The Saguaro Astronomy Club (Phoenix, AZ)
Night Sky Transparency Scale
Rating	What It Means:
0	completely cloudy, no stars seen (why are you out?)
1	more than 50% of the sky is cloudy
2	more than 25% of the sky is cloudy, less than 50%
3	more than 10% of the sky is cloudy, less than 25%
4	no clouds but hazy, only brightest stars seen down to 4th magnitude
5	somewhat hazy, some fainter stars seen, to mag 5;
	Milky Way visible only in brighter regions
6	not visibly hazy but Milky Way visible only in brighter regions
	(Sagittarius, Cygnus, Norma + Crux); stars seen to mag 5.8
7	fainter stars, equal to mag 6.0 are seen and the fainter parts of the
	Milky Way seen with averted vision, Zodiacal light seen with averted vision
8	stars fainter than mag 6.0 are just seen and fainter parts of the Milky Way
	are more obvious, Zodiacal light is seen with direct vision
9	stars fainter than mag 6.0 are seen with direct vision and so are faint portions
	of the Milky Way (Lyra, Libra), gegenschein seen with averted vision
10	overwhelming profusion of stars, Zodiacal light and the gegenschein form
	continuous band across the sky, the Milky Way is very wide and bright throughout

From the Saguaro Astronomy Club (SAC), www.saguaroastro.org

As quoted in Deep Sky Observing – The Astronomical Tourist,

Steven R. Coe, Springer, 2000