Douglas sea state scale
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.
Scale
| 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. |
Significant wave height
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.
Sea vs. swell
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.
See also
- · Beaufort wind scale — the wind force that drives the sea.
- · Distress frequencies — for reporting position when conditions deteriorate.
- · Reference index — full list of quick-reference pages.
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