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re: THAT THIN CLOUD? WILL IT GO AWAY?

Great article Lew!
PS, that's just what I had last night, too.
Went inside and got some sleep instead.
Wayne Tv
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Original Text
>From Lew Gramer <dedalus@latradedot com>, on 5/15/96 5:50 PM:
To: <meteorobs@latradedot com>

I found this in the EJASA archive (Electronic Journal of the Astronomical 
Society of the Atlantic). Something we should keep in mind when reporting 
cloud 
cover on our reports maybe?

[From EJASA Volume 6, Number 11 - June 1995. Reprinted without permission.]

                     THAT THIN CLOUD? WILL IT GO AWAY?
                            by Alister Ling
          (Reprinted from the April 1993 Hamilton Centre "Orbit")

       In short, no. Most observers have recognized this pattern: thin
    high clouds, or cirrus, lie along the horizon but disappear as night 
    falls.  Frequently, this is an illusion.
       As a meteorologist, I have watched thin cirrus on satellite imagery 
    march across reporting stations with nary a change overnight, yet the
    observer reports clear, or very scattered high cloud. This phenomenon 
has
    now been confirmed by a 4-year study of satellite imagery. There is no
    diurnal, or day to night change in semi-transparent cirrus. The only 
    exception to this is in summer, when cirrus formed by thunderstorms has 
    a tendency to evaporate gradually through the night.
       The illusion can be explained by an analogy to M57, the Ring nebula. 
    One can think of the nebula as a uniform spherical shell of gas that
    is mostly transparent. Looking through the center, we can see that 
there 
    is little absorption. However, as we look along an edge, our line of 
    sight encounters sufficient material to produce a noticeable veil.
       The same thing happens when we look at the sky. Overhead, the cirrus 
    is so thin it is in effect transparent, but as we look slantwise 
through 
    it towards the horizon, the cloud shows up. If it is dark, the stars 
    still shine through, but the Milky Way would lose much of its impact. 
    The situation is no different with haze during summer.
       Numerous times I've overheard observers saying how lucky we are
    to be free of haze overhead but are surrounded on all sides. If you 
    are willing to admit it to yourself, you will easily notice that 
    extended objects like M31 just don't have the detail you are used to 
    seeing. The best advice I can give for you is to accept what you've 
    got and observe high surface brightness objects like planets, clusters 
    and planetary nebulae. You have ventured into the night to relax, 
    so don't fret.