|
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 so they are often referred to as surface gravity waves. These series of surface gravity waves are not generated by the immediate local wind, instead by distant weather systems, where wind blows for a duration of time over a fetch of water. This is the primary definition of a swell as opposed to a locally generated wind wave, which is still under the influence of the mechanisms that created it e.g. Wind blowing over a puddle. 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 e.g. wavelengths are rarely more than 150 m in the Mediterranean. Swell wavelength, also, varies from event to event. Occasionally, swells which are longer than 700 m 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 formation== The great majority of large breakers one observes on a beach result from distant weather systems over a fetch of ocean. Five factors influence the formation of wind waves〔 p. 83.〕 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 for energy transfer,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. ===Sources of wind wave generation=== Sea water wave is generated by many kinds of disturbances such as Seismic events, gravity, and crossing wind. The generation of wind wave is initiated by the disturbances of cross wind field on the surface of the sea water. Two major Mechanisms of surface wave formation by winds (a.k.a.‘The Miles-Phillips Mechanism’) and other sources (ex. earthquakes) of wave formation can explain the generation of wind waves. However, if one set a flat water surface (Beaufort Point,0) and abrupt cross wind flows on the surface of the water,then the generation of surface wind waves can be explained by following two mechanisms which initiated by normal pressure fluctuations of turbulent winds and parallel wind shear flows. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Swell (ocean)」の詳細全文を読む スポンサード リンク
|