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That's a good site. Looking at that reminded me that in my youth I used a different scale which was popularised in Nortons Star Atlas (the original but now defunct astronomers bible).
Here's the abstract from my 1981 edition:
Antoniadi Scale. (a leading planetary observer apparently)
I. Perfect seeing, without a quiver;
II. Slight undulations, with moments of calm lasting several seconds;
III. Moderate seeing with larger air tremors;
IV. Poor seeing, Constant troublesome undulations;
V. Very bad seeing, scarcely allowing the making of a rough sketch.
I rather like it - it's not very scientific, but easy to apply.
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