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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.

Oceanographic Waves

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

Seismic waves are generated in the earth by the movements of earthquakes or explosions. Depending on the material traveled through, surface and internal waves move at variable velocities. Layers of the earth, including the core, mantle, and crust, have been discerned using seismic wave profiles. Seismic waves from explosions have been used to understand the subsurface structure of the crust and upper mantle and in the exploration for oil and gas deposits. Atmospheric waves are caused by differences in temperature, the Coriolis effect, and the influence of highlands.
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fetch

[fech]
(computer science)
To locate and load into main memory a requested load module, relocating it as necessary and leaving it in a ready-to-execute condition.
(oceanography)
The distance traversed by waves without obstruction.
An area of the sea surface over which seas are generated by a wind having a constant speed and direction.
The length of the fetch area, measured in the direction of the wind in which the seas are generated. Also known as generating area.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

fetch

a Doppelganger. [Irish Folklore: Leach, 376]
See: Doubles
Allusions—Cultural, Literary, Biblical, and Historical: A Thematic Dictionary. Copyright 2008 The Gale Group, Inc. All rights reserved.

fetch

1. Engineering the reach, stretch, etc., of a mechanism
2. Geography the distance in the direction of the prevailing wind that air or water can travel continuously without obstruction
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005

Fetch

A Macintosh program by Jim Matthews <Fetch@Dartmouth.edu> for transferring files using File Transfer Protocol (FTP). Fetch requires a Mac 512KE, System 4.1, and either KSP 1.03 or MacTCP.

Latest version: 2.1.2.

Fetch is Copyright 1992, Trustees of Dartmouth College.

ftp://ftp.Dartmouth.edu/pub/mac/Fetch_2.1.2.sit.hqx. ftp://src.doc.ic.ac.uk/computing/systems/mac/info-mac/comm/tcp.
This article is provided by FOLDOC - Free Online Dictionary of Computing (foldoc.org)

fetch

To locate the next instruction in memory (RAM) for execution by the CPU. See instruction cycle.
Copyright © 1981-2019 by The Computer Language Company Inc. All Rights reserved. THIS DEFINITION IS FOR PERSONAL USE ONLY. All other reproduction is strictly prohibited without permission from the publisher.