Swells are formed by the wind blowing across the surface of the ocean.
The size of the swell is determined by three factors.
- Strength—the intensity of the wind.
- Duration—the length of time the wind blows.
- Fetch—the distance over which the wind blows.
The longer and stronger the wind blows, and the greater the distance over which it blows, the larger the swell pattern will be as it travels across the ocean. This process is called swell propagation. At this stage, swells take on a distinct set of characteristics.
The following swell characteristics can give useful information to the lifesaver when assessing surf conditions.
- Swell period—the time (measured in seconds) between swell crests. Swells with a high period (longer time) indicate increased strength of the waves when breaking and increased time between sets.
- Significant wave height—the vertical distance (measured in metres) between the crest and the preceding trough of a wave that is about to break. It is indicative of the size of the wave face.
- Swell direction—the compass direction from which the swell is coming. Coastline that is more exposed to the incoming swell direction will experience larger surf conditions than more protected areas.
Swells can retain their energy over long distances. They reach saturation when a balance is reached between the generating force (wind) and the limiting force (gravity). They are also limited by loss of energy through white-capping (aka ‘white horses’).