The real issue is Brownian motion of the molecules keeps the gasses mixed.


Jim Logajan wrote:
"karl gruber" wrote:

The PHAK (A US governmet publication, so it can't be wrong!!!) pg. 2-1
states:

http://www.faa.gov/library/manuals/a...ilot_handbook/

The atmosphere is composed of 78 percent nitrogen, 21
percent oxygen, and 1 percent other gases, such as
argon or helium. As some of these elements are heavier
than others, there is a natural tendency of these heavier
elements, such as oxygen, to settle to the surface of the
earth, while the lighter elements are lifted up to the
region of higher altitude. This explains why most of the
oxygen is contained below 35,000 feet altitude.

Is this correct??

There is a tendency for the atmospheric components to "stratify" and were
it not for the continual agitation that comes from the Earth's rotation
and solar energy (i.e. weather), the Argon would long ago have settled
out as the lowest strata, Oxygen at the next strata, and Nitrogen above
the Oxygen.

Any fluid composed of several components would, absent external
perturbations, settle out into multiple layers in a gravitational field,
with higher density fluids at the lower levels and lower density fluids
at the higher levels. A home experiment in trying to mix oil with water
demonstrates this tendency. To keep the oil mixed with the water you have
to keep agitating it. If you don't, eventually the oil and water
separate.

The atmosphere is considered a fluid also, and this tendency exists there
too. But unlike an oil/water mix, the differences in density between O2
(Molecular Weight of 32), N2 (MW of 28), and Ar (MW of 40) are not large.
The lower atmosphere is considered "well mixed" and stays that way.

Hydrogen (H2, MW of 2) and Helium, MW of 2, are so much lighter than the
other atmospheric components that they rise to the "top" of the
atmosphere, there to be boiled off into space. But Helium is constantly
replenished by radioactive decay, and H2 is replenshed by processes such
as the oxidation of methane.

See also:
http://www.ccpo.odu.edu/SEES/ozone/class/Chap_6/6_2.htm