Sea State Atlas
Beaufort scale, wave state codes, and how to eyeball wind from what you see.
Beaufort scale (mariner's shorthand)
| Force | Wind (kt) | Description | Sea |
|---|---|---|---|
| 0 | < 1 | Calm | Mirror |
| 1 | 1–3 | Light air | Ripples |
| 2 | 4–6 | Light breeze | Small wavelets, glassy crests |
| 3 | 7–10 | Gentle breeze | Large wavelets, scattered whitecaps |
| 4 | 11–16 | Moderate | Small waves, frequent whitecaps |
| 5 | 17–21 | Fresh | Moderate waves, many whitecaps, spray |
| 6 | 22–27 | Strong breeze | Large waves, whitecaps everywhere, spray |
| 7 | 28–33 | Near gale | Sea heaps up, foam streaks with wind |
| 8 | 34–40 | Gale | Moderately high waves, foam in well-marked streaks |
| 9 | 41–47 | Strong gale | High waves, dense foam, crests rolling over |
| 10 | 48–55 | Storm | Very high waves, sea white with foam, visibility reduced |
| 11 | 56–63 | Violent storm | Exceptionally high waves, sea covered with foam patches |
| 12 | ≥ 64 | Hurricane | Air full of foam and spray, sea white, visibility very reduced |
WMO sea-state code (used in SYNOP reports)
| Code | Sig wave height (m) | Description |
|---|---|---|
| 0 | 0 | Calm (glassy) |
| 1 | 0–0.1 | Calm (rippled) |
| 2 | 0.1–0.5 | Smooth (wavelets) |
| 3 | 0.5–1.25 | Slight |
| 4 | 1.25–2.5 | Moderate |
| 5 | 2.5–4 | Rough |
| 6 | 4–6 | Very rough |
| 7 | 6–9 | High |
| 8 | 9–14 | Very high |
| 9 | > 14 | Phenomenal |
Significant wave height, explained
"Sig wave height" (Hs) = mean height of the highest one-third of waves. Individual waves can be much larger:
- Max expected in 3 h ≈ 1.6 × Hs.
- 1-in-1000 wave ≈ 1.9 × Hs.
- Rogue wave (Hmax > 2 × Hs) is rare but has been measured.
A 3 m Hs forecast means routine 3 m waves, but plan for occasional 5–6 m sets. Period matters too — 9 s waves at 3 m are uncomfortable; 5 s waves at 3 m are dangerous.