The Douglas sea scale classifies sea state in 10 degrees (0–9) by significant wave height. Degree 0: calm (0 m). Degree 3: slight (0.5–1.25 m). Degree 5: rough (2.5–4 m). Degree 7: high (6–9 m). Degree 9: phenomenal (>14 m).
Operational use
It is the standard scale for marine weather bulletins and coastal warnings. AEMET includes the Douglas sea state in its navigation bulletins. Unlike the Beaufort scale (measuring wind), Douglas directly evaluates the resulting wave conditions.