Before You Read

Where have you seen a water wave? How would you

describe the wave?

 

Waves

A wave is a rhythmic movement that carries energy through

matter or space. An ocean wave moves through seawater.

 

How are waves described?

Several terms are used to describe waves. The crest is the

highest point of a wave. The trough (TRAWF) is the lowest

part of a wave. The wavelength is the distance between the

crests or between the troughs of two adjoining waves. Wave

height is the distance from the trough of a wave to the crest.

 

The figure below illustrates each part of a wave.

138

1. Identify Highlight the

crests in the figure. Then

use another color to

highlight the troughs.

Wavelength

Wave

height

Wavelength

Crest Crest Crest

Trough Trough

How is a wave’s energy measured?

Waves carry energy. This energy can be measured. Half of

a wave’s height is called the amplitude (AM pluh tewd). To

measure the energy carried by a wave, the amplitude is

squared. A wave with twice the amplitude of another wave

carries four times (2 _ 2 _ 4) the energy. Small waves have

small amplitudes. Amplitude increases as waves grow larger.

Large waves can damage ships and coastal property.

 

How do waves move?

A bobber on a fishing line moves up and down in the water

as a wave passes. It does not move outward with the wave. It

returns to near its original position after the wave has passed.

Like the bobber, each molecule of water in a wave returns

to its original position after a wave passes. The molecule may

be pushed forward by the next wave, but it will return again

to its original position. The water in waves does not move

forward unless the wave is crashing onto shore. The water

molecules in a wave move around in circles, coming back to

about the same place. Only the energy moves forward.

Below a depth equal to about half the wavelength, water

movement stops. Below that depth, water is not affected by

waves.

Breakers As a wave reaches the shore, it changes shape. In

shallow water, friction with the ocean bottom slows water at

the bottom of the wave. The top of the wave keeps moving

because it has not been slowed by friction. Eventually, the

top of the wave outruns the bottom and it collapses. Water

tumbles over on itself, and the wave breaks onto the shore.

A breaker is a collapsing wave. After a wave breaks onto

shore, gravity pulls the water back to sea.

 

How do water waves form?

On windy days, waves form on lakes and oceans. When

wind blows across a body of water, wind energy is

transferred to the water. When wind speed is great enough,

water piles up forming a wave. As the wind continues to

blow, the wave grows in height.

Wave height depends on the speed of the wind, the

distance over which the wind blows, and the length of time

the wind blows. When the wind stops blowing, waves stop

forming. But waves that have already formed will continue

to move for long distances. Waves reaching one shore could

have formed halfway around the world.

 

 

Tides

Throughout the day, the level of the sea rises and falls.

This rise and fall in sea level is called a tide. A tide is caused

by a giant wave produced by the gravitational pull of the

Sun and the Moon. Although this wave is only 1 m or 2 m

high, its wavelength is thousands of kilometers long. As the

crest of this wave approaches the shore, sea level seems to

rise. This rise in sea level is called high tide. When the

trough of this huge wave nears the shore, sea level appears

to drop. This drop in sea level is referred to as low tide.

 

What is the tidal range?

As Earth rotates, Earth’s surface passes through the crests

and troughs of this giant wave. Many coastal areas, such as

the Atlantic and Pacific coasts of the United States, have two

high tides and two low tides each day. But because ocean

basins vary in size and shape, some coastal locations, such

as many along the Gulf of Mexico, have only one high and

one low tide each day. A tidal range is the difference

between the level of the ocean at high tide and the level of

the ocean at low tide.

 

Why do tidal ranges vary in different locations?

Most shorelines have tidal ranges between 1 m and 2 m.

However, tidal ranges can be as small as 30 cm or as large

as 13.5 m.

The shape of the seacoast and the shape of the ocean

floor both affect the ranges of tides. A wide seacoast allows

water to spread out farther. At high tide, the water level

might only rise a few centimeters. In a narrow gulf or bay,

the water cannot spread out. The water will rise many

meters at high tide. A narrow gulf or bay will have a greater

tidal range than a smooth, wide area of shoreline.

 

What are tidal bores?

Sometimes, a rising tide enters a river from the sea. If the

river is narrow and shallow and the sea is wide, a wave

called a tidal bore forms. A tidal bore can have a breaking

crest or it can be a smooth wave.

Tidal bores usually are found in places with large tidal

ranges. When a tidal bore enters a river, its force causes

water in the river to reverse its flow. Waves in a tidal bore

might reach 5 m in height and speeds of 65 km/h.

 

 

How does the Moon affect tides?

The Moon and the Sun exert a gravitational pull on Earth.

The Sun is much bigger than Earth, but the Moon is much

closer. The Moon has a stronger pull on Earth than the Sun.

Earth and the water in Earth’s oceans respond to this pull. The

water bulges outward as the Moon’s gravity pulls it. This results

in a high tide. The process is shown in the figure below.

The Moon’s gravity pulls at Earth. This creates two bulges

of water. One bulge is on the side of Earth closest to the

Moon. The other bulge is on the opposite side. The high

tide on the side of Earth near the Moon happens because

the water is being pulled away from Earth towards the

Moon. The high tide on the side of Earth opposite the

Moon happens because the gravitational pull on that part of

Earth is greater than the pull on the water on that side of

Earth. The areas of Earth’s oceans that are not toward or

away from the Moon are the low tides. As Earth rotates, the

bulges follow the Moon. This results in high and low tides

happening around the world at different times.

 

 

What effect does the Sun have on tides?

The Sun’s pull can add to or subtract from the

gravitational pull of the Moon. Occasionally during Earth’s

revolution, Earth, the Sun, and the Moon are lined up

together. Then, the Moon and the Sun both pull at Earth. The

combined gravitational pull results in spring tides on Earth.

During spring tides, high tides are higher and low tides are

lower than usual. When the Sun, Earth, and the Moon form

a right angle, high tides on Earth are lower than usual. Low

tides are higher than usual. These are called neap tides.

 

C Classify Cut a sheet of

paper into eight note cards.

Use the cards to record

important information about

waves and tides. Use the heavy paper.