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wave, in the earth sciences
wave, in oceanography, an oscillating movement up and down, of a body of water caused by the frictional drag of the wind, or on a larger scale, by submarine earthquakes, volcanoes, and landslides. In seismology, waves moving though the earth are caused by the propagation of a disturbance generated by an earthquake or explosion. In atmospheric science, waves are periodic disturbances in the air flow.
In a body of water, waves consist of a series of crests and troughs, where wavelength is the distance between two successive crests (or successive troughs). As waves are generated, the water particles are set in motion, following vertical circular orbits. Water particles momentarily move forward as the wave crest passes and backward as the trough passes. Thus, except for a slight forward drag, the water particles remain in essentially the same place as successive waves pass. The orbital motion of the water particles decreases in size at depths below the surface, so that at a depth equal to about one half of the wave's length, the water particles are barely oscillating back and forth. Thus, for even the largest waves, their effect is negligible below a depth of 980 ft (300 m).
The height and period of water waves in the deep ocean are determined by wind velocity, the duration of the wind, and the fetch (the distance the wind has blown across the water). In stormy areas, the waves are not uniform but form a confusing pattern of many waves of different periods and heights. Storms also produce white caps at wind speeds c.8 mi per hr (13 km per hr). Major storm waves can be over a half mile long and travel close to c.25 mi per hr (40 km per hour). A wave in the Gulf of Mexico associated with Hurricane Ivan (2004) measured 91 ft (27.7 m) high, and scientists believe that other waves produced by Ivan may have reached as much as 132 ft (40 m) high.
Waves of similar heights, known as extreme storm waves (often called rogue waves), most commonly occur in ocean regions of strong ocean currents, which can amplify wind-driven waves when they flow in opposing directions; sandbanks may also act to focus wave energy and give rise to such waves. An extreme storm wave is a wave that is more than twice as high as the surrounding waves. It also often is steeply sided and often comes from a different direction than the surrounding waves. In some cases, such waves can exceed 100 ft (30 m) in height, and sometimes are capable of causing a ship to sink.
When waves approach a shore, the orbital motion of the water particles becomes influenced by the bottom of the body of water and the wavelength decreases as the wave slows. As the water becomes shallower the wave steepens further until it “breaks” in a breaker, or surf, carrying the water forward and onto the beach in a turbulent fashion. Because waves usually approach the shore at an angle, a longshore (littoral) current is generated parallel to the shoreline. These currents can be effective in eroding and transporting sediment along the shore (see coast protection; beach).
In many enclosed or partly enclosed bodies of water such as lakes or bays, a wave form called a standing wave, or seiche, commonly develops as a result of storms or rapid changes in air pressure. These waves do not move forward, but the water surface moves up and down at antinodal points, while it remains stationary at nodal points.
Internal waves can form within waters that are density stratified and are similar to wind-driven waves. They usually cannot be seen on the surface, although oil slicks, plankton, and sediment tend to collect on the surface above troughs of internal waves. Any condition that causes waters of different density to come into contact with one another can lead to internal waves. They tend to have lower velocities but greater heights than surface waves. Very little is known about internal waves, which may move sediment on deeper parts of continental shelves.
Just as a rock dropped into water produces waves, sudden displacements such as landslides and earthquakes can produce high energy waves of short duration that can devastate coastal regions (see tsunami). Hurricanes traveling over shallow coastal waters can generate storm surges that in turn can cause devastating coastal flooding (see under storm).
Seismic and Atmospheric Waves
wave, in physics
Classification of Waves
Waves may be classified according to the direction of vibration relative to that of the energy transfer. In longitudinal, or compressional, waves the vibration is in the same direction as the transfer of energy; in transverse waves the vibration is at right angles to the transfer of energy; in torsional waves the vibration consists of a twisting motion as the medium rotates back and forth around the direction of energy transfer. The three types of waves are illustrated by an example in which a coil spring is held stretched out by two persons. If the person holding one end pulls a few coils toward himself and releases them, a longitudinal wave will travel along the spring, with coils alternately being pressed closer together, then stretched apart, as the wave passes. If the first person then shakes his end up and down or from side to side, a transverse wave will travel along the spring. Finally, if he grabs several coils and twists them around the axis of the spring, a torsional wave will travel along the spring.
A wave may be a combination of types. Water waves in deep water are mainly transverse. However, as they approach a shore they interact with the bottom and acquire a longitudinal component. When the longitudinal component becomes very large compared to the transverse component, the wave breaks.
Parameters of Waves
The maximum displacement of the medium in either direction is the amplitude of the wave. The distance between successive crests or successive troughs (corresponding to maximum displacements in the same direction) is the wavelength of the wave. The frequency of the wave is equal to the number of crests (or troughs) that pass a given fixed point per unit of time. Closely related to the frequency is the period of the wave, which is the time lapse between the passage of successive crests (or troughs). The frequency of a wave is the inverse of the period.
One full wavelength of a wave represents one complete cycle, that is, one complete vibration in each direction. The various parts of a cycle are described by the phase of the wave; all waves are referenced to an imaginary synchronous motion in a circle; thus the phase is measured in angular degrees, one complete cycle being 360°. Two waves whose corresponding parts occur at the same time are said to be in phase. If the two waves are at different parts of their cycles, they are out of phase. Waves out of phase by 180° are in phase opposition. The various phase relationships between combining waves determines the type of interference that takes place.
The speed of a wave is determined by its wavelength λ and its frequency ν, according to the equation v=λν, where v is the speed, or velocity. Since frequency is inversely related to the period T, this equation also takes the form v=λ/T. The speed of a wave tells how quickly the energy it carries is being transferred. It is important to note that the speed is that of the wave itself and not of the medium through which it is traveling. The medium itself does not move except to oscillate as the wave passes.
Wave Fronts and Rays
The general term applied to the description of a disturbance which propagates from one point in a medium to other points without giving the medium as a whole any permanent displacement.
Waves are generally described in terms of their amplitude, and how the amplitude varies with both space and time. The actual description of the wave amplitude involves a solution of the wave equation and the particular boundary conditions for the case being studied. See Wave equation, Wave motion
Acoustic waves, or sound waves, are a particular kind of the general class of elastic waves. Elastic waves are propagated in media having two properties, inertia and elasticity. Electromagnetic waves (for example, light waves and radio waves) are not elastic waves and therefore can travel through a vacuum. The velocity of the wave depends on the medium through which the wave travels. See Electromagnetic wave
["WAVE: A Model-Based Language for Manipulator Control", R.P. Paul, Ind Robot 4(1):10-17, 1979].
wave(1) See iOS 8 Wave prank.
(2) A ripple or undulation. All electromagnetic radiation, including radio signals, light rays, x-rays, and cosmic rays, as well as sound, behave like rippling waves in the ocean. To visualize a wave, take a piece of paper and keep drawing a line up and down while pulling the paper perpendicular to the line. Modulate the line by making it different lengths as you draw it with the paper moving, and notice the resulting pattern. See wave-particle duality and wavelength.