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The Douglas scale (also called the WMO sea state code) describes the state of the sea in ten degrees, 0 through 9, by the height of waves at the centre of a 10-minute observation window. It was devised in the 1920s by Captain H. P. Douglas of the British Admiralty and is now the standard sea-state report in routine SYNOP and METAREA bulletins.
Sea state describes the sea. Beaufort describes the wind. They correlate, but loosely — a fresh breeze on a sheltered fetch will give a much smoother sea than the same wind on an open ocean swell. Always report sea state and swell separately when log-keeping.
| Degree | Description | Metres | Feet | Beaufort | Sea criterion |
|---|---|---|---|---|---|
| 0 | Calm (glassy) | 0 | 0 | 0 | Sea like a mirror; no wind ripples. |
| 1 | Calm (rippled) | 0–0.10 | 0–⅓ | 1 | Small ripples without crests; surface scaled. |
| 2 | Smooth (wavelets) | 0.10–0.50 | ⅓–1⅔ | 2–3 | Small wavelets, glassy crests, not breaking. |
| 3 | Slight | 0.50–1.25 | 1⅔–4 | 3–4 | Large wavelets; crests begin to break, scattered whitecaps. |
| 4 | Moderate | 1.25–2.50 | 4–8 | 4–5 | Small waves becoming longer; fairly frequent whitecaps. |
| 5 | Rough | 2.50–4.00 | 8–13 | 5–6 | Moderate waves with longer form; many whitecaps, some spray. |
| 6 | Very rough | 4.00–6.00 | 13–20 | 6–7 | Large waves begin forming; foam blown in streaks; airborne spray. |
| 7 | High | 6.00–9.00 | 20–30 | 8–9 | Heaped sea; dense streaks of foam; spray may reduce visibility. |
| 8 | Very high | 9.00–14.00 | 30–46 | 10–11 | Long overhanging crests; sea takes on a white appearance. |
| 9 | Phenomenal | > 14.00 | > 46 | 12 | Air filled with foam and spray; sea completely white; visibility seriously reduced. |
The wave-height columns above are significant wave height (Hs) — the mean of the highest one-third of waves in the observation period. Individual waves can be substantially higher: in fully-developed seas, the maximum wave in a 3-hour window is roughly 1.9 × Hs, and rogue waves of 2 × Hs or more are not uncommon in the open ocean.
Wind sea is generated by the wind currently blowing over the observation area. Swell is wave energy that has propagated out from a distant generation area. A weather routing report typically gives sea height, swell height, swell period, and swell direction separately because the two combine non-linearly in the resulting motion of the vessel.