Ocean waves and tides

Naseer Ahmed Chandio · #1 Thursday, December 14, 2006

Ocean waves and tides

Tides and Ocean WavesNext to their vastness, the most striking feature of the oceans and other large bodies of water is the constant motion of their surfaces. Waves—ripples, ridges, and hollows moving over the water are the cause of this choppy, rolling, or otherwise disturbed appearance.
The tides are the regular rise and fall of the ocean. They are seen most prominently along coasts and in harbors and bays.

OCEAN WAVES

Wind is the most common cause of waves. Waves generated by the wind may range in height from less than an inch to as much as 60 feet (18 meters). Waves breaking against a shore are called surf.
Other waves are caused by such geologic disturbances as earthquakes and volcanic eruptions beneath the oceans. Waves formed by underwater earthquakes are known as seismic sea waves, or as tsunamis, their Japanese name (see Earthquake).
Tsunamis are sometimes incorrectly called tidal waves, but they have no relationship to the tides. Near seacoasts, tsunamis may become very large and cause great destruction, but in the deep open sea they cannot be detected by the eye.

Wave Formation and Wave Measurement

Waves are formed as variations in air pressure above the water force gusty winds downward upon the water's surface, displacing the water. In addition, the wind tends to drag water particles along in the direction of its movement—that is, to exert a shearing force.
Waves are generated by the combined effect of the downward and shearing forces of the wind upon the surface of the water. Scientists have not yet been able to determine to any certain degree the exact nature of this composite action.

Waves are characterized by their dimensions. The highest part, or top, of a wave is called the crest, or peak; and the lowest part, or hollow, is called the trough. A wave's dimensions are its height, which is the vertical distance between the crest and the trough; its wavelength, which is the distance between one wave and the next, measured from crest to crest or trough to trough; and its period, which is the time required for the wave to pass a fixed point.
When the wavelength measures less than the depth of the water, the waves formed are called deepwater waves, or short waves. Wind-generated waves on the open sea or very large lakes, such as Lake Michigan in the United States, are waves of this kind. When the wavelength is greater than the depth of the water, shallow-water waves, or long waves, are formed. Tsunamis are shallow-water waves because they may have wavelengths of 100 miles (161 kilometers) or more, while the ocean's greatest confirmed depth is only about 6.8 miles (10.9 kilometers).
In the area where they are generated to their maximum height by the wind, waves are called seas, or forced waves. They tend to be very steep and to break or curl over. As waves travel away from their generating area, they become longer and smoother. In this state they are called swell, or free waves. Waves that originate in stormy Antarctic waters may appear as swell off tropical shores thousands of miles away. The speed at which a wave travels depends upon its period and the depth of the water over which it moves.

Wave Forms

Waves encountered in the oceans are so irregular that no two are exactly alike. Because of the complexity of the forces that operate to form waves, scientists often find it convenient to visualize and to speak of waves in terms of an ideal wave form.
In the ideal wave, water particles follow a nearly circular path in a vertical plane—they move upward as the wave approaches, downward as the crest passes, and then return to their original position. In the real ocean where seas are irregular, however, the motion of water particles in a wave is also irregular, and their path is not completely circular. The particles move slightly in the direction of wave travel. This movement is called the mass transport of water. The velocity of transport is only a very small fraction of a wave's speed. All waves in which water particles move in near-circular paths are known as progressive waves.
To the eye, progressive waves give the effect of water traveling from one point to another. For example, when a pebble is dropped into a quiet pool, water appears to travel toward the edge of the pool from the point where the pebble struck. In reality, the water travels very little; what is seen is the wave traveling along the surface of the water. A leaf floating on the pool would merely bob up and down as each wave passed beneath it and would progress very little along the water's surface.
Another type of wave is the standing wave. Standing waves may be formed by two similar progressive waves traveling across the same area at the same speed but in opposite directions. A standing wave can be constructed in a fish tank by slightly tilting the tank, then setting it back on its stand. The water levels in the ends of the tank will rise and fall opposite each other while the water in the middle of the tank remains at a fairly constant level. One type of standing wave, called a seiche, may occur in large lakes and in such semi-enclosed bodies of water as bays and sounds.

Wave Forecasting

An irregular sea is a composite of many ideal wave forms. A spectrum of the waves may be determined by analyzing a sea to discover the simple wave forms of which it is made. Through study of wave spectra, sea conditions in areas far removed from a point of observation may be predicted. A wave spectrum for a given area can also be determined from knowledge of wind conditions. It is then possible, for instance, to predict how many waves more than 10 feet (3 meters) high may be encountered there, or even the maximum height of the waves.
In the near future, high-speed electronic computers may enable scientists to predict the spectrum of the waves for almost any area of the oceans. The computers will require only data on wind speeds and directions. Wave forecasting is important to both Naval and merchant vessels. It helps ships' captains avoid areas where heavy seas may be encountered, thus assuring smoother rides for passengers and safer transport for cargoes. The United States Navy and many modern steamship companies use wave forecasting as an aid in routing their vessels.

THE TIDES

Tides occur in all bodies of water. In small, enclosed areas such as lakes and ponds, however, the rise and fall of the water are slight and usually pass unnoticed. Where the oceans meet the land along seacoasts and in bays and harbors, the tides are more prominent. In mid-ocean the difference between high water and low water is perhaps two or three feet. Along the shores of continents, especially in gradually narrowing bays, the difference may be much greater. The highest tide in New York Harbor is about 5 1/2 feet (1.7 meters). In Boston (Mass.) Harbor the water rises as much as 11 feet (3 meters). In the Bay of Fundy, between New Brunswick and Nova Scotia, Canada, the tide can rise 53 1/2 feet (16.3 meters) from low water, and even more during exceptional storm conditions.

High Tide and Low Tide

In most places, the tide rises and falls twice a day, reaching a maximum height called high tide on each rise and a minimum level called low tide on each fall. It takes a little more than six hours for rising waters to reach high tide and approximately another six hours for falling waters to reach low tide. This sequence is called the tidal cycle. The complete cycle takes 12 hours and 25 minutes and is then repeated. The amount of change in the water level during a tidal cycle is known as the tidal range.
In some parts of the world there is only one complete tidal cycle in a day. Such tides are called daily tides; they are observed chiefly in Alaska and in the Gulf of Mexico.
Other areas have mixed tides; where two high tides occur with only a slight low tide between them, followed by a prominent low tide. Mixed tides are seen primarily in the Pacific Ocean. Daily tides and mixed tides occur where the shape of the coastline affects the tidal waters.
Closely related to the rise and fall of the tide is tidal current, the horizontal flow of water produced by the tide. In the open sea, tidal currents flow in circular paths, constantly changing their direction. In such semi-enclosed waters as harbors, bays, and the mouths of rivers, tidal current becomes a to-and-fro motion of the water. In certain areas tidal currents may reach high velocities.

The Astronomical Tides

For many centuries the tides have been familiar to sailors and the inhabitants of seacoasts. They were not understood, however, until the 17th century, when Isaac Newton proposed the law of gravitational attraction (see Newton). According to this law, the tides are caused by the gravitational attraction of the moon and the sun on the Earth. For this reason they are called the astronomical tides.
The sun's gravitational effect upon the oceans is less than half that of the moon. This may seem strange, because the moon is smaller than the sun. The moon, however, is much closer to the Earth than is the sun, and thus it has greater influence upon the tides. The other planets in the solar system are too far away to have any appreciable effect upon the Earth's tides.

The attractive forces of the moon and the sun operate to pile up ocean waters in a wave directly beneath them, forming at the same time a similar wave on the opposite side of the Earth. These tide waves have a wavelength of one half the circumference of the Earth, and as the Earth rotates they try to follow the moon and the sun.
In doing so, they would cause a high tide whenever they reached a continent. If the ocean covered the entire globe, such direct tides would occur every 12 hours and 25 minutes under the gravitational attraction of the moon and every 12 hours under the attraction of the sun.
The direct tides, however, are influenced by the depth of ocean basins, by the Earth's rotation, and by the continental boundaries they touch, so the high tides do not follow the moon and the sun exactly. The actual tides are therefore indirect tides. A full understanding of the actual tides has not yet been reached by scientists.
When there is a full moon or a new moon, the sun and the moon are in line with the Earth, and their gravitational attractions are added together. This causes higher-than-normal high tides, which are called spring tides though they have nothing to do with the season of the year.
When the moon is in its first and last quarters, the sun and the moon are at right angles to one another, and the attractive force of the sun partially offsets the attractive force of the moon. This causes lower-than-normal high tides, called neap tides.

Predicting Astronomical Tides

It is difficult to predict astronomical tides solely from observation of the movements of the moon and the sun. To overcome this difficulty, careful measurements of the rise and fall of the water level are made over a long period of time.
Instruments called tide gauges are placed at each point from which a tide prediction is needed. When subjected to a process known as harmonic analysis, tide-gauge records yield information that can be directly related to the movements of the moon and sun. Tides can then be predicted from the known future motion of the moon and sun.

Meteorological Tides and Bores

Certain types of weather can produce a tidelike rise or fall of the water level. Such effects are called meteorological tides. They may be caused by changes in air pressure, by wind-driven water piling up along coasts, and even by rain or melting snow. In enclosed bodies of water, meteorological tides may completely mask the astronomical tides.
In the ocean, the meteorological effects on tides are related to the large ocean currents, such as the Gulf Stream in the Atlantic and the Kuroshio Current in the Pacific. These current systems are considered to be fairly constant. In adjacent and marginal seas, storms may from time to time cause storm tides involving sea-level elevations of up to several meters. Under normal conditions the wind velocity decreases after several hours. Such an elevation of sea level is neither permanent nor periodic.
For a long time it was assumed that the mass of the ocean water is constant. Since a considerable amount of frozen water is stored in polar areas, however, and given proper variation of the climate of the Earth, the amount of ice and the elevation of sea level will change. Another potential cause of sea-level change is the gradual, vertical motion of the land.
When a coastline is so formed that a river flows to the sea through a funnel-shaped outlet, the high tide may enter the river as a wall of water called a bore. Bores occur in some European rivers, in the Petitcodiac River at the northern end of the Bay of Fundy, in the Amazon River in Brazil, and in the Fuchun Jiang in China.

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