Beaufort Wind Scale
| Wind speed | Waves height | ||||||
|---|---|---|---|---|---|---|---|
| km/h | m/s | kt | mph | m | ft | ||
| 0 Bf | calm | 0-1 | 0-0.2 | 0-1 | 0-1 | 0 | 0 |
| 1 Bf | light air | 1-5 | 0.3-1.5 | 1-3 | 1-3 | 0.1 | 0.25 |
| 2 Bf | light breeze | 6-11 | 1.6-3.3 | 4-6 | 4-7 | 0.2-0.3 | 0.5-1 |
| 3 Bf | gentle breeze | 12-19 | 3.4-5.4 | 7-10 | 8-12 | 0.6-1 | 2-3 |
| 4 Bf | moderate breeze | 20-28 | 5.5-7.9 | 11-16 | 13-18 | 1-1.5 | 3.5-5 |
| 5 Bf | fresh breeze | 29-38 | 8.0-10.7 | 17-21 | 19-24 | 2-2.5 | 6-8.5 |
| 6 Bf | strong breeze | 39-49 | 10.8-13.8 | 22-27 | 25-31 | 3-4 | 9.5-13 |
| 7 Bf | near gale | 50-61 | 13.9-17.1 | 28-33 | 32-38 | 4-5.5 | 13.5-19 |
| 8 Bf | gale | 62-74 | 17.2-20.7 | 34-40 | 39-46 | 5.5-7.5 | 18-25 |
| 9 Bf | strong gale | 75-88 | 20.8-24.4 | 41-47 | 47-54 | 7-10 | 23-32 |
| 10 Bf | storm | 89-102 | 24.5-28.4 | 48-55 | 55-63 | 9-12.5 | 29-41 |
| 11 Bf | violent storm | 103-117 | 28.5-32.6 | 56-63 | 64-72 | 11.5-16 | 37-52 |
| 12 Bf | hurricane | >=118 | >=32.7 | >=64 | >=73 | 14+ | 45+ |
Douglas Sea Scale
This actually has two scales; one for the wind generated waves, and the other for the background swell. Ideally one should use both scales to fully describe the conditions.
There is a good discussion about the difference between waves and swell on the New Zealand Met Service Blog.
State of the sea (wind sea)
| Degree | Height (m) | Description |
|---|---|---|
| 0 | no wave | Calm (Glassy) |
| 1 | 0 - 0.10 | Calm (Rippled) |
| 2 | 0.10 - 0.50 | Smooth |
| 3 | 0.50 - 1.25 | Slight |
| 4 | 1.25 - 2.50 | Moderate |
| 5 | 2.50 - 4.00 | Rough |
| 6 | 4.00 - 6.00 | Very Rough |
| 7 | 6.00 - 9.00 | High |
| 8 | 9.00 - 14.00 | Very High |
| 9 | 14.00+ | Phenomenal |
Swell
| Degrees | Description |
|---|---|
| 0 | No Swell |
| 1 | Very Low (short and low wave) |
| 2 | Low (long and low wave) |
| 3 | Light (short and moderate wave) |
| 4 | Moderate (average and moderate wave) |
| 5 | Moderate rough (long and moderate wave) |
| 6 | Rough (short and heavy wave) |
| 7 | High (average and heavy wave) |
| 8 | Very high (long and heavy wave) |
| 9 | Confused (wavelength and height indefinable) |
Wave length and height classification
Wave length
- Short wave 100 m -
- Average wave 100 – 200 m
- Long wave 201 m +
Wave height
- Low wave 2 m -
- Moderate wave 2 – 4 m
- High wave 4.01 m +
If wave height is 1.0 m and swell height is 3.0m , then what is the Douglas Sea Scale?
The Douglas scale has two parts – sea state (ie waves) and swell. Ideally one should quote both, so in this case the sea conditions would be described as Sea: 3 (slight), Swell: 4 (moderate).
Having said that, in many cases the swell part is not quoted. This seems to cause a lot of problems for charter companies who warranty certain performance characteristics in certain conditions, such as the ability to do X knots in good sea conditions. By this they, apparently, usually mean a Sea State of 3. But does that ignore the swell? Recent litigation on this has led to a consensus that a Sea State of 3, when quoted alone without reference to swell, would allow for a combined sea height (waves plus swell) of 2 meters. So, for your example, if you were really going to quote only one number, then probably you should quote 4. However, it is clearly much better to quote separately the sea and swell states. Unfortunately, even Met offices rarely do this. The Italian forecasts, for example, quote only the expected Sea State.
Thank you for the explanation but I am not clear on when there is no express reference to swell, how the combined sea height will be determined and what it exactly means. Could you make further explanations about it?
Unfortunately, as you can see from the fact that there are lawsuits about it, it is not clear what exactly a sea state, without a swell state, actually means!
In your previous comment you say that ‘recent litigation on this has led to a consensus that Sea State of 3, when quoted alone without reference to swell, would allow for a combined sea height (waves plus swell)’, that’s why i asked what combined sea height means. For examples if height of waves 1m and height of swell 2m , will we say that height in total is 3m, so that conditions in overall is douglas sea scale 5? I do not think that this is what they mean in that dispute!