Lesson 10: Tides
Physical Oceanography
Last lesson we learned about waves
How do you differentiate between a deepwater and shallow-water wave?
What is the period of a wave?
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Figure 1: Tide predictions for Dauphin Island, 3/1/11
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Water Level (ft)
Time
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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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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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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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
Ph
oto
: N
AS
A
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“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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What causes tides?
The tide rises when coastline enters the bulge and falls when it rotates out
High tide
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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 tides10
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 affect marine life
Organisms that live near the shore have adapted to the rise and fall of the tide
We’ll meet some of these creatures in today’s activity!
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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.
Good luck!
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
Student activity
In today’s activity, you will meet some marine organisms that live in the intertidal zone: the area between the low tide water mark and high tide water mark
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