UNIT 4 LESSON 16 WAVES AND TIDESOcean waves have a rising and falling motion, but they are caused by wind, gravity, or impacts.

OBJECTIVES

Explain how waves are generated and classified. Describe a wave in terms of height, wavelength, and wave period. Explain the role of the moon in the generation of Earth's tides. Describe tidal frequency and variation.VOCABULARY

crest highest part of a wave

fetch distance over which wind blows without interruption or change of direction

tide alternate rising and falling of the ocean caused by the gravitational pull of the moon and the Sun

trough lowest part of a wave

Lesson Outline

Surface waves are caused by wind. Impact waves are caused by a disturbance. The size and power of a wind-generated wave is determined by wind speed and

fetch. Waves are usually classified by their wave period, which is the length of time it

takes two consecutive crests to pass a point. Short waves travel faster than long waves.

Tides are very long waves generated by the moon's gravitational pull. There are two high tides and two low tides in each tidal cycle. Spring tides happen when the Sun, moon, and Earth are aligned. Neap tides happen when the Sun and moon are at a right angle to Earth.

Wave Generation

In the absence of wind, the ocean's surface is calm and flat. When the wind blows, it creates tiny ripples called wavelets.

The wavelets move in the same general direction as the wind. 

If the wind is strong enough, the wavelets will grow into stable waves, and as the waves travel toward the shore, they slow down and increase in height. Eventually, the front of the wave collapses on itself.

The size and power of a wind-generated wave is determined by wind speed and fetch. 

The fetch is the distance over which the wind blows without interruption or significant change in direction. 

Generally, the size of the fetch is more important than wind speed. 

A strong wind with a short fetch will create smaller and less powerful waves than a weaker wind with a long fetch. 

The longer the fetch, the more powerful the wave is.

You can see the effect of fetch length in a lake or pond. These bodies of water rarely have large waves because the waves have so little distance over which to form. These waves are sometimes called forced waves, because they are tied to the generating force of the wind.

Not all waves are generated by wind; a disturbance from an outside force causes impact waves. 

A tsunami is a giant impact wave caused by an underwater earthquake or volcanic eruption. They are sometimes called free waves, because they exist free from any generating force.

Wave Classification

Waves are described by their height, wavelength, and wave period. The highest part of a wave is the crest and the lowest part is the trough. Wave height is the vertical distance from the crest to the trough. Wavelength is the horizontal distance between the crest of one wave and the

crest of the next one. The frequency of a wave is the wave period.

It measures the time it takes for the crests of two successive waves (one wavelength) to pass a fixed point.

Waves are usually classified by their wave period. The smallest waves, called capillary waves, have periods less than one-tenth of

a second. You can create capillary waves in your kitchen by blowing on a bowl of hot soup. The longest waves are tides, which have 12-hour and 24-hour wave periods. The shorter the wavelength is, the shorter the wave period will be, and the faster

the wave moves.

Waves are also classified by the depth of the water they travel in. Deep-water waves travel in water deeper than one-half their wavelength. Shallow-water waves travel in water shallower than one-twentieth of their

wavelength. The wavelength of a shallow-water wave is at least 11 times the depth of the

water beneath it.

Tide Generation

The gravitational attraction between Earth's oceans and the moon causes special waves called tides.

Tides are very long-period waves which move through the oceans in a predictable cycle.

They begin in the ocean and move toward the coastline. High tide occurs when the crest of the wave reaches the shore. Low tide happens when the trough reaches the shore. The tidal range is the difference in height between high tide and low tide.

How does the moon affect Earth's water?

According to Newton's Law of Universal Gravitation, all objects exert a gravitational pull on other objects.

The moon exerts a gravitational pull on Earth. It isn't strong enough to affect Earth's solid rock, but it does influence Earth's water.

On the side of Earth facing the moon, the gravitational force is strongest. It pulls the water on the "near side" of Earth, creating a tidal bulge. On the opposite side, or "far side," of the planet, Earth's rotation spins water outward. The moon's gravity is not as strong, so the ocean forms another tidal bulge.

Tidal Frequency and Variation

[Diagram of the effect of Moon pulling Earth’s tides]

Gravitational forces between the Earth and the moon cause the Earth's oceans to bulge, and since the Earth rotates, any given location will pass through tidal bulges once every twenty-four hours and fifty minutes. If the moon were stationary, the tidal cycle would be twenty-four hours long. But, the moon revolves around the Earth every twenty-seven days. This adds about fifty minutes to each tidal cycle.

High tide occurs when a coastal area passes through a bulge. Low tide happens

when a coastal area is halfway between the bulges. There are two high tides and two low tides every lunar day, because the Earth has two tidal bulges. High tides happen every twelve hours and twenty-five minutes.

The same is true for low tides, and it takes six hours and twelve point five

minutes for a high tide to change to a low tide. The change from low tide to high

tide takes the same amount of time. The tidal cycle is twenty-four hours and fifty minutes long. A single tidal cycle has two high tides and two low tides.

[Full moon over a beach] The moon is the primary cause of tides. However, the Sun's gravitational attraction creates tidal variations. During a full or new moon, the Sun, moon, and the Earth are aligned, and their combined gravitational pull causes a larger bulge. The larger the bulge, the higher the high tides and the lower the low tides.

These very high and very low tides are called spring tides. During a first or third

quarter moon, the Sun and moon are at right angles as compared to the Earth. The gravitational pull of one offsets the pull of the other, causing a smaller bulge. A smaller bulge means a smaller tidal range, and these smaller tides are called neap tides.

Phases of the Moon