]> Often the greatest magnitude at a given point of any spatially and temporally varying physical quantity governed by a wave equation; can also mean the spatial part of a time-harmonic wave function. The rate of recurrence of any periodic phenomenon, often associated with waves of all kinds. Without qualification frequency often means temporal frequency, the rate of recurrence of a time-varying function, but could mean spatial frequency, the rate of recurrence of a space-varying function. Spatial frequency is the reciprocal of the repeat distance (sometimes the wavelength). The dimensions of (temporal) frequency are inverse time. A common unit for frequency is cycle per second, formerly abbreviated cps, but superseded by hertz, abbreviated as Hz. A general term referring to the radiation reflected from, or scattered back through, a given surface in response to radiation incident on the surface with the same wavelength or wavelength range. A measure of the extinction due to scattering of monochromatic radiation as it traverses a medium containing scattering particles. Wave propagation is any of the ways in which waves travel through a medium (waveguide). With respect to the direction of the oscillation relative to the propagation direction, we can distinguish between longitudinal wave and transverse waves. Another useful parameter for describing the propagation is the wave velocity that mostly depends on some kind of density of the medium. For electromagnetic waves, propagation may occur in a vacuum as well as in a material medium. scattering of waves (photons) that reverse the direction of propagation Back Scattering A change of direction and possibly amplitude of an electromagnetic, acoustic, or any other wave propagating in a material medium, homogeneous on the scale of the wavelength, as a consequence of spatial variation in the properties of the medium. In a broad sense, the process by which matter is excited to radiate by an external source of electromagnetic radiation, as distinguished from emission of radiation by matter, which occurs even in the absence of such a source. Absorption The process in which incident radiant energy is retained by a substance. Longitudinal waves are waves that have vibrations along or parallel to their direction of travel. They include waves in which the motion of the medium is in the same direction as the motion of the wave. Mechanical longitudinal waves have been also referred to as compressional waves or pressure waves A transverse wave is a wave that causes vibration in the medium in a perpendicular direction to its own motion. For example: if a wave moves along the x-axis, its disturbances are in the yz-plane. In other words, it causes medium disturbances across the two-dimensional plane that it is travelling in. Contrary to popular belief, transversal waves do not necessarily move up and down. wave in pressure field. Gravitational Wave Same as gravity wave Gravity Wave A wave disturbance in which buoyancy (or reduced gravity) acts as the restoring force on parcels displaced from hydrostatic equilibrium. An unstable wave in a system of two homogeneous fluids with a velocity discontinuity at the interface. 1. Any wave motion in which no form of energy other than kinetic energy is present. In this general sense, Helmholtz waves, barotropic disturbances, Rossby waves, etc., are inertia waves. 2. More restrictedly, a wave motion in which the source of kinetic energy of the disturbance is the rotation of the fluid about some given axis. In the atmosphere a westerly wind system is such a source, the inertia waves here being, in general, stable. A similar analysis has been applied to smaller vortices, such as the hurricane. See inertial instability A wave that propagates in density-stratified fluid under the influence of buoyancy forces. A wave in fluid motion having its maximum amplitude within the fluid or at an internal boundary (interface). The concepts of internal and external waves originated in the study of gravity waves in homogeneous incompressible fluids, and it makes no difference in the dynamics of the wave whether the static stability of the fluid is concentrated in a free surface or in an interface. However, internal waves in a fluid with continually varying density have maximum amplitudes and nodal surfaces within the fluid itself, so that these are properly distinguished from external waves. Compare surface wave. A waveform disturbance that arises from Kelvin?Helmholtz instability. A type of low-frequency gravity wave trapped to a vertical boundary, or the equator, which propagates anticlockwise (in the Northerm Hemisphere) around a basin. Rossby (or planetary) waves are large-scale motions in the ocean or atmosphere whose restoring force is the variation in Coriolis effect with latitude. The waves were first identified in the atmosphere in 1939 by Carl-Gustaf Arvid Rossby who went on to explain their motion. Rossby waves are a subset of inertial waves With regard to atmospheric circulation, a progressive wave in the horizontal pattern of air motion with dimensions of cyclonic scale, as distinguished from a long wave. A wave that is stationary with respect to the medium in which it is embedded, for example, two equal gravity waves moving in opposite directions. Waves with a restoring force arising from variations in depth. The stretching or compression of displaced columns of water generates anomalous vorticity tending to drive them back to their original position. Rayleigh waves, also called ground roll, are surface waves that travel as ripples similar to those on the surface of water. The existence of these waves was predicted by John William Strutt, Lord Rayleigh, in 1885. They are slower than body waves, roughly 70% of the velocity of S waves, and have been asserted to be visible during an earthquake in an open space like a parking lot where the cars move up and down with the waves. Reports among seismologists suggest that the apparent motion may be due to distortion of the human eye during shaking. Anecdotally, placing people on shake tables causes the room to appear to ripple. In any case, waves of the reported amplitude, wavelength, and velocity of the visible waves have never been recorded instrumentally. Ocean surface waves that are nearly two-dimensional, in that the crests appear very long in comparison with the wavelength, and the energy propagation is concentrated in a narrow band around the mean wave direction. A seiche is a standing wave in an enclosed or partially enclosed body of water. Seiches and seiche-related phenomena have been observed on lakes, reservoirs, bays and seas. The key requirement for formation of a seiche is that the body of water be at least partially bounded, allowing natural phenomena to form a standing wave. An ocean wave with its length sufficiently large compared to the water depth (i.e., 25 or more times the depth) A type of seismic surface wave having a horizontal motion that is shear or transverse to the direction of propagation. Its velocity depends only on density and rigidity modulus, and not on bulk modulus. It is named after A. E. H. Love, the English mathematician who discovered it. P waves are longitudinal or compressional waves, which means that the ground is alternately compressed and dilated in the direction of propagation. In solids these waves generally travel slightly less than twice as fast as S waves and can travel through any type of material. In air, these pressure waves take the form of sound waves, hence they travel at the speed of sound. Typical speeds are 330 m/s in air, 1450 m/s in water and about 5000 m/s in granite. P waves are sometimes called primary waves. When generated by an earthquake they are less destructive than the S waves and surface waves that follow them, due to their lesser amplitudes. Body waves are seismic waves that travel through the lithosphere. Two kinds of body waves exist: P-waves and S-waves. Surface waves are analogous to water waves and travel just under the Earth's surface. They travel more slowly than body waves. Because of their low frequency, long duration, and large amplitude, they can be the most destructive type of seismic wave. There are two types of surface waves: Rayleigh waves and Love waves. Theoretically, surface waves can be understood as systems of interacting P and/or S waves. S waves are transverse or shear waves, which means that the ground is displaced perpendicularly to the direction of propagation. In the case of horizontally polarized S waves, the ground moves alternately to one side and then the other. S waves can travel only through solids, as fluids (liquids and gases) do not support shear stresses. Their speed is about 60% of that of P waves in a given material. S waves are sometimes called secondary waves, and are several times larger in amplitude than P waves for earthquake sources.