Douglas Sea Scale
The Douglas Sea Scale was devised by the English Admiral H.P. Douglas in 1917, while he was head of the British Meteorological Navy Service. Its purpose is to estimate the sea's roughness for navigation. The Douglas Scale consists of two codes, one for estimating the state of the sea (fresh waves attributable to local wind conditions), the other for describing sea swell (large rolling waves attributable to previous or distant winds).
The Douglas Sea Scale is expressed in one of 10 degrees.
Degree 0—no measurable wave height, calm sea
Degree 1—waves >10 cm., rippled sea
Degree 2—waves 10–50 cm., smooth sea
Degree 3—waves 0.5–1.25 m., slight sea
Degree 4—waves 1.25–2.5 m., moderate sea
Degree 5—waves 2.5–4 m., rough sea
Degree 6—waves 4–6 m., very rough sea
Degree 7—waves 6–9 m., high sea
Degree 8—waves 9–14 m., very high sea
Degree 9—waves >14 m., phenomenal sea
It was difficult to relate the existing wind scale designed by Sir Frances Beaufort in 1805 to a ship's features, especially as sails were replaced with the rigid structures of powered ships. The Douglas Sea Scale standardized the many variations being used by ship captains from many nations.