TIDES

What is a tide?

• A tide is the periodic rise and fall of a body of water due to gravitational interactions between the sun, moon and Earth

• The wavelength of an average tide can be up to 17,000 km (over 10,500 miles). Do you think tides are deepwater or shallow water waves?

• Shallow water! Recall that shallow-water waves have Depth < 1/20 Wavelength and deepwater waves have Depth > 1/2 Wavelength

• The ocean would have to be deeper than 5,250 miles for an ocean tide to be a deepwater wave!

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1.Gravitational pull of the moon and sun

2.Centripetal force of the rotating Earth

Tides are generated by:

Tides are generated by:• the gravitational pull of the moon and sun

- moon has 2x greater gravitational pull than the sun - sun is 10 million x more massive than the

moon and is 390 times farther away

“Bulge” of Earth

What causes tides?

The gravitational pull of the sun and the moon causes “bulges” on Earth that move as we rotate

Sun

Earth

Moon

Gravitational pull

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Centripetal force

CENTRIPETAL

GRAVITATIONAL FORCE

GRAVITATIONAL & CENTRIPETAL

What causes tides?

The tide rises when coastline enters the bulge and falls when it rotates out

High tide

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• High water: a water level maximum ("high tide") • Low water: a water level minimum ("low tide") • Tidal range: the difference between high and low tide

Description of tides

Intertidal zoneHigh tide

Low tide

Alma at High Tide

Alma at Low Tide

Earth-Moon-Sun positions and the monthly tidal cycle

Spring TideHighest high tide and lowest low tide

Neap TideModerate tidal range

Different types of tide

Sun

Earth

Moon

• Spring tides occur when the sun and moon are in a straight line

• The tidal range is typically highest during spring tides12

Different types of tide

• Neap tides occur when the sun and moon form a right angle with Earth

• Low tides are typically higher and high tides are lower

• Why is the moon’s pull greater than the sun’s?

Earth

Moon

Sun

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Tides are also important physical forces in our ocean

• Gravitational interactions between the sun, moon and earth are the primary causes of tides

• Different positions of the sun and moon create two different types of tides: spring tides and neap tides

• Tides play an important role in the life cycle of many marine organisms

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6 ft

Tidal Range

56 ft

The Bay of Fundy: Site of the world’s largest tidal range

• Tidal energy is focused by shape and shallowness of bay

• Maximum spring tidal range in Minas Basin = 17 meters (56 feet)

Tidal extremes: The Bay of Fundy

The Bay of Fundy in Canada is known for its large tidal range (differences of over 50ft have been recorded!)

High tideLow tide

Photo: NASA

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Tidal patterns vary around the world

• Some places have one low tide and one high tide per day (diurnal tides)

• Other places have two high and low tides per day approximately equal in size (semidiurnal tides)

• If the two high and low tides of a semidiurnal tide are unequal in size, they are call mixed semidiurnal tides

• The US has examples of semidiurnal tides (East Coast), diurnal tides (some areas of the Gulf of Mexico) and mixed tides (Pacific Coast)

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Tide exercise: Can you identify the different types of tides?

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• You will be presented with three figures and corresponding questions

• See if you can determine which figures represent diurnal, semidiurnal and mixed semidiurnal tides.

Figure 1: Tide predictions for Dauphin Island, 3/1/11

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Water Level (ft)

Time

Figure 1 shows a diurnal tide: one high and one low tide

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Water Level (ft)

Time

First low tide~8am

First hightide~8pm

Figure 2: Tide predictions for Woods Hole, MA, 8/25/10

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Water Level (ft)

Time

Figure 2 shows a semidiurnal tide: two high and low tides of equal heights

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Water Level (ft)

Time

First low tide~4am

First high tide~9am

Secondhigh tide~9pm

Second low tide~4pm

Tidal range2ft

Tidal range2ft

Figure 3: Tide predictions for Seattle, WA (Puget Sound), 5/1/11

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Water Level (ft)

Time

Figure 3 shows a mixed semidiurnal tide: two high and low tides of unequal heights

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Water Level (ft)

Time

First high tide~4am

Second high tide~6pm

First low tide~11am

Secondlow tide~11pm

Tidal range10ft

Tidal range4ft