Sky Visibility

Transparency

ScoreDescriptorDescriptionVisual test
0Clouded outPossible light precipitation.No stars visible.
1Very PoorCloud covering most of sky.Momentary patches of clear sky.
2PoorPatchy cloud or heavy haze.Alpha and Beta Crux visible.
3Somewhat clearCirrus or Moderate haze.α, β, γ Crux visible.
4Partly clearSlight haze.α, β, γ, δ Crux visible.
5ClearNo clouds.α, β, γ, δ, ε Crux visible.
6Very ClearNo clouds, little sky glow, very little light pollution.Milky way visible, all stars of Crux visible plus Coal Sack.
7Extremely ClearNo clouds, minimal sky glow, Dark skies.ω Centauri and 47 Tucanae visible to naked eye.

Seeing

ScoreDescription
1Perfect, no movement of the image even at high magnification (100X plus).
2Slight quiver with calm moments lasting several seconds.
3Moderate quiver with larger tremors, blurring of image.
4Poor constant undulation of image.
5Very Bad not able to make even a rough sketch of image.

Bortle Scale

ClassTitleDescription
1Excellent
dark-sky site
Many constellations, particularly fainter ones, are barely recognizable amid the large number of stars.
Limiting magnitude with 12.5″ reflector is 17.5.
2Typical truly
dark site
Milky Way is highly structured.
Many Messier objects and globular clusters are naked-eye objects.
Limiting magnitude with 12.5″ reflector is 16.5.
3Rural skyLight pollution and clouds are evident at the horizon.
The Milky Way still appears complex.
Limiting magnitude with 12.5″ reflector is 16.
4Rural/
suburban
transition
Pollution domes visible in several directions.
The Milky Way is still impressive, but lacks detail.
Limiting magnitude with 12.5″ reflector is 15.5.
5SuburbanLight pollution is visible in most directions.
Clouds are noticeably brighter than the sky.
The Milky Way is very weak or invisible near the horizon
Limiting magnitude with 12.5″ reflector is 15.
6Bright
suburban
Light pollution makes the sky within 35° of the horizon glow gray.
White clouds anywhere in the sky appear fairly bright.
Background surroundings are easily visible.
The Milky Way is only visible near the zenith.
Limiting magnitude with 12.5″ reflector is 14.5.
7Suburban/
urban
transition
Light pollution makes the entire sky light grey. Clouds are bright.
Strong light sources are evident in all directions.
The Milky Way is nearly or totally invisible.
Limiting magnitude with 12.5″ reflector is 14.
8City The sky is light gray or orange.
Stars forming familiar constellation patterns may be weak or invisible.
Limiting magnitude with 12.5″ reflector is 13.
9Inner-cityWhite.
Source of above information: Wikipedia