The Douglas Sea Scale is a scale which measures the swell of the sea and the height of the waves:
| Beaufort number (force) | Wind Speed | Wave height (feet) | WMO* description | Effects observed on the sea | Effects observed on land | |
|---|---|---|---|---|---|---|
| knots | mph | |||||
| 0 | under 1 | under 1 | - | Calm | Sea is like a mirror |  |
| 1 | 1 - 3 | 1 - 3 | 0.25 | Light air | Ripples with appearance of scales; no foam crests | |
| 2 | 4 - 6 | 4 - 7 | 0.5 - 1 | Light breeze | Small wavelets; crests of glassy appearance, not breaking |  |
| 3 | 7 - 10 | 8 - 12 | 2 - 3 | Gentle breeze | Large wavelets; crests begin to break; scattered whitecaps |  |
| 4 | 11-16 | 13-18 | 3½ - 5 | Moderate breeze | Small waves, becoming longer; numerous whitecaps |  |
| 5 | 17-21 | 19-24 | 6 - 8 | Fresh breeze | Moderate waves, taking longer form; many whitecaps; some spray |  |
| 6 | 22-27 | 25-31 | 9½-13 | Strong breeze | Larger waves forming; whitecaps everywhere; more spray |  |
| 7 | 28-33 | 32-38 | 13½-19 | Near gale | Sea heaps up; white foam from breaking waves begins to be blown in streaks |  |
| 8 | 34-40 | 39-46 | 18-25 | Gale | Moderately high waves of greater length; edges of crests begin to break into spindrift; foam is blown in well-marked streaks |  |
| 9 | 41-47 | 47-54 | 23-32 | Strong gale | High waves; sea begins to roll; dense streaks of foam; spray may begin to reduce visibility |  |
| 10 | 48-55 | 55-63 | 29-41 | Storm | Very high waves with overhanging crests; sea takes white appearance as foam is blown in very dense streaks; rolling is heavy and visibility is reduced |  |
| 11 | 56-63 | 64-72 | 37-52 | Violent storm | Exceptionally high waves; sea covered with white foam patches; visibility further reduced |  |
| 12 | 64 and over | 73 and over | 45 and over | Hurricane | Air filled with foam; sea completely white with driving spray; visibility greatly reduced |  |
