Air density question?? Pilot's Handbook ofAeronautical knowledge

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??

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

Best,
Karl
"Curator" N185KG

I have a book titled "The Atmosphere" (duh) and it breaks the constituents
down as you have noted. Nothing about change in composition with altitude.
How long has it been since the PHAK was rewritten, if ever? I remember using
it back in the 60s.

Bob Gardner

"karl gruber" wrote in message
...
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??

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

Best,
Karl
"Curator" N185KG

The new version is available on-line from the FAA website,
it was re-written just a few years ago.

I was taught that the "air" is basically a uniform mixture
from seal level to outer space, with some variation for
local conditions. There is more water vapor over the ocean
than a large land mass, there is more dust below 10,000
feet. There is more ozone at 40-50 km because that is where
it is made. There is more man-made stuff below 18,000 feet
because we make it at the surface and the different layers
don't mix well. Volcanoes and thunderstorms transport stuff
to high altitudes.

Link http://www.faa.gov/pilots/training/handbook/


--
James H. Macklin
ATP,CFI,A&P

"Bob Gardner" wrote in message
...
|I have a book titled "The Atmosphere" (duh) and it breaks
the constituents
| down as you have noted. Nothing about change in
composition with altitude.
| How long has it been since the PHAK was rewritten, if
ever? I remember using
| it back in the 60s.
|
| Bob Gardner
|
| "karl gruber" wrote in message
| ...
| 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??
|
| I always thought that only the density changes until a
very high altitude.
|
| Best,
| Karl
| "Curator" N185KG
|
|
|
|

"karl gruber" wrote in message
...
[...] 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:

"homosphe [...] 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/gl...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

Peter Duniho wrote:
"karl gruber" wrote in message
...
[...] 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.


Karl mistakenly omitted to leave in a hyphen. It's thus Page 18, not
21, but 2-1 is more precise

Ramapriya

wrote in message
ups.com...
Karl mistakenly omitted to leave in a hyphen. It's thus Page 18, not
21, but 2-1 is more precise

That's okay. I wasn't questioning whether the quoted text actually appeared
in the Handbook (I have no reason to doubt that it does). I was just
pondering out loud the oddity that I never actually noticed the text before,
in spite of having (presumably) read it.

"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

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

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

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

"2.3.1 Turbulent Diffusion and the Homosphere -- While the molecular oxygen
is heavier than molecular nitrogen, the two gases do not stratify in our
atmosphere according to their weights. The gases don't stratify because
parcels of air are thoroughly mixed into a uniform soup by wind currents,
convection, and large-scale circulation patterns. These stirring processes
are such that there is very little variation in the atmosphere for gases like
nitrogen and oxygen. Such mixing is known as turbulent diffusion and it is
very important from the surface up to about 120 km. This region is known as
the homosphere, the region of the atmosphere where gases are uniformly mixed.

2.3.2 The Heterosphere -- Above 120 km, gases begin to stratify according to
molecular weight. Air is so thin at this altitude that individual molecules
are able to accelerate to high speeds before bumping into another molecule.
The lighter gases accelerate more than the heavier gases, and as a result,
the atmosphere begins to stratify according to their molecular weight. This
region above 120 km is called the heterosphere, the region of the atmosphere
where gases stratify according to their molecular weight."

A home experiment in trying to mix oil with water
demonstrates this tendency.

Try the same experiment with water and alcohol. The reason the oil and
water separate is that they are not miscible, and (of course) once
separated, the denser one sinks(*).

If you do the water/alcohol experiment, you'll find that whatever
alcohol does not end up in your stomach stays mixed with the water
fairly evenly. It would take a deep bucket to show a density variance.
Simply being warm mixes the stuff up enough to keep them mixed.

Otherwise, you are correct in the rest of your post, and it is (one of
the reasons) why the present atmospheric concentrations are not the same
as the original ones. The lighter stuff (Hydrogen and helium) end up
lost in space. The planets with more gravity hold on to those gasses a
bit better.

Jose
--
The monkey turns the crank and thinks he's making the music.
for Email, make the obvious change in the address.