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Waves

Waves

The ocean surface is in continual motion. Waves are the result of disturbance of the water surface; waves themselves represent a restoring force to calm the surface. The standard example is the rock-in-the-pond scenario. The rock provides the disturbing force, and generates waves that radiate outward, eventually losing their momentum and dissipating their energy so that the pond returns to calm.

Characteristics of Waves

Wave characteristics include a crest at the top and a trough at the bottom. The difference in elevation between the crests and trough is the wave height. The distance between the crest or the troughs of waves is termed the wavelength. The ratio of wave height to wavelength is the wave's steepness.

A cohesive force, termed capillarity, holds the water molecules of the ocean surface together, allowing insects and debris to be supported. Capillarity is the initial restoring force for any body of water. The major disturbing force in the open ocean is wind. As winds begin to blow across the surface, they create pressure and stress. Small, rounded waves, called capillary waves, begin to form. These "ripples" have very short wavelengths, less than 1.74 centimeters (0.7 inch). For these small waves, capillarity is the restoring force that smoothes the surface.

As winds increase, capillary wave development increases and the sea surface becomes rough. This presents perfect conditions for the wind to catch more surface area of the wave, transferring increased energy to the water. As the young wave grows in height, gravity replaces capillarity as the restoring force, and the wave becomes a gravity wave with wavelengths exceeding 1.74 centimeters. These waves now exhibit the standard profile of a progressive wave.

Waves at the surface of the ocean and lakes are orbital progressive waves. This type of wave forms at the boundary of two liquids of different density, in this case air and water. The wave form moves forward with a steady velocity, so it is called "progressive." The water itself moves very little: Like the crowd in a football stadium doing "the wave," individual particles of water move up and then down, but do not follow the moving wave form. The complete motion of the water particles is a circle, so that a small object floating on top of the wave actually describes a circle as the wave goes underneath it.

Wave period is the length of time it takes for a wave to pass a fixed point (crest to crest). The speed of a wave is equal to the wavelength divided by the wave period. Wave steepness is defined as the ratio of the wave height to the wave length. When the wave builds and reaches a steepness greater than a ratio of 1:7, the wave breaks and spills forward. The wave has actually become too steep to support itself and gravity takes over. Breakers are normally associated with shorelines, where they are known as surf, but can occur anywhere in the ocean.

The passage of a wave only affects the water down to the wave base, which is half the wave length. Below that depth there is negligible water movement. This is the part of the water column that submarines use for "clear sailing." Waves in water deeper than half their wavelengths are known as deep water waves. Their speed in meters per second can be approximated by the equation Speed = gT/2π, where T is the wave period and g is the acceleration due to gravity (9.8 meters per second squared).

Shallow water waves are those moving in water less than one-twentieth the depth of their wavelength. Waves approaching shallow water at a shoreline are in this category. In these waves, the orbits of water particles are flat ellipses rather than circles. Shallow water wave movements can be felt at the bottom, and their interaction with the bottom affects both wave and sea floor. Shallow water waves include both seismic sea waves (tsunamis) generated by earthquakes at sea, and tide waves generated by the attraction of the Moon and the Sun on the ocean. Both of these wave types have such long wavelengths that average ocean depths are easily less than one-twentieth that value. The speed of shallow water waves decreases as the water depth decreases; it is equal to 3.1 times the square root of depth. Transitional waves have wave lengths between 2 and 20 times the water depth; their speed is controlled in part by water depth and in part by wave period.

Breaking Waves.

As waves approach landmasses, the wave base begins to contact the sea floor and the wave's profile begins to change. This friction slows the circular orbital motion of the wave's base, but the top continues at its original speed. In effect, the wave begins leaning forward on its approach to shore. When the wave's steepness ratio reaches 1:7, the wave's structure collapses on top of itself, forming a breaker.

A spilling breaker is the classic rolling wave coming up a gradually sloping sandy beach. The long incline drains the energy of the wave over a large area.

A plunging breaker approaches a steeper beachfront and forms a curling crest that moves over a pocket of air. The curling water is traveling faster than the slowing wave base, and the water outruns itself with nothing beneath for support.

Along oceanfronts with steep inclines or cliffs, a wave's energy is expelled in a very short distance, often with great force. These surging breakers develop and break right at the shoreline, proving dangerous and sometimes fatal to unsuspecting beachgoers. The tremendous energy dissipated at the ocean-level interface results in enormous erosion and deposition.

Wave Refraction, Reflection, and Diffraction

Seldom do wave fronts approach the shore parallel to the beach. Rather, their direction of approach varies according to the prevailing winds and the contour of the oceanfront. As a wave approaches a straight shoreline at an angle, one part of the wave base may begin to feel the bottom first and begins to slow before the rest of the wave. This causes the wave crest to bend towards the shore, termed refraction, allowing waves to break more closely parallel to the beachfront than was their original direction. Along irregular shorelines, waves also refract, but tend to converge on headlands, causing erosion of sediments; they disperse in bays, causing deposition.

As waves contact the oceanfront, not all their energy is expelled. The wave will tend to reflect back to sea at an angle equal to its approach. The reflected waves may form wave interference patterns with the original incoming wave fronts.

Wave diffraction is the creation of a wave around an obstacle and depends on the interruption of the obstacle to provide a new point of departure for the wave. As waves approach a chain of islands, some of the approaching wave's energy is directed through the spaces between the islands. These spaces serve as a starting point for new waves that spread across the ocean surface beyond the island chain.

Formation of Waves at Sea

Most waves are formed by wind, usually by storm systems. Unlike storm systems that are observed over land, ocean storm systems can be quite large, some exceeding 805 kilometers (500 miles) in diameter. These systems break up as they approach land, but over the ocean there is little to affect them. The wind transfers its energy to the water through wave-building directly under the storm system in an area of mixed wave types simply termed "sea." Factors that affect the amount of energy transferred to the waves depend on wind speed, the duration of time the wind blows in one direction, and the "fetch," the distance over which the wind blows in one direction.

Sea-wave heights determine the amount of energy transferred. Normal sea-wave heights average less than 2 meters (6.6 feet) but have been observed reaching 10 meters (33 feet.) Once the wave steepness reaches the critical 1:7 ratio of wave height to wavelength, the wave breaks and openocean breakers are formed, termed whitecaps.

At a given wind speed, there is a maximum wind duration and fetch which allow the waves to be fully developed. This "fully developed sea" is in equilibrium and is defined as the maximum size to which waves can grow under given conditions of wind speed, duration, and fetch. At this point, the waves of a fully developed sea will gain as much energy from the wind as they lose to gravity as breaking whitecaps.

Storm-Generated Waves: Swell

The most intense wave generating activity is where the winds are strongest, directly under the storm system. As waves radiate out from the center, the winds decrease near the margins of the storm system. The waves soon begin to outpace the wind speeds; waves with the longest wave lengths travel fastest; these large waves traveling away from a storm are called swell.

Swell waves are long-crested, uniformly symmetrical waves that have traveled outside the area of their origin. Swell waves expel little energy and travel vast areas of the ocean, fanning out from approaching storm systems. Wave dispersion begins to take effect and the swell waves becomes grouped by their wavelength. Waves with longer wavelengths travel faster and soon outrun the slower waves with shorter wavelengths. The long-wavelength waves do not have steep wave heights but move out of the generating area first, with wave groups of progressively shorter wavelengths following. This procession is termed a "swell wave train" and can travel long distances, breaking on distant shores.

As storm systems approach shore from far at sea, swell will begin to break, forming long, low rolling surf. Medium size swell follows with taller, curling breakers. As the storm system nears shore, the swell comes in high and fast with plunging breakers and crashing surf.

Interference.

As swell wave trains fan out across the Earth's oceans, waves from different storm systems will eventually meet and collide, causing interference and interesting wave behavior. When swell wave trains collide they can produce several types of interference.

Constructive interference occurs when two swell wave trains have the same wavelength and they combine in-phase. There is no affect on wavelength, but wave height increases

Destructive interference occurs when the wave crest of one swell combines with the wave trough of another. The energy from these swells cancels each other out and the surface becomes calmer.

Commonly, however, swell wave trains combine in mixed interference, producing unpredictable and complex wave patterns and heights. This type of interference may produce rogue waves, extremely large unpredictable waves that can be very dangerous to ships.

On rare occurrences in the open ocean, an unusually large wave may develop. These rogue waves are massive, single waves that can reach extreme heights of 15 to 30 meters (50 to 100 feet) or more. It is believed that one cause for rogue waves is overlap of multiple waves that produce an extremely large wave; they tend to occur most frequently downwind of islands and shoals. If storm winds push waves against a strong ocean current, rogue waves can develop. In the Agulhas Current off the southeastern coast of Africa, Antarctic storms push waves northeast into the oncoming current. Rogue waves have destroyed many ships in this region, capsizing them, smashing bow or stern, or lifting them amidships to snap the keel.

Internal Waves

Internal waves are disturbances that occur at the boundary between two water masses of different density. The wave heights can be quite large, sometimes exceeding 100 meters (330 feet) and may be formed by tidal movement, turbidity currents, wind stress, or passing ships. The surface expression of the waves is minimal, but if the crests approach the surface they affect the reflection of light from the water. Excellent photographs of internal waves have been taken from the space shuttle. As internal waves approach a landmass, they build up and expend their energy as turbulent currents.

Kelvin Waves.

Kelvin waves in the western Pacific Ocean are internal waves that form near Indonesia and travel east toward the Americas whenever the west-to-east trade winds diminish. A typical Kelvin wave is 10 centimeters high, hundreds of kilometers wide, and a few degrees warmer than surrounding waters. Scientists pay careful attention to these Kelvin waves because they may be precursors of the next El Niño.

Tsunamis (Seismic Sea Waves).

Seismic waves are formed when a severe shock such as an earthquake affects the ocean. The largest seismic sea wake known from geologic history is the one created by the impact of the K-T meteor 65 million years ago. The 10-mile-wide asteroid hit Earth at 72,000 kilometers (45,000 miles) per hour and created a wave estimated to be 914 meters (3,000 feet) high that traveled throughout Earth's oceans. Seismic waves are also referred to as tsunamis, their Japanese name. Sometimes they are incorrectly called tidal waves; they are not associated with the tides.

Tsunamis typically have wave lengths of 200km, which makes them shallow water waves even in the ocean. They travel extremely fast in open water, 700 km/h (435 m/h). These waves have insignificant wave heights at sea, but in shallow coastal waters they can exceed 30m (100 ft). They may travel thousands of kilometers across the ocean nearly unnoticed until they reach land. Earthquakes in the Aleutian Trench regularly send large seismic waves across the Pacific Ocean, affecting Hawaii and the coastlines of the North Pacific Ocean.

Seiche Waves.

The seiche phenomenon relates to the rocking of water in a confined space at a resonant frequency. When disturbed, water in a pan, bathtub, lake, harbor, or ocean basin will slosh back and forth at a particular resonant frequency. The frequency will alter with changes in the amount of water and the size and shape of the confined space. This is one type of standing wave rather than a moving progressive wave. Seiche wave periods can last for a few minutes to more than a day and have extremely long wavelengths. Even so, damage from seiche waves is rare because wave height in the open ocean generally is only a few inches.

Storm Surge.

Another phenomenon, storm surge, is associated with weather and is very dangerous. The air pressure over a section of the ocean affects the sea level. Sea level under a strong high-pressure system is pushed downward to a level several centimeters below normal sea level. Conversely, under an area of extreme low pressure, such as a hurricane or tropical storm, a mound of water develops and is pushed along by the storm front. As the storm system approaches land, the mound of seawater becomes a mass of wind-driven, elevated water, usually associated with large storm waves.

Storm surges are most dangerous when they coincide with high tides. They are responsible for the majority of flooding and destruction associated with hurricanes. Ninety percent of people killed by hurricanes are killed by storm surge. Severe hurricanes can produce storm surge to 12 meters (40 feet) in height.

see also Beaches; Coastal Ocean; Energy from the Ocean; Marginal Seas; Tsunamis; Tides.

Ron Crouse

Bibliography

Garrison, Tom. Oceanography, An Invitation to Marine Science. New York: Wadsworth Publishing Company, 1996.

Prager, Ellen J., with Sylvia A. Earle. The Oceans. New York: McGraw-Hill, 2000.

Summerhayes, C. P., and S. A. Thorpe. Oceanography, An Illustrated Guide. New York: John Wiley & Sons, 1996.

Thurman, Harold V., and Alan P. Trujillo. Essentials of Oceanography. Englewood Cliffs, N.J.: Prentice Hall, 1999.

Internet Resources

NOAA Wavewatch. National Oceanic and Atmospheric Administration. <http://polar.wwb.noaa.gov/waves>.

SURFING

The excitement of flying down the face of a monster wave lures more than 2 million surfers to American beaches annually. As swell approaches the beach and the breaker begins to form, wavelength decreases and wave height grows. The surfer must paddle the board with energy to match the speed of the oncoming wave. If timing is right, the surfer can get on the board just as the curl of the plunging breaker begins to form.

When swell comes onshore at an angle, a long curling breaker can form that rolls down the length of the beach. Surfers praise these breakers. A skilled surfer can catch an extended ride inside the curl.

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WAVES

WAVES. President Franklin D. Roosevelt signed Public Law 625 establishing a program for women in the U.S. Navy, as an integral part of the naval reserve, on 30 July 1942. The navy's newest members served for the duration of the war plus six months. On 2 August, Mildred McAfee, president of Wellesley College, became the director of the navy's female reserve and the first female naval officer with the rank of lieutenant commander. To avoid nicknames such as “sailorette,” Elizabeth Raynard, a member of the Naval Advisory Council that developed the women's program, recommended the official nickname WAVES, an acronym for Women Accepted for Volunteer Emergency Service.

Women were recruited from nearly every state. Officers were trained at Smith College in Northampton, Massachusetts. The navy organized training schools for yeomen, radiomen, and storekeepers, located respectively at Oklahoma A&M College, the University of Wisconsin, and Indiana University in Bloomington. In February 1943, a naval station for enlisted recruits was commissioned at Hunter College in New York. WAVES could apply for more billets and were assigned to more locations than their predecessors, the 11,275 yeomen (female) who served temporarily during World War I. WAVES worked at naval shore establishments across the United States as chauffeurs, cryptologists, recruiters, and stenographers. They also filled nontraditional billets as air traffic controllers, link trainers, mechanics, and parachute riggers. About one‐third of the WAVES served in the communications and aviation communities. By 1944, the need to relieve men stationed in Alaska and Hawaii led the navy to amend the original bill that had limited WAVES to duty within the continental United States.

Nothing in the legislation prevented the recruiting of black women, yet the navy did not admit them into the WAVES until 19 October 1944. The next day, the U.S. Coast Guard also announced that African Americans could join its female reserve program, but the Women's Marines Corps remained all‐white until 1949. Two black women, Frances E. Wills and Harriet Ida Pickens, were sworn into the U.S. Navy on 13 November 1944 and were added to the last class of WAVES officer candidates to be trained. Receiving their commissions on 25 December, they became the first black female officers in the navy. The first black enlisted recruits reported to Hunter in January 1945. By 30 July, the WAVES had reached a peak strength of 86,000.

The performance of the WAVES and the other 150,000 women serving in the military services—SPAR, WAC (Women's Army Corps), WASP (Women Air Service Pilots), and the U.S. Marine Corps Women's Reserve—persuaded officials that women should have a permanent place in the peacetime military. Senator Margaret Chase Smith of Maine achieved that goal with the Women's Armed Services Act of 1948. WAVES continued to serve, particularly during the Korean War and the Vietnam War. In 1972, Capt. Robin Quigley, assistant chief of naval personnel for women, wrote a memo discontinuing the navy's official use of the term WAVES, recommending the more accurate description Women in the Navy. This change reflected the navy's policy of integrating women. By then, the women‐in‐ships program had begun and the aviation community was opening up more jobs to women.
[See also African Americans in the Military; Women in the Military.]

Bibliography

Joy B. Hancock , Lady in the Navy, 1972.
Jean Ebbert and and Marie‐Beth Hall , Crossed Currents, Navy Women from World War I to Tailhook, 1993.

Regina T. Akers

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Waves

Waves (Women Appointed for Voluntary Emergency Service), U.S. navy organization, created (1942) in World War II to release male naval personnel for sea duty. The organization was commanded until 1946 by Mildred Helen McAfee. Waves served in communications, air traffic control, naval air navigation, and clerical positions in the United States, Hawaii, Alaska, and the Caribbean. Recruiting ended in 1945, with a peak enrollment of 86,000. Waves forces were reduced when the war ended. After the passage (1948) of the Women's Armed Service Integration Act, women were enlisted into the regular navy, though they continued to be known as Waves for some time.

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waves

waves See GEOMETRIDAE.

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WAVES

WAVES (or Waves) (weɪvz) US Navy Women Accepted for Volunteer Emergency Service

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Waves

WAVES

Waves which are involved in many aspects of life, are disturbances that propagate through a medium with a definite speed. A wave of light conveys information to the eyes, a wave of sound brings music to the ears, a water wave rolling onto a beach can topple the swimmer, and electromagnetic waves cook food (microwave oven), and carry television reception. It takes energy to create the wave disturbance, and how the wave travels through the medium (elastic or damped) is quite variable.

There are two important categories of waves, mechanical and electromagnetic. Mechanical waves require a material medium. Water waves in a pond require water, sound waves in a room require air, a stadium "wave" must have people, and a wave produced by plucking a guitar string needs a string. Mechanical waves are produced when the medium is disturbed. A stone dropped in a pond disturbs the water, a spoken word results from a pressure disturbance, the guitar player disturbs the string by plucking it, and the sports fan disturbs the audience by standing up. The disturbance moves through the medium with a speed that is determined by the properties of the medium. In a stringed musical instrument, for example, the mass of the string and how tightly it is stretched determine the speed. That is why there are different size strings and a mechanism for tightening them.

A stone dropped in a pond pushes the water downward, which is countered by elastic forces in the water that tend to restore the water to its initial condition. The movement of the water is up and down, but the crest of the wave produced moves along the surface of the water. This type of wave is said to be transverse because the displacement of the water is perpendicular to the direction the wave moves. When the oscillations of the wave die out, there has been no net movement of water; the pond is just as it was before the stone was dropped. Yet the wave has energy associated with it. A person has only to get in the path of a water wave crashing onto a beach to know that energy is involved. The stadium wave is a transverse wave, as is a wave in a guitar string.

When air in a room is disturbed by a person speaking the molecules of the air have movements that are along the path of the wave. If you were to draw a line from the speaker's mouth to your ear, the movement of the molecules would be along this line. This type of wave, called an acoustical wave, is said to be longitudinal. The pleasant sounds of music are produced by acoustical waves. On the other hand, destruction by a bomb blast also is caused by acoustical waves. Instead of oscillating up and down, molecules in the acoustical (or compression) wave bunch together as the wave passes. It is not a transverse wave.

Imagine something floating in a pan of water with both the float and the water motionless (Figure 1A). If you were to continually bob the float up and down, you would see a continuous train of crests and troughs moving away from the float (Figure 1B). The separation of adjacent crests is called the wavelength, and given the lower case Greek lambda (λ) for a symbol. The time it takes one of the crests to travel a distance equal to the wavelength is called the period, and given the symbol T. The speed that the crest moves is just the distance traveled divided by the time so that v = 1/T. The period measured in seconds is just the time for one cycle of the oscillation. The reciprocal of the period is the number of cycles per second and is called the frequency, symbol f. The speed of the wave can also be written v = f λ.This simple relationship is a very important feature of all types of waves.

When waves encounter something in their path, they may bounce off (reflect) somewhat like a ball bouncing off a wall. A sound echo is produced by sound waves reflecting from something, a building, for example. We see our image in a mirror because of reflection of light. Reflection as well as other phenomena involving light led to the notion that light is a wave. It is decidedly different from a sound wave because there is no material medium required to sustain its motion. Light traveling to Earth from a distant star encounters virtually no matter until it reaches Earth's atmosphere. Modeling light as a wave has many virtues and uses.

Light is an example of electromagnetic waves. Electromagnetic waves propagate electric and magnetic fields. All electromagnetic waves travel with the same speed in a vacuum. This speed is given the symbol c and has a value 3.00 × 10 8 m/s (300,000,000 m/s). The general equation v = f λ becomes c = f λ for electromagnetic waves. Because c is the same for all electromagnetic waves in vacuum, f λ is constant. Therefore, when the frequency of an electromagnetic wave increases, the wavelength must decrease. The range of wavelengths of electromagnetic waves is staggering: about 10 -14 m (roughly the diameter of the nucleus of an atom) to about 1,000,000 m (approximately the distance from New York to Chicago). Because f λ is constant, the frequency varies from a high of about 10 22 Hz to a low of about 10 3 Hz.

Electromagnetic radiation is classified according to ranges of wavelength. Although the classifications are not precise, they are useful. For example, visible light to which our eyes are sensitive has wavelengths between about 4 × 10 -7 m and 7 × 10 -7 m. Microwaves have much longer wavelengths, about 0.001 m to 0.3 m.

Energy streams to Earth from the sun by electro- magnetic waves. Photosynthesis depends on solar energy, and humans and animals rely on photosyn- thesis for food. In this sense, solar energy is not a luxury, it is essential.

All objects, including the sun and incandescent light bulbs, emit electromagnetic radiation. The radiant energy and the type of radiation depend on the temperature. When an electric toaster is switched on, the heating element begins to glow and one can feel the radiant energy increase as the temperature increases. Visible radiation is produced by both the sun and a light bulb because the temperature of both is roughly 6,000 K. When the bulb cools to room temperature, about 300 K, the visible radiation disappears for all practical purposes. Nevertheless, the bulb at 300 K is still radiating, but the radiation is predominantly infrared that our eyes do not sense.

Radiation from the sun includes significant ultra- violet and infrared radiation in addition to visible radiation. Contributions of each type to the radiation that reaches Earth's surface are reduced significantly through absorption in Earth's atmosphere. The rate at which solar radiation falls on a square meter of Earth's solid surface is called the solar insolation. Solar insolation depends on the time of day, the day of the year, and where the square meter is located on Earth's surface. During an eight-hour period, the solar insolation in midwestern United States is roughly 600 W/m 2. If the 600 watts striking a 1 m 2 surface could be collected and converted to electricity, there would be enough electricity to operate six typical 100-watt light bulbs.

Solar energy can be used in many ways: heating buildings and houses, providing heat for producing steam for a turbogenerator in an electric power plant, producing electricity with photovoltaic cells, and so on. But since solar energy is not highly concentrated, it takes a considerable area of collection to even heat a small home, which makes it uneconomical and impractical in many situations.

One of the most interesting and fascinating methods of electronic communication uses electromagnetic waves of light. The light is literally piped around in tiny bundles of transparent strings called optical fibers. Because the frequency of the light is thousands of times greater than the frequency of radio and television waves, a much higher concentration of information is permitted. The telephone industry makes great use of transmission of information via optical fibers. Using this technology allows information to be transmitted between North America and Europe via optical fiber cables strung beneath the surface of the Atlantic Ocean.

The back-and-forth motion of a child's swing is an example of an oscillating system, oscillator, for short. Set into motion, these systems oscillate at a frequency determined by properties of the oscillator. For example, the length of a child's swing and the acceleration due to gravity (g) determines the frequency of the back-and-forth motion of a child's swing. Periodically feeding energy into such a system at the same rate as the natural frequency gradually increases the energy of the oscillator. This is the phenomenon of resonance. Children achieve resonance with a swing by "pumping" at the resonant frequency of the swing. Molecules are microscopic oscillators that can be "pumped" by electromagnetic waves. If the frequencies match, the molecules absorb energy. Water molecules have a resonant frequency that falls in the microwave region of the electromagnetic spectrum. Water-based materials exposed to microwaves of the proper frequency absorb energy and heat up. This principle is exploited in a microwave cooker. When food on a plastic plate is placed in a microwave oven. the food warms because of its water content, and the plate does not warm because it is essentially devoid of water.

Ozone is a molecule formed from binding together three oxygen atoms. Gaseous ozone readily absorbs selected frequencies of ultraviolet radiation because ozone molecules have a resonant frequency in the ultraviolet region of the electromagnetic spectrum. A natural concentration of ozone exists twenty to thirty kilometers above Earth's surface. Ozone molecules absorb nearly all ultraviolet radiation from the sun having a wavelength less than about 0.3 micrometers. Accordingly, the natural ozone layer protects humans from harmful effects such as skin cancer. Human-made products migrating into the ozone layer can react with ozone molecules and convert them to other forms that do not absorb ultraviolet radiation. Chlorinated fluorocarbons, CFCs for short, once commonly used in refrigerators and aerosol spray cans, were found to be depleting the ozone, and in 1978 were banned from use in aerosol sprays. Banning the use of CFCs has not totally solved the ozone depletion problem.

Carbon dioxide is a molecule formed from binding together two oxygen atoms and one carbon atom. Gaseous carbon dioxide readily absorbs selected frequencies of infrared radiation because carbon dioxide molecules have a resonant frequency in the infrared region of the electromagnetic spectrum. Coal, natural gas, and oil all have carbon as a component, and carbon dioxide is produced when they are burned. It is a fact that the concentration of carbon dioxide in the atmosphere has been increasing since the Industrial Revolution. The carbon dioxide has little effect on solar radiation penetrating Earth's atmosphere. Earth absorbs the solar energy and, like any object, emits electro- magnetic radiation. The temperature of Earth is more than 5,000 K lower than the sun, and Earth's radiation is mostly infrared. Carbon dioxide can absorb energy from the infrared radiation, resulting in a temperature increase of Earth. This is the socalled "greenhouse effect."

Coping with the greenhouse effect is a very difficult sociopolitical problem. A greenhouse effect existed on Earth long before the Industrial Revolution. Had it not, Earth's surface would be much colder than it is now. The introduction of gases absorbing infrared radiation only enhances the greenhouse effect. Carbon dioxide is not the only gas of importance; water vapor and methane, for example, are also of concern. The industrial world is dependent on coal, oil, and natural gas, so it is not easy to quit burning them to stop producing carbon dioxide.

The radiation one sees from a neon advertising sign or a laser is not produced by accelerated electric charges or by thermal effects. It comes from atoms or molecules that have absorbed energy and given this energy up as photons. A photon is a unit of electro- magnetic energy having energy equal to Planck's constant (6.63 × 10 -34 joule.seconds) times the frequency of the radiation (E = hf ). The frequency of the radiation is determined by the magnitude of energy associated with atoms and molecules. Radiation from atoms and molecules is usually in the infrared, visible, and ultraviolet regions of the electromagnetic spectrum. Electromagnetic radiation also results from energy transitions in the nucleus of an atom. However, the nuclear energy scale is roughly a million times larger, making the frequency of the radiation correspondingly larger. This radiation is called gamma radiation.

All electromagnetic radiation has energy to some extent. Absorption of electromagnetic energy can be put to good use, as in photosynthesis or a microwave oven. On the other hand, electromagnetic energy can do damage. Skin cancer can result from absorption of ultraviolet radiation from the sun. This is why young and old are encouraged to stay out of the summer sun between the hours of 10 A.M. and 2 P.M. Gamma radiation is more energetic and more penetrating than ultraviolet, and the damage it can do is not confined to the surface of the skin. This is why some are concerned about the gamma radiation produced by radioactive nuclei in the spent fuel of a nuclear reactor. Ironically, gamma radiation is used to treat certain types of cancer.

Joseph Priest

See also: Atmosphere; Climatic Effects; Communications and Energy.

BIBLIOGRAPHY

Hobson, A. (1995). Physics: Concepts and Connections. Englewood Cliffs, NJ: Prentice-Hall.

Serway, R. A. (1998). Principles of Physics, 2nd ed. Fort Worth, TX: Saunders College Publishing.

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Waves

Waves

Wind creates waves. As an air current (moving stream of air) moves over an undisturbed water surface, friction between air and water creates a series of waves that move across the surface. The size of the waves depends on the wind speed, the duration of the wind, and the distance over which the wind blows. (The distance of open water surface that the wind blows over is called the fetch.) A week-long tropical storm in the Pacific Ocean might produce waves as tall as three-story houses; a ten-minute gust blowing across a small lake might make waves that are only a few inches tall.

Waves move away from their point or area of origin in widening circles, like ripples moving away from a pebble dropped into a pool. In an ocean basin (the deep ocean floor), waves from many different wind events are moving across the sea surface at any given moment. When sets of waves meet they interact to form new patterns. By the time they reach the coastline, waves have been affected by many wind events.

Ocean waves may appear that the water is moving forward but in actuality the water is moving in a circle as the water molecules lift and fall. (A molecule is the smallest unit of a substance that has the properties of that substance.) Imagine floating in the ocean in a raft. When a wave approaches, you rise and fall as it passes, but you don't move toward the beach. The same thing happens to the water molecules below you. As a wave arrives, the water particles rise and fall in small circles as the wave passes, but they are not carried forward. The highest point the wave reaches is called the crest. The lowest point of the wave is called the trough. The wavelength is the distance from one crest to the next. The water molecules closest to the surface move in the largest circles, and deeper water moves less. Molecules below a depth known as wave base are undisturbed by a passing wave. Wave base is equal to half the horizontal distance between wave crests, or one-half a wavelength.

Breaking waves

Waves change form when they approach a coastline. When the seafloor is shallower than the wave base, it interferes with the circular motion of the water at the bottom of the wave. Waves that were broad, gentle swells in the open ocean grow taller and their crests get closer together. Eventually, the wave grows too tall to support itself and it breaks; the wave crest collapses over the front of the waves. Spilling breakers that gradually become more steep and then crumble typically form along shallowly sloping shores. Plunging breakers that grow tall and curl sharply generally pound steep coastlines. Big waves start to break farther from shore than smaller waves because they have deeper wave bases.

Surfing the Perfect Wave

If you want to know about waves, ask a surfer. The conditions that produce perfect surfing waves are rare, and the sport of surfing is also a study of subtle patterns of wind, weather, and waves along coastlines. The classic 1964 surf movie, Endless Summer, follows two surfers as they follow summer around the globe—from California, to Africa, Australia, New Zealand, Hawaii, and back to California—in search of the perfect wave. Surfers also rely on their knowledge of waves and coastal hazards to keep them safe.

Here are some useful terms that surfers commonly use:

  • Barrel, tube: The barrel, or tube, is the hollow front of a breaking wave. Sometimes the crest of the wave curls all the way down to the water enclosing the surfer in a spinning tube of water. This is a "totally tubular" ride.
  • Break: A break is a line of breaking waves. Surfers wait to catch waves just seaward (in the direction of the sea) of the break. A beach break, where waves break on the sandy seafloor in front of a beach, is a good place to learn to surf. Reef breaks, where waves break on offshore reefs or rock shoals, and point breaks around rocky headlands are strictly for experts.
  • Lefts, rights, and peaks: A left is a wave that breaks from right to left when viewed from the beach, and right is a wave that breaks from left to right. (This is wave refraction.) Surfers ride lefts to the left and rights to the right. A peak is a wave that breaks almost parallel to the beach and surfers can ride this in either direction.
  • Onshore/offshore winds: For surfing, winds blowing onto the shore (onshore breezes) are bad and winds blowing off of the shore (offshore breezes) are good. Wind blowing toward the beach knocks the crests of breaking waves over and they crumble into "foamies." Wind blowing away from the beach stabilizes the curl of the breaking waves and helps create barrels.
  • Pipeline: This is the classic Hawaiian surf break. Also called the Banzai Pipeline, this most-photographed break on the north shore of the island of Oahu has huge waves with perfect, massive barrels.
  • Rip: This strong, shallow current that can drag swimmers and surfers far out to sea is usually quite narrow and, unlike undertow, will not drag you underwater. If you are caught in a rip current, don't panic.Swim parallel to the beach to escape instead of straight back against the current.

Water from breaking waves sloshes forward up the beach. It returns in an outgoing current along the seafloor called undertow. The forward and back motion of water in the surf zone (area of rough water next to the land, where ocean waves hit the shore) between the breaking waves and the beach is called swash. Like water in a washing machine, water in a surf zone endlessly cycles between the breakers and the beach. Beaches are subjected to relentless swashing that breaks down all but the most resistant sediments (sand, gravel, or silt). The mineral quartz is particularly strong, and beach sand is often composed of identical quartz grains that waves have rounded into perfect spheres and sorted by size.

Wave refraction

Waves bend when they reach coastlines. It is extremely rare for wind to blow exactly toward a perfectly straight coastline, and waves almost always approach shorelines at an angle. Wave bending or refraction occurs because the end of a wave that reaches shallow water first slows down and breaks before the deeper end. Water moving in the surf zone flows sideways along the beach from the direction of the approaching wave, and gravity pulls the returning water directly downhill. Water and sediment thus move in a zigzag pattern that carries them along the beach. Wave refraction produces longshore currents, which are currents that flow parallel to coastlines in shallow water. If you have ever dropped your towel on the beach and gone for a swim only to discover that you have been carried away from your towel, you have experienced a longshore current.

Wave refraction also brings the eroding power of waves onto headlands, the jagged, rocky, narrow strips of land that extend into the ocean. Longshore currents carry the eroded sediment away from headlands and deposit it in bays. Waves thus, straighten irregular coastlines by wearing down the headlands and filling the bays. A typical arc-shaped bay with headlands at each end has two longshore currents that flow from the headlands toward each other. The shallow, strong, outgoing current that forms at the tip of the bay where they meet is called a rip current. Rip currents also form where large waves pile water between a sand bar (a ridge of sand built up by currents) and a beach. Rip currents, can be dangerous to swimmers because they can form or become strong suddenly.

Waves and longshore currents can also mold sand into strings of barrier islands (a long, narrow island parallel to the mainland) formed from sediment deposits. These islands are often called depositional coastlines. In the Gulf of Mexico, waves have washed sand from the Mississippi River Delta. Longshore currents have deposited the sand in a long streamer of barrier islands along the Louisiana and Texas coastlines. Tidal inlets (inlets maintained by the tidal flow) separate barrier islands from each other and shallow bays called lagoons separate them from the mainland. The barrier island of the United States eastern seaboard, included the Outer Banks of the Carolinas, formed in a similar fashion. Wave patterns and coastline conditions are constantly changing, and coastline features are continuously remolded. The waves from a large hurricane, for example, can completely destroy a barrier island, beach houses and all, and reshape a new one in a matter of days.

Laurie Duncan, Ph.D.

For More Information

Books

Berger, Melvin, and Berger, Gilda.What Makes an Ocean Wave?: Questions and Answers About Oceans and Ocean Life. New York: Scholastic Reference, 2001.

Open University. Waves, Tides, and Shallow Water Processes. Burlington, MA: Butterworth-Heinemann, 2001.

Warshaw, Ed. Surfriders: In Search of the Perfect Wave. New York: Harper-Collins, 1998.

Periodicals

Lawton, Graham. "Monster of the Deep (rogue waves)." New Scientist (June 30, 2001): p. 28.

Websites

"About Water Levels, Tides and Currents." NOAA/NOS Center for Operational Oceanographic Products and Serviceshttp://www.co-ops.nos.noaa.gov/about2.html (accessed on August 17, 2004).

"Surfing Waves." Surfing Waves.comhttp://www.surfingwaves.com/ (accessed on May 4, 2004).

"Why Tides?" Fitzgerald Marine Reservehttp://www.sfgate.com/getoutside/1996/jun/tides.html (accessed on August 17, 2004).

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