Forecasting Dark Skies

What tools do we have available to forecast sky darkness? By darkness, I
mean how inky black a the sky at a particular viewing site will be well
after dark with no moon.

Let me explain. I have observed from my rural location three times in the
past 10 days. During those periods there has been zero cloud cover and no
change in the visible levels of light pollution on the horizon. Observing
periods were the same: from 22:00L to 01:00L.

The first period I could easily see 5 stars in the bowl of the Little Dipper
and the dust lanes in the Milky Way extended all the way to Cygnus. I could
also see the full extension of the Milky Way into Ophiuchus. M4, M7 & M8
were easily seen with the naked eye. They sky was not inky black but it was
a very dark gray and there was very good contrast. The moon was 2 hours away
from rising. Transparency was forecast at 4 out of 5.

The second period the sky was a medium gray. The Milky Way was washed out,
although I could see the extension into Ophiuchus. It looked like the sky
does with a rising quarter moon. M4 and M8 were invisible to the naked eye,
although M7 could just be made out. I could make out 4 stars in the Little
Dipper. Transparency was forecast at 3 out of 5.

The third period was the worst. The sky was a light gray. The Milky Way was
barely visible with no extension into Ophiuchus and no dust lane into
Cygnus. M7 was not visible to the naked eye. I could make out 3 stars in the
Little Dipper. Forecast transparency was 3 of 5. It almost looked like the
full moon was up.

In all these cases there was no cloud cover and transparency was similar. I
do have some medium light pollution from Dallas (80 miles) in my
southwestern sky, so I am assuming that this is the culprit and that
different levels of air clarity have "reflected" the pollution throughout my
skies on the bad viewing days.

When choosing a night to view (or in choosing what objects to view on a
particular night) it would be nice to be able to forecast how dark my skies
will be. Any ideas? Thanks.

-John


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