sky condition and 'transparency'
on the 1 to 10 scale it is measured as:

1-3 Very Bad
4-5 Poor
6-7 Good
8-10 Excellent

Or the Antoniadi scale:

I - perfect seeing, without a quiver.
II - Slight undulations, with moments of calm lasting several seconds.
III - Moderate seeing, with large tremors.
IV - Poor seeing, with constant troublesome undulations.
V - Very bad seeing, scarcely allowing the making of a rough sketch.

Besides seeing, there is another measure used to describe the sky
condition: 'transparency' - it is the darkness/clearness of the sky,
measured from 1 to 6. It is about equal to the magnitude of the
faintest star visible near the zenith with the naked eye. So 6 means perfectly transparent sky.

The following 2 images will give you better idea of how stars might
appear at different seeing conditions:

1: this image shows a double star, seeing is worsening towards right
2: diffraction pattern of a star used to measure seeing.

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