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Veikko M{kel{: "Re: pearlescent clouds" (Jul 3, 11:21):
>
> Yes, mother-of-pearl (or nacreous) clouds needs some special
> conditions:
>
> * extra-ordinary cold temperature in stratosphere (in 17-30
> kms altitude). About -90C...-120C.
>
> * They have seen on lee side side of mountains, normally
> in north-south direction, because strong winds usually blow
> only from west in northern hemisphere. As Marko told,
> to mountains caused wavy wind, which both raises humidity
> to stratosphere and cools the layer.
>
>
> Is there such of conditions in Philippines? I guess these
> temperatures are perhaps possible also tropic stratosphere,
> or isn't it?
>
I doubt that. The reason why polar stratosphere is so cold during winter
is a sum of several factors. First of all, sun doesn't warm up the air
masses much, because it is under the horizon most of the time (all the
time at higher latitudes). Second, there are a lot less disturbances in
stratospheric flow, resulting to a more stable and nearly latitudinal
(east-west) flow, preventing the little mixing that might happen with
warmer low latitude air masses. In northern hemisphere there are strong
synoptic scale waves that can propagate upwards and mess things up
pretty good, but at the southern hemisphere the system is even more
stable. At tropics, sun is warming middle and upper stratosphere almost
equally during winder and summer, resulting to a nearly constant
temperatures, which are not that low. Actually the tropical tropopause
is colder that most the tropical stratosphere (if my memory hasn't
failed me miserably). In fact the tropical tropopause is widely known as
"cold trap", because it "traps" most of the water vapour by condensing
it. The little amount of vater vapour that survives and gets to the
stratospere is actually half of the total sources of humidity in
stratosphere, the second half is methane that produces water as it is
photodissociated in the upper stratosphere and in the mesosphere.
Mother-of-pearl clouds are seen when the polar vortex, as it is called,
is strong and undisturbed, because these are the times when temperatures
can easily drop below minus 130 degrees, or so. Mountain waves are not
needed to supply water, as was mentioned befure. In fact, they do not do
this. What they do is that they cause the stable air masses fluctuate up
and down, resulting to a temperature fluctuations. They also transport
energy upwards, and may interact with the basic flow.
To put is simply, I can't see how there could be cold enough and humid
enough in tropical stratosphere to cause mother-of-pearl clouds. I think
it could be something associated to the cold trap itself. The clouds
could be tropospheric clouds, but could still easily be around 15
kilometers above the ground, as the tropical troposphere is quite high.
> Only areas, I know, where to mother-of-pearl clouds exists
> are Scandinavia, Scotland and northern coasts of Antartica.
> There could be more.
>
Now that the man-supplied (methane generated) extra vater vapour is
making stratosphere more humid, I expect that mountain waves shoundn't
be any longer needed for mother-of-pearl clouds. Thus all the polar
regions are potential regions of existence.
Interesting point, by the way, is that mother-of-pearl clouds were seen
first time at 1870 and noctilucent clouds at 1885. Both of these clouds
need low temperatures and relatively high humidity to form in otherwise
so dry upper atmosphere. Methane is a major source of humidity in
stratosphere and in mesosphere and methane emissions are more than twice
of what they were during preindustrial age. It may well be that these
beautiful clouds were the first signs that the human kind is changing
the climate.
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