Wind waves are the result of local winds blowing across the water’s surface. Hence, the properties of these surface waves depend on the wind speed, the contact distance over the water (the so-called fetch) and the duration. The wave height and wavelength of wind waves have a certain amount of randomness, which makes them quite choppy and unorganised, and they tend to break.
If the waves are propagating away from their origin, they sort themselves by their speed. Since waves with a shorter wavelength lose more energy (through dissipation) while propagating, the distant swell waves have much longer wavelengths with a smoother crest. Swell waves can travel thousands of kilometres almost unchanged in height and period. Thus, swell waves often occur even without any local winds: a preferable situation for wave surfers. For example, swell waves for “big wave surfing” in Hawaii are usually generated from large winter storms in the Northern Pacific Ocean.
The actual wave height at a certain location is usually a combination of swell and wind waves. The significant wave height (SWH) is the average of the highest one-third of all measured waves within a given period. However, much higher waves can occur with maximum individual wave heights of even two times the significant wave height.
The sea state is a crucial factor not only for water sports. Breaking waves with a height of only 3% of the boat hull length can already lead to the capsizing of some boats if they hit the broadside. Therefore, steersmen need to manoeuvre the boats at the correct angle to the wavefronts. This might get quite tough within “cross-sea” in which the propagation directions of wind waves and swell differ. On the other hand, a nice downwind course with a tiny swell from behind can be quite an enjoyable ride.
Our weather maps are the perfect tool to plan such rides, since they provide detailed information about the different map types, and you have the option to compare the forecasts of different models to detect possible uncertainties in the forecast. You can find the model selection in the lower right corner of our weather maps. The model selection is now extended with the two new models, provided by Copernicus Marine Services:
- MEDWAM (first screenshot) covers the Mediterranean Ocean with 4km resolution for up to 10 forecast days.
- IBIWAM (second screenshot) with a spatial resolution of 5km covering Iberian Biscay Irish Area in hourly resolution and for a forecast time range of 9 days.
In addition to our weather maps, you are also recommended to consult our Sea&Surf meteogram.
