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.
Transparency is a measure of absorption and scattering: haze, humidity, dust, thin high cloud. Poor transparency dims everything, and it dims faint things disproportionately. A galaxy at magnitude 10 disappears into the sky background long before Jupiter looks any different.
Seeing is turbulence. Air of different temperatures mixes along your line of sight and the image boils. It is measured in arcseconds — the apparent diameter a point source is smeared into. Under 1 arcsecond is superb and rare. Two to three arcseconds is ordinary. Four or more and high magnification is pointless.
The two frequently disagree, and the reason is instructive. The clearest, most transparent nights often follow a cold front, when the air is dry and clean — and moving. That same moving air is turbulent, so transparency is excellent and seeing is terrible. Conversely a stagnant, humid summer night under a high-pressure ridge can have steady air and dreadful transparency: perfect for planets, useless for galaxies.
This is why one number cannot serve every observer, and why the window finder reweights the same forecast depending on what you intend to do.
REVIEWED 2026-08-12 · SOURCES: MET Norway, sun and moon positions computed on our own servers