The Douglas Sea State Scale
Beaufort measured the wind. Douglas measured what the wind had done to the sea.
Origin
Devised by Vice-Admiral Henry Percy Douglas in 1921, while he was Director of the British Naval Meteorological Service.
The catch
Wind and sea do not agree. A gale that has just begun leaves a calm sea, and a storm a thousand miles away arrives as heavy swell under a clear sky. Douglas is a second scale precisely because Beaufort could not describe both.
No waves at all. The surface is unbroken.
Ripples without crests.
Small wavelets, no breaking.
Large wavelets, some crests beginning to break.
Moderate waves with breaking crests and scattered foam.
Large waves, extensive foam crests, some spray.
Sea heaps up. Foam blown in dense streaks along the wind.
High waves with long overhanging crests. Visibility affected.
Very high waves. The surface takes on a white appearance.
The sea is completely covered with foam and spray. Air filled with driving water.
Douglas is in fact two scales used together: one for wind sea, generated locally, and one for swell, arriving from distant weather. A navigator reporting sea state gives both, since a two-metre wind sea and a two-metre swell present quite different problems to a ship. The scale remains in international use for marine observation and forecasting.