Re: [ATM] asymptote to ambient

[Mark Holm <mdholm@telerama.com>]

Donald Good wrote:
> Air temperature cannot fall below the dew point.

Yes, of course.  I miss wrote.  Thanks for correcting my error.

Your explanation of a fan's effect on a radiatively cooled surface near 
dew point is what I meant to convey.

There is one minor exception.  If the air is really clean, no dust, 
pollen, sea salt crystals, etc., it is possible for the temperature to 
fall a way below the formal dew point without condensation.  This is the 
phenominon known as super cooling.  It isn't too common at lower 
altitudes, where amateur astronomers usually set up shop, because the 
air isn't clean enough.

The thermodynamics of supercooling are fairly well known.  It occurs 
because a microscopic droplet of water actually has higher energy and 
higher vapor pressure than a larger one.  Because of this excess energy 
required to form the first tiny droplets, air can cool below the 
temperature at which it is in equilibrium with larger droplets, the 
temperature known as the dew point.  If you have nice clean, supercooled 
air, and you introduce something on which water can condense, or 
something that lowers the local condensation energy, you can get rapid 
condensation.  This is the mechanism at work in the cloud and bubble 
chambers physicists used to use for viewing charged particle tracks. 
Ionizing radiation produces ions, charged atoms and molecules, in air. 
   The electic charge of the ions attracts water molecules, lowering the 
condensation energy.  It is also the mechanism behind the idea of cloud 
seeding.  Tiny salt crystals sprayed into supercooled air, give water 
molecules a low energy surface on which to start condensing.

Super cooling and super heating also occur with other phase changes.  I 
once had a nice new glass pitcher of water at the back of my 
refrigerator.  When I looked in, it was liquid.  When I picked it up, a 
fraction of the water instantly froze into a slush of ice.  The slush 
filled the whole pitcher until it floated to the top.  This had to be 
supercooling, because ordinary freezing would have made a solid layer 
starting at the top, or perhaps the sides, not a slush through the whole 
volume.  It won't happen in a used pitcher, because the small scratches 
that naturally accumulate on the surface of used glass provide the low 
energy sites for water molecules to begin freezing.

I once was doing  a vapor pressure curve of benzene as a P Chem lab.  We 
were supposed to pull vacuum on the apparatus in order to measure the 
small vapor pressure as the benzene cooled down.  I got a bit clever, 
and backed the vacuum pump with the house vacuum line.  It let me go a 
fair ways farther down in both vacuum and temperature than my class 
mates.  (You learned lots of good tricks like that reading the old 
Scientific American, Amateur Scientist department.)  The apparatus was 
clean and smooth enough that I got several degrees of super cooling 
before the benzene suddenly froze, all in an instant.  Unfortunately, 
the apparatus did not allow the measurement of the vapor pressure of a 
solid, so I could not compare the vapor pressure of the solid to the 
vapor pressure of the super cooled liquid.

Same occurs with boiling.  If you try to boil water in a clean new glass 
container, it will "bump", boil very unevenly and violently.  It is 
because the water superheats until it gets hot enough to overcome the 
energy barrier of forming the first tiny bubble, then the excess energy 
in the superheated water allows much water to vaporize very quickly, and 
the bubble expands almost explosively.  Chemists add small porous 
"boiling stones" to liquids they wish to boil without bumping.  They do 
the same thing as cloud "seeds", allowing the liquid to boil nearer to 
its formal boiling point, without superheating.

Mark Holm
mdholm@telerama.com


_______________________________________________
ATM mailing list http://www.atmlist.net/