Swells are collections of waves produced by storm winds ranging hundreds of miles out to sea.
In other words, a swell in the context of an ocean, sea or lake, is a series of mechanical waves that propagate along the interface between water and air and are often referred to as surface gravity waves.
These series of surface gravity waves are not wind waves, which are generated by the immediate local wind, but instead are generated by distant weather systems, where wind blows for a duration of time over a fetch of water.
More generally, a swell consists of wind-generated waves that are not—or are hardly—affected by the local wind at that time.
Features of swell waves
1 . Swell waves often have a long wavelength. Occasionally, swells which are longer than 700 m occur as a result of the most severe storms.
2 . Swells have a narrower range of frequencies and directions than locally generated wind waves, because swell waves have dispersed from their generation area, have dissipated and therefore lost an amount of randomness, taking on a more defined shape and direction.
3 . Swell direction is the direction from which the swell is coming. It is measured in degrees (as on a compass), and often referred to in general directions, such as SW swell.
Factors influencing the formation of swell waves
1 . Wind speed or strength relative to wave speed — the wind must be moving faster than the wave crest for energy transfer; stronger prolonged winds create larger waves
2 . The uninterrupted distance of open water over which the wind blows without significant change in direction (called the fetch)
3 . Width of area affected by fetch
4 . Wind duration — the time over which the wind has blown over a given area
5 . Water depth
Significance of swell waves
They are coveted by surfers looking to catch a big wave. However, they are not so revered by boaters as large swells can capsize ships.
But Swells were used by Polynesian navigators in the past to maintain course when no other clues were available, such as on foggy nights.
