Observers use the words seeing and transparency for different atmospheric qualities. Seeing describes how steadily the atmosphere preserves fine angular detail. Transparency describes how efficiently light from an astronomical object reaches you through the atmosphere.
Seeing controls image steadiness and fine detail
Turbulent air with changing temperature and density bends incoming light by slightly different amounts from moment to moment. Through a telescope, stars may twinkle or swell and planetary detail may blur and sharpen repeatedly. Poor seeing can make high magnification unproductive even when the sky looks perfectly clear.
Planetary observing, lunar observing, double-star work and high-resolution imaging are particularly sensitive to seeing because they depend on small angular details.
Transparency controls contrast and limiting faintness
Water vapour, aerosols, haze, smoke and thin cloud scatter or absorb light. Reduced transparency brightens or whitens the sky background and makes faint galaxies and nebulae harder to separate from it. Deep-sky observing often benefits more from excellent transparency than from exceptionally steady seeing.
The two conditions can move independently
A night after a weather front may have exceptionally transparent air but strong turbulence. Another night may have stable air that produces crisp planetary detail while humidity or haze reduces deep-sky contrast. A useful observing plan therefore considers both qualities instead of treating “clear” as a complete description.
How this affects equipment choices
The Telescope Setup Calculator can show a mathematically high magnification, but the atmosphere may prevent that power from revealing more detail. The Pixel Scale Calculator also asks for seeing because camera sampling should be interpreted in relation to the angular blur imposed by the atmosphere.