 |
 |
Humidity
Name: Alden
Status: other
Age: 60s
Location: N/A
Country: N/A
Date: 2001
Question:
What is the name of this natural process?
Replies:
How are the water molecules spaced within the air, do they form a regular
geometrical arrangement of points over an area?
The solubility of water vapor in air has absolutely nothing to do with the
molecular structure or chemical properties of air. The water molecules
simply bounce around in exactly the same manner as the other molecules in
the air - principally nitrogen, oxygen, carbon dioxide, and argon. For the
most part, the molecules of the different gases and vapors in air are
randomly distributed and continuously intermixing. Sometimes, some of the
molecules will stick together in clusters for a while, but only a minor
fraction at ordinary temperatures and pressures.
Incidentally, the water molecule is the LIGHTEST of all those molecules
listed.
Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
Last inquiry first: Water molecules that are in the vapor (gaseous) state
are more or less randomly distributed in air. They do not associated with
one another to any appreciable extent. Of course, air over a body of water
is likely to contain more water vapor than air over a desert, but so long as
there is air movement, the water molecules will be randomly distributed. In
fact, that is the equilibrium condition. A region containing a higher
concentration of water molecules will tend to lose water molecules to a
region containing a lower concentration. This is due to the random
collisions that tend to "spread" molecules, any molecules equally in all
directions unless there is some external factor(s) inhibiting this diffusion
process.
The amount of water vapor in the air does not depend upon the air (except to
a minor extent due to the solubility of atmospheric gases in liquid water)
but depends upon the vapor pressure of the liquid water. The vapor pressure
of water depends only on the temperature (again, assuming the water is free
of solutes). When the amount of water vapor in the air above a pool of
liquid water reaches the amount corresponding to the vapor pressure, we say
the air is saturated or that the relative humidity is 100% (different words,
same meaning).
Transpiration (the movement of water vapor due to the flow of air),
convection (the movement of water vapor due to a difference in temperature
(hot air
with its water tends to rise because it is less dense), and diffusion (the
movement of water vapor molecules due to random collisions with other
molecules) are the mechanisms for the "mixing" of water molecules.
The equilibrium condition for a mixture of any gases is equal concentration
of each component gas. The reason that "heavy" gases appear to "sink" or
"hover" closer to the ground is the fact that convection and diffusion are
usually slow processes, so what we are observing is a non-equilibrium
condition. The force of gravity on an individual molecule is extremely small
because of the small mass of the individual molecules.
When we talk about a system (air + water vapor in this case) not being "at
equilibrium", understand we are not talking about the "speed" at which the
processes occur, but about the ultimate condition that the system. There are
many cases where non-equilibrium states may last a long time even though
they are not at equilibrium -- diamond with respect to graphite at room
temperature, wood in the presence of air with respect to water and CO2 at
room temperature are two such examples. Equilibrium refers to the ultimate
"possibility / impossibility" of a process, not the "speed" with which the
process occurs.
Vince Calder
Click here to return to the Weather Archives
| |
Update: June 2012
|
|
