape Earth's gravity. The lighter ones will escape more easily than the heavier ones (which is why we have essentially no H2 or He in our atmosphere), and the lower the temperature, the fewer of any type will have enough speed to escape.

This is all in general terms; I'm too lazy right now to look up the temperature for the exosphere and do the calculations for water vapor. But *some* fraction of the water molecules that make it up that high will have enough energy to escape.
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"Standard Atmosphere" goes right out the window.
http://www.srh.noaa.gov/jetstream/atmos/layers.htm
Thermosphere gets up to 2000•C but no temp listed for the exosphere (I recall reading 3,500 somewhere).
 
Keep in mind that escaping isn't the only way to lose water from the atmosphere. That high, photodissociation becomes significant. So does methane oxidation, which produces water. Ionization processes eventually become significant as well, as it becomes hard to find neutralization in very rarefied gasses. That makes magnetic fields play a role. Charged particles can leak out the poles much more easily than elsewhere.

It becomes a very dirty problem rather quickly. It's not the same weather we're used to at much lower altitudes.
 
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