|
meteoptic-l [ät] ursa.fi
message archive
This is meteoptic-l [ät] ursa.fi message archive.
Note, your can reply the messages on this page only if your
are already subscribed the list.
» To the end of the list/message
> I've been enjoying the discussions of sundogs, halos, and pearlescent
and
> irridescent clouds. What beautiful descriptions! I've also read some
things
> I don't understand, such as "tropopause".
Oor atmosphere can be divided in several "spheres", and the top of each
"sphere" is called a "pause". We have:
Troposphere 0 to 6-18 km Temperature decreases with altitude
Stratosphere 6-18 to ca 50 km Temperature constant or increasing
with altitude
Mesosphere ca 50 to ca 80 km Temperature decreasing with altitude
Termosphere 80 km and above Temperature increasing with altitude
The tropopause is the top of the troposphere, at 6-18 km altitude.
This altitude is latitude dependent -- it's lowest at the poles and
highest in the tropics.
The stratopause is the top of the statosphere, at ca 50 km altitude.
The mesopause is the top of the troposphere, at ca 80 km altitude.
The reason these "spheres" do exist are the heat sources in our
atmosphere:
- The ground absorps solar radiation, is heated, and will heat up
the troposphere from below. Within the troposphere all phenomena we
call "weather" happens. With a few exceptions, all our clouds are
tropospheric clouds (the exceptions are nacreous clouds which occur
in the lower stratosphere, and the noctilucent clouds which occur
close to the mesopause).
- The ozone layer absorbs UV solar radiation, and this causes a
maximum heating at about 50 km altitude -- that's why we have a
temperature maximum there. If our atmosphere had no ozone, it would
also have no stratosphere or mesosphere -- instead the termosphere
would start immediately above the tropopause. This is the situation
in the atmospheres of Venus and Mars: in both these atmospheres the
tropopause is much higher, at about 60-90 km altitude, and the
thermosphere starts immediately above the tropopause.
- The thermosphere is heated from above, by absorbing extreme UV
radiation and charged particles from the Sun. This part of the
atmosphere becomes very hot, ca 1000 to 2000 degrees, depending on
solar activity. Above ca 200 km there is little variation in
temperature with altitude - here the thermosphere is essentially
isothermal. There is no well-defined "thermopause", instead above
ca 800 to 1000 km altitude the thermosphere merges into the
magnetosphere, and then into interplanetary space.
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
|