Noctilucent Cloud

Hi all,

Can I just remind everyone that the northern hemisphere NLC season has
started. Further information is available at:

www.nlcnet.co.uk

Tom

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+--------------------------------------------------+
| Aurora info: http://www.baa-aurora.fsnet.co.uk |
| NLC info: http://www.nlcnet.co.uk |
+--------------------------------------------------+

Tom McEwan wrote:
Can I just remind everyone that the northern hemisphere NLC season has started. Further information is available at:

www.nlcnet.co.uk



Thanks. Something that eludes me every time I read about noctilucent
clouds is why they are only visible in a range of latitudes near the
Poles. I understand how you would have many more hours of twilight to
provide viewing opportunities, but that alone would not make them
impossible to see during the briefer twilight at lower latitudes. Do
the clouds themselves exist only over the higher latitudes (seems
unlikely when they are at 80km)? What aspect of the geometry am I
missing?

Tom

On 31 May 2005 13:23:43 -0700, "Tom Polakis"
wrote:

Thanks. Something that eludes me every time I read about noctilucent
clouds is why they are only visible in a range of latitudes near the
Poles. I understand how you would have many more hours of twilight to
provide viewing opportunities, but that alone would not make them
impossible to see during the briefer twilight at lower latitudes. Do
the clouds themselves exist only over the higher latitudes (seems
unlikely when they are at 80km)? What aspect of the geometry am I
missing?

I think it is because the Earth's shadow after sunset cuts through the
atmosphere at a more shallow angle towards the poles in summer.
Basically, the Sun continues to light up the air overhead much longer
after sunset. For the same reason, you can see satellites in much of the
sky during most of the night in the summer; in the winter you stop
seeing them not long after sunset until shortly before sunrise.

The geometry: imagine facing west at midnight. In the winter, or at low
latitudes, the Sun is basically underneath you, so the Earth's shadow is
cast straight overhead. Most of the sky is dark 'all the way up'. But in
summer, at high latitudes, the Sun is only a little below the horizon.
The Earth's shadow is now cast back behind you, towards the east (the
antisolar point is only a little above the eastern horizon). So you
still have sunlight hitting the air at the high altitudes where
noctilucent clouds form.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

On Tue, 31 May 2005 20:45:49 GMT, Chris L Peterson wrote:

The geometry: imagine facing west at midnight. In the winter, or at
low latitudes, the Sun is basically underneath you, so the Earth's
shadow is cast straight overhead.

Erm, at midnight (GMT) I think you mean facing north for the northern
hemisphere rather than west. What you the go one to say makes much
more sense provided you subsitute south for east.

Most of the sky is dark 'all the way up'. But in summer, at high
latitudes, the Sun is only a little below the horizon. The Earth's
shadow is now cast back behind you, towards the east (the antisolar
point is only a little above the eastern horizon). So you still have
sunlight hitting the air at the high altitudes where noctilucent
clouds form.

--
Cheers
Dave. pam is missing e-mail

On Tue, 31 May 2005 22:40:18 +0100 (BST), "Dave Liquorice"
wrote:

Erm, at midnight (GMT) I think you mean facing north for the northern
hemisphere rather than west. What you the go one to say makes much
more sense provided you subsitute south for east.

When I say midnight, I mean local midnight, not GMT. But yes, the Sun
will be at its lowest below the horizon to the north, not the west, and
the antisolar point will be to the south.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

On Tue, 31 May 2005 22:31:32 GMT, Chris L Peterson wrote:

When I say midnight, I mean local midnight, not GMT.

When I say "midnight (GMT)" it was with a UK centric meaning as this
is a uk group. B-) To differentiate it from midnight clock time as
that is not real midnight in the summer.

--
Cheers
Dave. pam is missing e-mail

I wrote:

Something that eludes me every time I read about noctilucent
clouds is why they are only visible in a range of latitudes near the Poles.


Chris Peterson wrote:

I think it is because the Earth's shadow after sunset cuts through the atmosphere at a more shallow angle towards the poles in summer. Basically, the Sun continues to light up the air overhead much longer after sunset...


But that shallow angle of the shadow is no different than than the
angle if you were standing on the equator. It lasts much, much longer
at high latitudes, as you say, but why can't a person standing on the
equator see noctilucent clouds during the half hour or so of optimal
twilight if they happen to be active at that time? Again, I don't see
anything special about the polar latitudes other than the obvious
difference in duration of twilight.

Tom

Tom Polakis wrote:
But that shallow angle of the shadow is no different than than the
angle if you were standing on the equator. It lasts much, much longer
at high latitudes, as you say, but why can't a person standing on the
equator see noctilucent clouds during the half hour or so of optimal
twilight if they happen to be active at that time? Again, I don't see
anything special about the polar latitudes other than the obvious
difference in duration of twilight.

It's colder near the poles. Maybe that facilitates the formation of
clouds that are best illuminated during twilight.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

On 31 May 2005 15:40:55 -0700, "Tom Polakis"
wrote:

But that shallow angle of the shadow is no different than than the
angle if you were standing on the equator. It lasts much, much longer
at high latitudes, as you say, but why can't a person standing on the
equator see noctilucent clouds during the half hour or so of optimal
twilight if they happen to be active at that time? Again, I don't see
anything special about the polar latitudes other than the obvious
difference in duration of twilight.

Do NLCs happen at all latitudes to begin with? Even so, the observation
window may be much less than 30 minutes at low latitudes. At the equator
the Sun sets very quickly- there is hardly any twilight. That might also
mean a different temperature profile to the atmosphere that makes NLC
formation difficult. I don't know that you can't see them at the
equator; maybe they are just a lot more rare than they are at higher
latitudes (where they are fairly rare as it is)?

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Chris L Peterson wrote:

Do NLCs happen at all latitudes to begin with?


That's what one would think is happening, but from the site of the
original poster:

"They form at very high altitudes - around 82 km above sea level - and
are, thus, a quite separate phenomenon from normal weather or
tropospheric cloud."

At that height, it wouldn't seem like latitude would matter.



Even so, the observation window may be much less than 30 minutes at low latitudes. At the equator the Sun sets very quickly- there is hardly any twilight.


Just ran the planetarium software, and it shows that even on the
equator, the period of visibility (sun's altitude between -6 and -16
degrees, according to that site) is 45 minutes. That's a short window,
but I don't see it as impossible to see noctilucent clouds from the
equator.

I still don't get it!

Tom