page_title: Swell (ocean)
section_title: Formation
hierarchical_section_title: Swell (ocean) / Formation
caption_attribution_description: Easterly swell at Lyttelton Harbour, New Zealand
context_page_description: 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 thus are often referred to as surface gravity waves. These 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. Swell waves often have a long wavelength, but this varies due to the size, strength, and duration of the weather system responsible for the swell and the size of the water body. Swell wavelength also varies from event to event. Occasionally, swells that are longer than 700m occur as a result of the most severe storms. 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. Swell direction is the direction from which the swell is coming.
context_section_description: Large breakers observed on a beach may result from distant weather systems over a certain distance of the uninterrupted ocean. Five factors influence the formation of wind waves which will go on to become ocean swell:
  Wind speed or strength relative to wave speed – the wind must be moving faster than the wave crest (in the direction in which the wave crest travels) for net energy transfer from air to water; stronger prolonged winds create larger waves
  The uninterrupted distance of open water over which the wind blows without significant change in direction (called the fetch)
  Width of area affected by fetch
  Wind duration – the time over which the wind has blown over a given area
  Water depth
  All of these factors work together to determine the size of wind waves:
  Wave height (from trough to crest)
  Wave length (from crest to crest)
  Wave period (time interval between arrival of consecutive crests at a stationary point)
  Wave propagation direction
  A fully developed sea has the maximum wave size theoretically possible for a wind of a specific strength, duration, and fetch. Further exposure to that specific wind could only cause a loss of energy due to the breaking of wave tops and formation of "whitecaps".  Waves in a given area typically have a range of heights.  For weather reporting and for scientific analysis of wind wave statistics, their characteristic height over a period of time is usually expressed as significant wave height. This figure represents an average height of the highest one-third of the waves in a given time period (usually chosen somewhere in the range from 20 minutes to twelve hours), or in a specific wave or storm system. The significant wave height is also the value a "trained observer" (e.g. from a ship's crew) would estimate from visual observation of a sea state. Given the variability of wave height, the largest individual waves are likely to be somewhat less than twice the reported significant wave height for a particular day or storm.
caption: Swell near Lyttelton Harbour, New Zealand
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