Transparency
Astronomy Sky transparency Atmospheric transparency Sky clarity
Astronomical transparency describes how readily light passes through the atmosphere. It helps determine whether faint stars, nebulae and galaxies can be seen.
The sky as a more or less clear window
Thin clouds, haze and aerosols can scatter or absorb light. Water vapour absorbs especially in certain infrared bands. Good transparency means little attenuation in the wavelength range being considered, so transparency also depends on wavelength.
Clear, steady and dark are different things
- Transparency: How much light from the object gets through?
- Seeing: How steady and sharp is the image?
- Sky brightness: How much do moonlight, artificial light and natural sources brighten the background?
A sky can therefore be very clear but still bright. The stars are not the problem; the competing lighting is.
Why faint objects disappear first
A galaxy spreads its light across an area. If that light is weakened or the background becomes brighter, contrast decreases. A bright planet may remain easy to see in the same conditions.
What forecasts can tell us
Cloud, aerosols and humidity provide clues, but surface visibility alone cannot describe the whole atmospheric path. Near the horizon, you also look through more air than overhead. A high observing target therefore helps even when transparency is good.