A measure of how dark the sky is on a given night. Transparency is
affected by the amount of humidity and dust in the atmosphere, as well
as by the amount of light pollution. The four stars in the bowl of the
Little Dipper are magnitudes 2.2, 3.1, 4.3, and 5.0. If all four can be
seen most nights without using averted vision (after your eyes have had
10 minutes or so to become dark adapted), and you can clearly see the
faint outline of the Milky Way, the transparency would be rated 5 and
your observing site is probably dark enough to let you use a 10" scope
without being overly affected by light pollution. If you have to use
averted vision to see the fourth star, you may be limited to an 8"
scope. If only three of the Little Dipper stars can be seen consistently
(the faintest being magnitude 4.3), the transparency would be rated 4,
and light pollution will probably limit you to a 6" scope. A
transparency of 4 is only fair for deep sky observing. A transparency of
5 is much more satisfactory with an 8" or larger scope.