During a storm, the sea surface generally looks very chaotic, with lots of short, steep waves of varying heights. Away from very windy areas, the sea surface has long, rolling peaks of a uniform shape. For this reason, we differentiate between two types of surface waves: wind waves and swell.
Wind waves refer to short-period waves that are still being created by winds or are very close to the area in which they were generated. Wind waves are a local manifestation of the energy that has been transferred to the sea from the wind. The energy travels downwind away from the source area, like ripples that form when a stone is dropped in a pond, or from the bow wave from a ship.
Swell refers to waves that have moved out of the generating area, far from the influence of the winds that created them. The stronger the winds at the source area, the larger the swell and further it will travel. The longer that the wind blows in the source area, the longer the swell will persist, even long after the wind has ceased or changed direction. The farther swell travels, the more dispersion and nonlinear wave-wave interactions operate, resulting in well-sorted, long-period waves.
Since wave energy becomes more organized as it travels long distances, swell is more uniform and regular than wind waves. The wave energy leaving the fetch area is concentrated at the center of the fetch exit region and is in the direction of the prevailing wind, although it continues to spread out once it leaves the area.
Changing surface wind variables, such as speed, direction, fetch, length, and duration, tend to create waves of different types. That’s why the sea surface is best described by mathematical functions like wave spectra (the characteristic shape of the spectrum of waves) and statistical parameters like significant wave height (Hs, defined as the average height of the highest one third of all waves). Significant wave height correlates well with average wave height reported by trained observers.
For more information, see the COMET lessons on the MetEd website, Wave Life Cycle I: Generation and Wave Life Cycle II: Propagation & Dispersion.