A swell, also sometimes referred to as ground 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 under the predominating influence of gravity, and thus are often referred to as surface gravity waves. These surface gravity waves have their origin as wind waves, but are the consequence of dispersion of wind waves from distant weather systems, where wind blows for a duration of time over a fetch of water, and these waves move out from the source area at speeds that are a function of wave period and length. More generally, a swell consists of wind-generated waves that are not greatly affected by the local wind at that time. Swell waves often have a relatively long wavelength, as short wavelength waves carry less energy and dissipate faster, 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, and varies from event to event, and from the same event, over time. Occasionally, swells that are longer than 700 m occur as a result of the most severe storms. Swell direction is the direction from which the swell is moving. It is given as a geographical direction, either in degrees, or in points of the compass, such as NNW or SW swell, and like winds, the direction given is generally the direction the swell is coming from. Swells have a narrower range of frequencies and directions than locally generated wind waves, because they have dispersed from their generation area and over time tend to sort by speed of propagation with the faster waves passing a distant point first. Swells take on a more defined shape and direction and are less random than locally generated wind waves.
Formation
Large breakers observed on a shore may result from distant weather systems over the ocean. Five factors work together to determine the size of wind waves which will become ocean swell:
Wind 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 water surface in the fetch Wind duration – the time over which the wind has blown over the fetch Water depth A wave is described using the following dimensions:
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 Wave length is a function of period, and of water depth for depths less than approximately half the wave length, where the wave motion is affected by friction with the bottom.
A fully developed sea has the maximum wave size theoretically possible for a wind of a specific strength and fetch. Further exposure to that specific wind would result in a loss of energy equal to the energy input giving a steady state, due to the energy dissipation from viscosity and breaking of wave tops as "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 time interval 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 significant wave height.
Sources of wind-wave generation
Wind waves are generated by wind. Other kinds of disturbances such as seismic events, can also cause gravity waves, but they are not wind waves, and do not generally result in swell. The generation of wind waves is initiated by the disturbances of the crosswind field on the surface of the water. For initial conditions of a flat water surface (Beaufort Scale 0) and abrupt crosswind flows on the surface of the water, the generation of surface wind waves can be explained by two mechanisms, which are initiated by normal pressure fluctuations of turbulent winds and parallel wind shear flows.
Surface wave generation by winds
From "wind fluctuations": Wind wave formation is started by a random distribution of normal pressure acting on the water from the wind. By this mechanism, proposed by O.M. Phillips in 1957, the water surface is initially at rest, and the generation of the wave is initiated by turbulent wind flows and then by fluctuations of the wind, normal pressure acting on the water surface. Due to this pressure fluctuation arise normal and tangential stresses that generate wave behavior on the water surface. The assumptions of this mechanism are as follows:
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