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.