The Douglas Sea Scale is a scale which measures the swell of the sea and the height of the waves:
- Degree 0—no measurable wave height calm sea
- Degree 1—waves >10 cm. rippled sea
- Degree 2—waves 10–50 cm. smooth sea
- Degree 3—waves 0.5–1.25 m. slight sea
- Degree 4—waves 1.25–2.5 m. moderate sea
- Degree 5—waves 2.5–4 m. rough sea
- Degree 6—waves 4–6 m. very rough sea
- Degree 7—waves 6–9 m. high sea
- Degree 8—waves 9–14 m. very high sea
- Degree 9—waves >14 m. phenomenal sea
Waves are primarily caused by the wind and its action on the surface of the water. Their height depends on how long the wind has been blowing and also on the strength of the wind. Waves formed by the wind blowing locally are termed "sea". Waves formed by the wind blowing at a distance from the place of observation are termed "swell".
Some waves result from earthquakes or underwater seaquakes and on approaching shallow water they become abnormally high and begin to break with great violence causing enormous devastation and loss of life. They are termed "tsunami" and we will all remember the tragic waves caused my a seaquake near Sumatra on Dec 26th, 2004, which claimed the lives of nearly 300,000 people in South-East Asia.
The following terms are frequently used in connection with waves:
- the length of a wave, that is the horizontal distance from crest to frest or trough to trough. If the distances between the crests of waves are far apart, the sea is termed "a long sea". When the crests are close together the sea is termed "a short sea", like for example in the Baltic Sea.
- the height of wave, that is the vertical distance from trough to crest.
- the period of a wave, that is the time between the passages of two successive wave crests or troughs past a fixed point.
- the velocity of a wave, that is the rate at which the crest travels.
| 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 |  |
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