Glossary

Amphidromic point	The point of rest in the tidal oscillations of an ocean basin.
Amplitude	The distance between the crest of a wave and the equilibrium level of the sea; for a sine wave, half the distance between crest and trough.
Barycenter	The point at which the total mass of a group of bodies can be thought to concentrate when acted upon by external forces.
Berm	A flat terrace on a beach, caused by wave action.
Blob of water	A volume of water about the size of your thumb.
Coriolis effect or force	An apparent force that seems to act on a body when observed from a rotating frame of reference.
Cotidal line	The line on which high tide occurs within an ocean basin at a chosen moment.
Cross sea	The disturbed state of the sea when a swell crosses wind-driven waves at an angle.
Diffraction	A spreading of a wave front as it passes through a narrow aperture, such as a channel between islands.
Dispersion	A variation of wave speed with wavelength. Long gravity waves travel faster than shorter gravity waves.
Energy wave spectrum	The distribution of energy among waves of different wavelengths (or periods) in a sample.
Fetch	The distance of open water over which the wind has blown.
Forced wave	A wave that moves under the influence of an external force—for example, a wind-driven water wave.
Free wave	A wave that propagates at a characteristic speed when no external forces act upon it.
Fully developed sea	The state of the sea after the wind blows steadily for a long time over a large area. The waves eventually reach a point of equilibrium with the wind.
Gradient	The change of a quantity (such as velocity or temperature) with a change in another quantity (such as horizontal or vertical distance).
Gravity wave	An ocean wave in which gravity is the dominant restoring force.
Group of waves	The envelope of the crests of several similar wave trains that overlap and interfere. A group is a pattern that travels at a slower speed than the phase speeds of its constituent waves.
Gyre	A rotating body of water that fills a whole ocean basin, turning clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.
Hindcast	An attempt to reproduce the properties of the sea in a past storm, using a numerical model with wind observations as input.
Interference	The interaction of two or more wave trains that results in a pattern of enhanced and depressed wave heights.
Kilometers to miles conversion	1 kilometer = 5/8 mile, or about 0.6 mile
Knot	1.15 mph
Meters to feet conversion	1 meter = 3.28 feet = 39.37 inches
Period	The time interval between the arrival of consecutive crests at a stationary point.
Phase	The present state of a system that is oscillating in a cycle. A full moon, for example, is one of the phases of the moon.
Phase speed	The rate (in, say, kilometers per hour) that a representative point on a wave profile (such as the crest or trough) travels. The speed is determined by the wavelength or period of the wave.
Refraction	The turning of a wave caused by a variation of wave speed along the wave front. Speed variations are commonly caused by variations in the depth of the ocean bottom near a shore.
Resonance	A state in which two oscillations have the same frequency. When a driving force acts on an oscillator at its resonant frequency, the amplitude of the oscillation increases.
Ripple or capillary wave	A wave whose dynamics are dominated by the effects of surface tension.
Rogue wave	A wave higher than expected in the present state of the sea. Specifically, a wave height at least twice the significant height.
Rossby wave	A complex wave that transports changes of rotation or vorticity across an atmosphere or ocean. The Coriolis force acts as a restoring force for the wave’s oscillations.
Self-similarity	The condition in which the whole is exactly similar to a part of itself: the whole has the same shape as one or more of the parts.
Shoaling wave	A wave that changes its speed and shape as a result of encountering water depths that are a small fraction of its wavelength.
Significant wave height	The mean wave height (trough to crest) of the highest third of the waves.
Slope or steepness	The dimensionless ratio of wave height to wavelength.
Slosh zone	The region on a beach where the water from a breaker runs up the slope of the beach.
Soliton	A single isolated group of waves that propagates without a change of shape; a wave “packet.”
Spectrum of waves	A graph that shows how the wave energy in a sea is distributed among waves of different wavelengths or directions.
Stokes drift velocity	The speed with which a blob of water advances in the direction of propagation of the Stokes wave.
Stokes wave	A water wave of finite height that preserves its nonsinusoidal shape as it propagates despite the destructive effects of dispersion.
Storm surge	The mountain of water that storm winds push toward the shore.
Surface gravity wave	A wave formed at the interface between two media of different densities caused by the restoring force of gravity or buoyancy.
Swash zone	The area of beach covered by successive breaking waves.
Swell	A series of surface gravity waves not generated by the local wind but dispersed from a distant generation area. They often have a narrow range of long wavelengths.
Synthetic aperture radar (SAR)	A form of radar that uses the relative motion between an antenna and its target region to provide distinctive spatial signatures that are exploited to obtain finer spatial resolution than is possible with conventional beam-scanning means.
Tsunami	A high-speed, low-amplitude, long-wavelength gravity wave that is triggered by a submarine seismic event or by the eruption of a marine volcano.
Wave group	A collection of waves of similar wavelengths that interfere to produce a succession of enhanced peaks.
Wave height	The vertical distance between a wave crest and the neighboring trough.
Wavelength	The distance between crests of a wave.
Wave steepness	The height of a wave divided by its wavelength.
Weak wave	A wave whose height is very much smaller than its wave-length, for which Airy theory applies.