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.