See also: Meteorological satellitesĪs sea waves move out of the generating area into a region of weaker winds, a calm, or opposing winds, their height decreases as they advance, their crests become rounded, and their surface is smoothed. As a result, it appears that directional wave spectra S ( f,θ) may be obtained from SAR images, but the procedure is not yet fully understood. The backscattering is from sea-surface roughness on scales comparable to the radar wavelength (several centimeters or inches), but longer-wavelength components appear in the SAR image by hydrodynamic interaction, electromagnetic modulation, and effects of wave motion. The synthetic-aperture radar (SAR >1 GHz) obliquely irradiates a patch of ocean surface (about 100 km or 60 mi in size) and uses pulse timing and phase information in the backscattered signal to obtain spatial resolution approaching 10 m (30 ft). The deformation of the reflected pulse carries information about significant wave height H 1/3 in the irradiated patch of ocean (several kilometers in diameter). The radar altimeter (13.5 GHz) observes the reflection of pulses directed vertically. Two radar techniques are presently used for wave measurements from satellites. Also, laser or narrow-beam radar ranging may be used to measure profiles of the sea surface. Ranges of 50–500 km (30–300 mi) are feasible, but with over-the-horizon sky-wave systems relying on ionospheric reflection, ranges beyond 3200 km (2000 mi) have been achieved.įrom aircraft, stereo-photographs can be taken of the sea surface and analyzed photogrammetrically, but this is a laborious process. Ground-based high-frequency (3–30 MHz) radar systems can provide information on wave height and direction from the backscattered signal. Consequently, a freak wave can have a lifetime of a minute or two. In such a wave, the effects of nonlinearity can compensate for those of dispersion, allowing a solitary wave to propagate almost unchanged. But nonlinear effects are bound to be significant in a large wave. According to linear theory, waves with different periods propagate with different speeds in deep water and hence the wave components remain in phase only briefly. Many large ships in this region have been severely damaged by such waves.īecause actual ocean waves consist of many components with different periods, heights, and directions, occasionally a large number of these components can, by chance, come in phase with one another, creating a freak wave with a height several times the significant wave height of the surrounding sea. For example, when swell from a Weddell Sea storm propagates northeastward into the southwestward-flowing Agulhas Current off South Africa, high steep waves are formed. When waves propagate into an opposing current, they grow in height. Northwest of Hawaii, on February 7, 1933, the Navy tanker USS Ramapo encountered the largest open-ocean wind waves ever reliably observed with heights that were reported to be at least 34 m (112 ft). Off the west coast of Canada, there have been several measurements of individual waves with heights around 30 m (100 ft). Such systems of very low atmospheric pressure form in the Gulf of Alaska, the region around Iceland and the Weddell Sea. The highest wind waves are produced by large intense storm systems that last for a day or longer. Equations ( 1 ), ( 2 ), and ( 3 ) assume that the waves present were generated by local wind, with no significant swell present. Such waves, generated by distant storms, are called swell. Waves with long periods (typically 10 s or more) can travel thousands of kilometers with little energy loss. See also: Storm surge Tide TsunamiĪpplies, if a wind blows steadily over a known fetch for a period of time.īecause of viscosity, surface waves lose energy as they propagate, short-period waves being dampened more rapidly than long-period waves. Like tides, they are ubiquitous in the ocean and continue to travel well beyond their area of generation. Wind-generated waves, having periods from a fraction of a second to tens of seconds, are called wind waves. Wakes are waves resulting from relative motion of the water and a solid body, such as the motion of a ship through the sea or the rapid flow of water around a rock. Earthquakes or other large, sudden movements of the Earth's crust can cause waves, called tsunamis, which typically have periods of less than an hour. Storm surges are individual waves produced by the wind and dropping barometric pressure associated with storms they characteristically last several hours. They have long periods, usually 12.42 h for the strongest constituent. Tides are ocean waves induced by the varying gravitational influence of the Moon and Sun. Surface gravity waves may be classified according to the nature of the forces producing them.
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