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> Thanks for the explanation, Paul! So the "pauses" are the transition
areas
> between the "spheres" where the chemical composition of the lower sphere
is
> changing into that of the upper sphere. Do I have that right?
No -- there is no sudden change in chemical composition at either pause
except at the homopause. Yep -- another "pause", and we also have the
"neutropause".....
If we use temperature as an indicator, we get these "spheres" and
"pauses":
Troposphere
Tropopause, 6-18 km
Stratosphere
Stratopause, ca 50 km
Mesosphere
Mesopause, ca 80 km
Thermosphere
If we instead use chemical composition, we get:
Homosphere
Homopause, ca 120 km
Heterosphere
In the homosphere the chemical composition of dry air changes very
little. The amount of water vapor changer a lot of course, but
that's all. When we enter the heterosphere the chemical composition
begin tyo change, first a little and then drastically: the UV
radiation will first split up oxygen (O2) molecules into atomic
oxygen (O). Also the mean free path (= the mean distance travelled
by a molecule/atom between collisions with other molecules) becomes
very large. THis means that gases no longer mix but start to behave
as if they were independent of each other. This causes the density
of heavier molecules to drop off more rapidly than the density of
lighter molecules, which means that at higher altitudes the
atmosphere will contain more of these lighter molecules: first the
helium (He) density will become dominant at perhaps 200 km altitude,
and still higher, at 300-400 km, the very thin atmosphere we have
there will consist almost purely of atomic hydrogen (H) !!!
Finally we can use ionization as an indicator, then we get:
Neutrosphere
Neutropause, 50-90 km
Ionosphere
The altitude of the neutropause varies a lot and is dependent on
latitude, season, time-of-day, and solar activity. Occasionally,
during severe solar activity, the ionosphere may penetrate down to
about 30 km altitude.
> Of course if that is true, then what about the ozone layer? Is the ozone
> actually part of the tropopause?
No. Ozone is a minor constituent, and our atmopshere never contains
more than a fraction of a percent ozone. The reason ozone is so
important is (in the stratosphere) because it absorbs solar UV radiation
so strongly, and (in the troposphere) because ozone very easily reacts
with other chemical compounds. Like all minor constituents, the amount
of ozone varies strongly all through the atmosphere.
> Are nacreous clouds and noctilucent clouds ever visible to us on the
> ground?
YES!!!! In July and August noctilucent clouds are commonly visible,
usually at least several times per summer, sometimes much more.
Nacreous clouds are more rare, but they are visible from the ground
too.
Paul Schlyter, Swedish Amateur Astronomer's Society (SAAF)
Grev Turegatan 40, S-114 38 Stockholm, SWEDEN
e-mail: pausch_at_saaf.se psr_at_home.ausys.se
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