The Antoniadi scale is a system used by amateur astronomers to categorise the weather conditions when viewing the stars at night.
The scale is on a 5 point system, with one being the best seeing conditions and 5 being worst. The actual definitions are as follows:
I. Perfect seeing, without a quiver.
II. Slight quivering of the image with moments of calm lasting several seconds.
III. Moderate seeing with larger air tremors that blur the image.
IV. Poor seeing, constant troublesome undulations of the image.
V. Very bad seeing, hardly stable enough to allow a rough sketch to be made.
Note the scale is usually indicated by use of a Roman numeral or an ordinary number.
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