Transparency vs seeing
Transparency is how much light gets through the atmosphere. Seeing is how steady it is. They are independent, they often move in opposite directions, and which one matters depends entirely on what you are pointing at.
Build tonight’s air.
Try the extremes. Smaller seeing values mean steadier air; more aerosol means less light gets through.
Add Moon, altitude & cloud
Explore what the air changes.
Move a slider to compare the same target.
Illustrative, magnified effects—not a telescope photograph or a calibrated prediction. AOD is aerosol optical depth at 550 nm; ALR is artificial-to-natural sky brightness. Moonlight assumes the Moon is above the horizon. Cloud amount does not tell us where each cloud is.
Why stars blur—and how observatories correct it.
This ESO animation explains adaptive optics. Our lab illustrates atmospheric effects; it does not simulate an observatory’s correction system.
Original video and description · Credit: ESO · CC BY 4.0. No endorsement implied.
The clip loads from ESO only when you choose Play. No autoplay or third-party player.Visual explanation: a reference star reveals atmospheric distortions; a deformable mirror counteracts them to sharpen the image.
Faint structure and high-power detail both hold.
Post-front air often reveals galaxies while planets boil.
Background glow hurts galaxies, but stable detail survives.
Use low power, bright targets, or wait for another night.
What transparency changes
Transparency is about how much light the atmosphere absorbs and scatters away. Astronomers quantify it as extinction, measured in magnitudes lost per airmass. The Institute of Astrophysics of the Canaries breaks the total into three parts: Rayleigh scattering by air molecules, scattering by aerosols such as dust and pollution, and a small amount of molecular absorption by ozone and water vapour.
Those components add up to roughly 0.28 magnitudes per airmass at sea level under standard conditions, falling to about 0.16 at 2 km altitude — which is a large part of why observatories are built on mountains. Extinction is also strongly wavelength dependent, hitting blue light far harder than red.
What seeing changes
Seeing is a different thing entirely: turbulence. Air of differing temperature mixes along your line of sight and the image boils. It is defined as the angular full width at half maximum of the long-exposure point spread function, quoted in arcseconds, and it is governed by the Fried parameter, the width of atmosphere over which the wavefront stays roughly coherent. For most sites that parameter sits around 5 to 10 cm; at ESO's Paranal observatory it can reach 40 cm, which is why its standard reference seeing is 0.83 arcseconds.
Why they disagree
The two frequently disagree, and the reason is worth knowing. The clearest, most transparent air often arrives behind a cold front — dry, clean, and moving. That same movement is turbulence, so transparency is excellent while seeing is poor. A stagnant, humid night under high pressure can do the reverse: steady air, mediocre transparency. Good for planets, poor for galaxies.
This is why one number cannot serve every observer, and why the window finder reweights the same forecast according to what you intend to point at.
Sources and review notes · reviewed 2026-09-15
- Instituto de Astrofísica de Canarias — Atmospheric Extinction and Aerosol Optical Depth
- Isaac Newton Group of Telescopes — Atmospheric extinction
- ESO — On the Difference between Seeing and Image Quality (The Messenger 141)
- ESO Paranal — Astronomical Site Monitor
- Sky & Telescope — Transparency and Atmospheric Extinction