Astronomical Transparency Scale
0 - Completely cloudy or precipitating.
1 - Very Poor - Mostly Cloudy.
2 - Poor - Partly cloudy or heavy haze. 1 or 2 Little Dipper stars visible.
3 - Somewhat Clear - Cirrus or moderate haze. 3 or 4 Little Dipper stars visible.
4 - Partly Clear - Slight haze. 4 or 5 Little Dipper stars visible.
5 - Clear - No clouds. Milky Way visible with averted vision. 6 Little Dipper stars visible.
6 - Very Clear - Milky Way and M31 visible. 7 Little Dipper stars visible.
7 - Extremely Clear - M33 and/or M81 visible.

Courtesy of the American Association of Amateur Astronomers
http://www.astromax.org/faq/aa01faq14.htm