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(meteorobs) Noctilucent Clouds

> Please pardon my ignorance...what are noctilucent clouds?? TNX! Steve Harrison

Well, this is not to become a definition from a science book, but I'll try
to explain in a few words what noctilucent clouds (NLC) are.

NLC were first described by Hermann Jesse of Germany at the end of the
last century, briefly after the eruption of the Cracatau volcano. 
They are white/bluish or silverish clouds that appear near the northern
horizon at times when the sun's elevation is between -6 deg and -16 deg.
This tells you that these are not 'usual' clouds, which were dark at
that time due to the large depression of the Sun, but something special
that is still in the sunlight. The reason for that was found out soon -
NLC are clouds that appear at extreme large altitudes.  Parallax
measurements have shown, that they show up in a small layer at
about 84 km  - that's almost where meteors appear!. 

NLC are not completely understand by now, but the is a number of
well-established fact about them:

* their altitude varies only a little. This is because there is a
temperature minimum at 84 km.
* they can be seen when the Sun's depression is between 6 and 16 deg. The
first limit comes from the bright twilight, the second results from the
'sunset' at 84 km.
* NLC appear only in mid-northern or -southern latitudes, e.g. between ~50
and 65 deg. Nearer to the Earth's equator the atmosphere does not become
cold enough up there, nearer to the pole it does not really getting dark
in the NLC season.
* NLC appear only in summer time, i.e. for northern observers between the
end of May and beginning of August. At those times you have the right Sun
depression for a long time and the atmosphere has the right temperature.

What is not yet known is what triggers these NLC to show up. There have
been investigation for connections between the solar cycle and NLC or
between meteors and NLC, but to my knowledge none of them was successful.

In 1994 we started an NLC observing program in Germany to improve
observational statistics, which may help to clarify these issues. On
average, there are 2-3 really bright NLC displays each season. However,
our observations from the last years show that almost every second night
you may have some faint NLC.

Finally, if you want to know how NLC look like you should browse through
some older isuues of Sky&Telescope. I have also put some 1995 photographs
in my homepage at http://www.snafudot de/~smo/nlc/serie.html
More pictures can be found at
http://www-imk.physik.uni-karlsruhedot de/~muehr/Wolken/Wo103.html
http://www-imk.physik.uni-karlsruhedot de/~muehr/Wolken/Wo104.html
A very rich NLC source is http://alcor.coloradodot edu/

Sirko

************************************************************************** 
*           Sirko Molau             *                    __              *  
*          Str.246 Nr.16            *             " 2B v 2B "            * 
*          D-13086 Berlin           *                                    * 
*        sirko.molau@dlrdot de         *                       Shakespeare  *
*      http://www.snafudot de/~smo     *                                    *
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