On Saturday, July 30, 2016 at 8:17:07 AM UTC-7,
osbo...@gmail.com wrote:
> What appears to be cloud-free air (virtually) always contains sub microscopic drops, but as evaporation exceeds condensation, the drops do not survive long after an initial chance clumping of molecules.
>
http://www.ems.psu.edu/~fraser/Bad/BadClouds.html
>
> I am wondering of the form of water we are considering. Which sounds like the form of water your working with, sub microscopic drops or chance clumping of molecules?But
>
> thanks...
Excellent website. It's good to see there are some genuine college professors addressing some of the popularistic myths of meteorology. But I don't think this professor has quite broken free from these myths. And that is a problem because unless you break free from all of these myths you will most likely be drawn back in to the seduction of the other myths.
So, what is going on?
Water molecules are constantly coursing back and forth between phases (another word for the three states: vapor, liquid, and solid).
This is mistaken. Vapor is not a phase change. Vapor is liquid suspended in air. Electrostatic charges are the force that suspends heavier microdroplets.
If more molecules are leaving a liquid surface than arriving, there is a net evaporation; if more arrive than leave, a net condensation. It is these relative flows of molecules which determine whether a cloud forms or evaporates, not some imaginary holding capacity that nitrogen or oxygen have for water vapor.
The rate at which vapor molecules arrive at a surface of liquid (cloud drop) or solid (ice crystal) depends upon the vapor pressure.
The rate at which vapor molecules leave the surface depends upon the characteristics of the surface. The number escaping varies with:
the phases involved --- molecules can escape from liquid more readily than from the solid (ice);
the shape of the boundary --- molecules escape more readily from highly curved (small) drops or ice crystals (convex);
the purity of the boundary --- foreign substances dissolved in the liquid or ice diminish the number of water molecules which can escape;
the temperature of the boundary --- at higher temperatures the molecules have more energy and can more readily escape.
And therein lies the origin of the myth. The temperature of a cloud droplet or ice crystal will be (nearly) the same as that of the air, so people imagine that somehow the air was to blame. But, if the (other gases of the) air were removed, leaving everything else the same, condensation and evaporation would proceed as before (the air was irrelevant to the behavior). To assign the behavior of water to an invented holding capacity of the air is like assigning your life's fortunes to an invented influence of the constellations (and as we all know, nobody does that anymore).
I'm not sure if I agree or disagree. Electrostatic charges cause microdroplets (which are heavier than air molecules) to be pulled up between the N2 and O2 of the atmosphere (this results in that parcel of air being heavier [not lighter]).
So, what do you tell your students?
What appears to be cloud-free air (virtually) always contains sub microscopic drops,
Yes. This is correct. (Except actually it's always, not virtually always.)
but as evaporation exceeds condensation, the drops do not survive long after an initial chance clumping of molecules.
Not true. The droplets always persist. There is no gaseous H2O (steam) in earth's atmosphere. Invisibility of vapor is not evidence that it contains gaseous H2O. That is a bullshit assumption that this professor has fallen for.
H2O is only gaseous at temperatures above it's boiling point. Anybody who suggests otherwise is a dumbass that should be deliberately ignored. Invisibility of moist air is not evidence that it contains gaseous H2O. Don't be gullible like this professor.
As air is cooled, the evaporation rate decreases more rapidly than does the condensation rate with the result that there comes a temperature (the dew point temperature) where the evaporation is less than the condensation and a droplet can grow into a cloud drop.
Kinda true, accept that there is never any truly gaseous (monomolecular) H2O in earth's atmosphere. Never. It's impossible. So condensation involve smaller (often invisibly smaller) microdroplets combining to form larger (often visibly larger--due to having a diameter longer than a photon) microdroplets. Condensation, therefore, never, ever, ever, ever involves a phase change.
Evaporation increases with temperature, not because the holding capacity of the air changes, but because the more energetic molecules can evaporate more readily (with, of course, the caveat that evaporation is also influenced by things other than temperature, as described above).
Temperature is an influence, pressure is an influence, electric charge is an influence, turbulence is an influence etc.
If that explanation is not simple enough for your students, just present the facts: when the temperature drops below the dew-point temperature, there is a net condensation and a cloud forms.
What does this prove though?
But don't ever teach nonsense by claiming that the air has a temperature-dependent holding capacity for water vapor.
I disagree. Air does have a holding capaity. And it is partly dependent on temperature and so there are other factors also.
The fact is droplets of H2O that are considerably heavier than either N2 or O2 particles do/does stay suspended. Something is holding it there and preventing it from falling due to gravity. IMO, it is obvious that electrostatic charges are involved. And I suspect the high surface tension of H2O is a factor.
A little history
The idea that it is the air which determines the amount of water vapor which can be present through some sort of holding capacity is an eighteenth century idea which was shown to be false both empirically and theoretically about two hundred years ago!
I think this author is oversimplifying something that is actually more complex than he suggests. What exactly is he talking about here? I can't figure it out.
The fact that it is still taught in our schools and defended by teachers and (gulp) professors, is a testimony to the mindless persistence of myth. A discussion of some of the history of this bankrupt idea is offered by Steven M. Babin .
Bad Clouds FAQ
Before writing me with a question about this page, please check the Bad Clouds FAQ to see if the issue has already been addressed satisfactorily.
I will look when I get a chance.