Antoniadi scale

A scale of five points, devised by the French astronomer Eugenios Antoniadi (1870-1944) and widely used by amateur astronomers for describing the quality of seeing. The points on the scale are defined as follows: I, perfect; II, slight undulations with periods of virtually perfect seeing lasting for several seconds; III, moderately good seeing, though with noticeable air movements; IV, poor seeing making observations difficult; V, very bad seeing that permits no useful observation.