TIDES

Tides

• The periodic short term changes in the height (rise or fall) of the ocean surface at a particular place

• Caused by a combination of–Distance between the Earth, moon and

sun–Gravitational force of the moon and the

sun and the motion of Earth– Moon’s influence 2x’s the sun’s (it is closer to

Earth)

Tides

• The longest of all waves• Wavelength = ½ Earth’s circumference• They are always in shallow water• And the fastest• Speed is proportional to wavelength• Tides move quickly

~1600 kilometers per hour (1000 mph)

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TIDES

Tides are extremely long waves caused by the gravitational attraction of the Moon and the Sun on Earth. They are also affected by the:

NOAA

1. shape and depth of the ocean floor

2. latitude (greater pull at equator)

3. volume of waterNASA

Earth-Moon System

• The moon’s gravity attracts the ocean toward it

• The Earth-Moon system revolves once a month (27.3 days) around the system’s center of mass

• The motion of Earth around the center of mass creates a bulge on the side of Earth opposite the moon

• Combination creates two tidal bulges daily

• Tides occur when Earth rotates beneath the bulge

• A tidal day is 24 hours 50 minutes long because the moon rises 50 minutes later each day

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The Earth and Moon revolve around each other and rotate on their axes.

Earth’s rotation = 24 hours (one day)

Moon’s rotation = 24 hours and 50 minutes (lunar day)

Tides occur 50 minutes later every day because of the rotation time difference.

At high tide (flow), the Moon pulls water out toward it. Centripetal force pulls water out on the opposite side - causing a tidal bulge.

As the Earth turns, locations move out of the center of gravity and low tides (ebb) result.

high tide

low tide

Moon

Variations in Tides

• The moon can also move 28 ½ degrees above or below the equator each month

• Tidal bulges are offset relative to this change

18Tidal range is the difference between high and low tide.

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The Bay of Fundy, Nova Scotia has the greatest range in the world ~53 feet.

Sun-Earth-Moon System

• Ocean responds to inertial and gravitational forces of the sun too

Spring Tides

• When the sun, Earth and moon are aligned, solar and lunar tides are additive

• Results in higher high tides and lower low tides

• Occur at two week intervals during new moons and full moons

Neap Tides

• When the sun, Earth and moon are at right angles, the solar will reduce the lunar tide

• Results in lower high tides and higher low tides

• Occur at two week intervals, arriving one week after the spring tide (during first quarter and last quarter moons)

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Tides are greater in North America in the winter because the Earth is slightly closer to the Sun in winter.

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Tides are also greater at times during the month when the moon and earth are closer together.

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Frequency is how often tides occur. There are some exceptions but basically there are 3 types:

1. Semi-diurnal = 2 equal high and low tides per dayeach 6 hours - 12 ½ minutes apart

ex: east coast of U.S.

2. Diurnal = 1 high and 1 low tide per day each 12 hours - 25 minutes apart ex: Gulf of Mexico, S.E. Asia

3. Mixed = 2 high tides and 2 low tides of unequal range each 6 hours - 12 ½ minutes apart ex: west coast of U.S.

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Importance of Tides

• Greatly influences coastal marine life• Different organisms have adapted to areas

within the intertidal zone• Ships use the tides to exit and enter port• Tidal power is the only marine energy

source that has been successfully exploited on a large scale