The movements of the sea surface over a given area and at a given moment are generally complex since they combine, on one hand, the undulations from the wind sea (waves caused by the wind), created by the progressive action of the wind, and, on the other hand, the waves coming from one or several different directions due to the propagation of one or several components of the swell (waves generated earlier in regions distant from the area observed). The total sea (that results in that area and at that moment from the combination of the swell and the wind sea) must therefore be described by numerical characteristics - in particular for wave height - and by details on its physical behaviour : for example, two swells that are crossing tend to give the sea surface an "indented" appearance that reveals a risk of instability for navigation. Wave heights grow with wind speed, as well as with its tendency to "accompany" within a low (the atmospheric pressure is lower than in the neighbouring zones at the same level ), the waves that it has generated. |
The data concerning wind speed, its duration of actions and the length of fetch (the length of the distance over which the wind blows above the sea without really changing direction or speed) also enable the determination of foreseeable sea state in terms of period (the time in seconds for the passage of two successive crests or troughs at a given point) of wave length (length in metres between two successive crests or troughs) and height. Nevertheless, the rigorous forecasting of the sea state over larger nautical areas and ranges requires the use of digital weather forecasting models that essentially calculate values of this state based on the results of the forecasted surface wind. These types of models provide, at each point of a grid covering a determined region of the ocean, the forecasted values of the mean significant wave height or H1/3 (from the height of a third of the highest waves) and their direction by differentiating between three types of forecasts : that of the wind sea generated in that area whose direction is that of the local wind, that of the swell reaching the considered grid point from one or several generating areas (the single or main component of the swell is then also represented on the maps), and lastly, the total sea that integrates the effects of the two previous factors. |