Douglas sea scale

A numerical scale classifying different sea states from 0 to 9, according to wave size. It was created by the English vice-admiral Sir Henry Percy Douglas (1876-1939) in 1919, during the period he directed the British Royal Navy's Hydrographic Service. At the lower end of the scale, grade 0 corresponds to a calm or flat sea, with no waves. At the upper end, grade 9 corresponds to an huge sea (sic), with waves over 14 meters.