Significant wave height, period, direction, sea surface temperature and currents over 72 hours using the Open-Meteo global marine model. Built for surfers, sailors and sport fishing.
Data from the global marine model (Open-Meteo Marine API) based on GFS-Wave / IFS-WAM. Resolution ~0.1°. Locations far from the coast or in inland seas may not return data.
The figure every marine model reports is neither the largest wave nor the average: it is the significant wave height, defined as the mean of the highest third of the waves in a record. It was chosen because it matches reasonably well the height an experienced observer estimates by eye from a boat. The practical consequence is that there are always waves bigger than the number in the forecast. Under the Rayleigh distribution, which describes a developed sea well, the largest wave expected in twenty minutes is about 1.5 times the significant height, over three hours 1.9 times, and the rule of thumb of «double» corresponds to an episode of some six hours. With 2 metres of significant height, a 3.5 metre set is within what a morning can bring.
Two seas of the same height can behave entirely differently depending on their period, which is the time between two crests. The power a wave train carries grows with the square of the height and also in proportion to the period, so a metre and a half at 14 seconds breaks with far more force than a metre and a half at 6. The difference has a name:
Wind sea (periods of 3 to 8 seconds): waves generated by the wind blowing right there. Disordered, with short crests and several directions at once. This is what spoils a swim and what makes a crossing uncomfortable.
Swell (periods from 10 seconds upwards): waves generated by a distant depression that have travelled hundreds or thousands of kilometres. In doing so they have sorted themselves into regular sets and lost the short components. These are what surfers look for and what overtops seafront promenades, because they arrive with energy even when there is no wind at the coast.
| Period | Type of sea | Origin | What to expect at the coast |
|---|---|---|---|
| 3-5 s | Local wind sea | Wind in the area itself | Choppy, shapeless break |
| 6-8 s | Mature wind sea | Wind a few tens of km away | Closed-out waves, awkward swimming |
| 9-12 s | Short swell | Depression 500-1,500 km away | Defined sets, surfable |
| 13-16 s | Long swell | Depression more than 2,000 km away | Powerful, ordered waves; currents |
| > 16 s | Very long swell | North Atlantic in midwinter | Overtopping, promenades closed |
Wave and wind direction are always given as where they come from, not where they are going: «northwesterly swell» means the waves arrive from the northwest. That is the convention in every forecast, and the opposite of the one used for ocean currents, which are given as the direction they flow towards. The sea temperature here is that of the surface, which in summer can be several degrees above the water at three metres if there is no wind mixing.
The marine model works on a grid of several kilometres and in open water. Everything that happens in the last hundred metres — the bank that makes the wave break, refraction against the headland, the rip current in the channel — falls below its resolution. That is why two beaches a kilometre apart can share a forecast and have very different conditions, and why the number is for deciding whether the trip is worth it, not for knowing how the break will be. For navigation, the official bulletin from Puertos del Estado and AEMET always takes precedence.
