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Ocean Waves (continued) the backscattered signal to obtain spatial resolution approach- ing 10 m (30 ft). The backscattering is from sea-surface rough- ness on scales comparable to the radar wavelength (several centimeters or inches), but longer-wavelength components appear in the SAR image by hydrodynamic interaction, electro- magnetic modulation, and effects of wave motion. 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. Sea state Sea state is the description of the ocean surface or state of the sea surface with regard to wave action. Wind waves in the sea are of two types: those still growing under the force of the wind are called sea: those no longer under the influence of the wind that produced them are called swell. Differences between the two types are important in forecasting ocean wave conditions. Sea Those waves which are still growing under the force of the wind have irregular, chaotic, and unpredictable forms. The unconnected wave crests are only two to three times as long as the distance between crests and commonly appear to be traveling in different directions, varying as much as 20° from the dominant direction. As the waves grow, they form regu- lar series of connected troughs and crests with wavelengths commonly ranging from 12 to 35 times the wave heights. Wave heights only rarely exceed 17 m (55 ft). The appearance of the sea surface is termed state of the sea. The height of a sea is dependent on the strength of the wind, the duration of time the wind has blown, and the distance of sea surface over which the wind has blown (fetch). Swell As sea waves move out of the generating area into a re- gion of weaker winds, a calm, or opposing winds, their height decreases as they advance, their crests become rounded, and their surface is smoothed. These waves are more regular and more predictable than sea waves and, in a series, tend to show the same form or the same trend in characteristics. Wave- lengths generally range from 35 to 200 times wave heights. The presence of swell indicates that recently there may have been a strong wind, or even a severe storm, hundreds or thousands of miles away. Along the coast of southern Califor- nia long-period waves are believed to have traveled distances greater than 8000 km (5000 mi) from generating areas in the South Pacific Ocean. Swell can usually be felt by the roll of a ship, and, under certain conditions, extremely long and high swells in a glassy sea may cause a ship to take solid water over its bow regularly. When swell is obscured by sea waves, or when the compo- nents are so poorly defined that it is impossible to separate them, it is reported as confused. In-between state Often both sea waves and swell waves, or two or more systems of swell, are present in the same area. When waves of one system are superimposed upon those of another, crests may coincide with crests and accentuate wave height, or troughs may coincide with crests and cancel each other to produce flat zones. This phenomenon is known as wave inter- ference and the wave forms produced are extremely irregular. When wave systems cross each other at a considerable angle, the apparently unrelated peaks and hollows are known as a cross sea. Breaking waves The action of strong winds (greater than 6.2 m/s or 12 knots) sometimes causes waves in deeper water to steepen too rap- idly. As the height-length ratio becomes too large, the water at the crest moves faster than the crest itself and topples forward to form whitecaps. As waves travel over a gradually shoaling bottom, the mo- tion of the water is restricted and the wave train is telescoped together. The wavelength decreases and the height first decreases slightly until the water depth is about one-sixth the deep-water wavelength and then rapidly increases until the crest curves over and plunges to the water surface below. Swell coming into a beach usually increases in height before break- ing, but wind waves are often so steep that there is little if any increase in height before breaking. For this reason, swell that is obscured by wind waves in deeper water often defines the period of the breakers. The zone of breakers, or surf, includes the region of white water between the outermost breaker and the waterline on the beach. If the sea is rough, it may be impossible to differentiate between the surf inshore and the whitecaps in deep water just beyond. Capillary waves Capillary waves, or ripples, occur at the interface between two fluids when the principal restoring force is surface tension. Ripples generated by wind on the ocean and lakes are impor- tant for the initiation of turbulence in both media, transfer of gases between air to water, and scattering of electromagnetic and sound waves. + ward ' s science