Before You Read
Where have you seen a water wave? How would you
describe the wave?
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.
1. Identify Highlight the
crests in the figure. Then
use another color to
highlight the troughs.
Crest Crest Crest
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
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.
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.