Re: Air density question?? Pilot's Handbook ofAeronautical knowledge

"karl gruber" <removeskywagon@xxxxxxxxx> wrote in message
news:e7nbbu$liu$1@xxxxxxxxxxxxxxxx
[...] This explains why most of the
oxygen is contained below 35,000 feet altitude.<<<<<

Is this correct??

I always thought that only the density changes until a very high altitude.

Huh. I have read both the most recent edition, and the previous edition, of
that book from cover to cover and I never noticed that passage. I must've
been distracted or something.

Anyway, I have been taught just as you have, and apparently most (if not
all) of everyone else. In an "ideal" situation, my understanding is that
gasses mix homogenously and without concern for relatively molecular
weights.

On the other hand, I would not at all be surprised to find that this is a
theoretical simplification, and that in reality molecular weights *do*
matter, and cause some slight variation in elemental makeup of the
atmostphere at different altitudes. Note that 35,000' is *well* above the
altitude at which anyone would be breathing ambient air. The usual "partial
pressure" discussion of breathing oxygen may well be accurate enough at the
altitudes at which it matters, and yet not relevant at 35,000'.

On the other other hand, a quick Google search sure turns up a number of
references that say exactly what all of us believed all along (more or
less):

"Below a height of about 86 kilometers, the three main gaseous elements,
which together account for about 99.9% of the total atmosphere, exist in
essentially constant proportion to the total"
http://www.ccpo.odu.edu/SEES/ozone/class/Chap_2/2_2.htm

"The proportions of gases, excluding water vapor, are nearly uniform up to
approximately 80 kilometers (km) above Earth's surface."
http://www.oralchelation.com/clarks/data/p1.htm

There even appears to be a word for the portion of the atmosphere in which
the composition is relatively constant:

"homosphere: [...] The homosphere is the region in which there is no gross
change in atmospheric composition, that is, all of the atmosphere from the
earth's surface to about 80 or 100 km"
http://amsglossary.allenpress.com/glossary/search?id=homosphere1

"The homosphere is the region of Earth's atmosphere which extends from the
Earth's surface to an altitude of about 80 km. The chemical composition of
the atmosphere remains constant in this region"
http://en.wikipedia.org/wiki/Homosphere

Even Wiki hasn't been able to decide whether the homosphere extends to 80 km
or 100 km:

"Below the turbopause at an altitude of about 100 km, the Earth's atmosphere
has a more-or-less uniform composition (apart from water vapor) as described
above; this constitutes the homosphere"
http://en.wikipedia.org/wiki/Heterosphere

Interestingly, whatever the height of the homosphere actually is, once one
actually gets above that height, oxygen content drops off quickly (as does
nitrogen, and any gas other than helium or hydrogen):

"Thus higher mass constituents, such as oxygen and nitrogen, fall off more
quickly than lighter constituents such as helium, molecular hydrogen, and
atomic hydrogen. Thus there is a layer, called the heterosphere, in which
the earth's atmosphere has varying composition. As the altitude increases,
the atmosphere is dominated successively by helium, molecular hydrogen, and
atomic hydrogen."
http://en.wikipedia.org/wiki/Heterosphere (same page as previous citation)

So, perhaps the main thing that's wrong about the text in the Handbook is
that they have the altitude wrong, and in fact are describing a phenomenon
that's basically irrelevant if you're not an astronaut. Other than that...

Pete


.

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