Tides

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• Lesson 10
• Tides
• Physical Oceanography

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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
Water Level (ft)
Time
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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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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 lt 1/20 Wavelength and deepwater waves
  have Depth gt 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!)

Photo NASA
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What causes tides?

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

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What causes tides?

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

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Different types of tide

• Spring tides occur when the sun and moon are in a
  straight line
• The tidal range is typically highest during
  spring tides

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Different types of tide
Earth

• 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 moons pull greater than the suns?

Sun
Moon
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Tides affect marine life

• Organisms that live near the shore have adapted
  to the rise and fall of the tide
• Well meet some of these creatures in todays
  activity!

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Tide exercise Can you identify the different
types of tides?

• 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!

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Figure 1 Tide predictions for Dauphin Island,
3/1/11
Water Level (ft)
Time
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Figure 1 shows a diurnal tide one high and one
low tide
First hightide8pm
Water Level (ft)
First low tide8am
Time
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Figure 2 Tide predictions for Woods Hole, MA,
8/25/10
Water Level (ft)
Time
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Figure 2 shows a semidiurnal tide two high and
low tides of equal heights
First high tide9am
Secondhigh tide9pm
Tidal range2ft
Tidal range2ft
Water Level (ft)
Second low tide4pm
First low tide4am
Time
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Figure 3 Tide predictions for Seattle, WA (Puget
Sound), 5/1/11
Water Level (ft)
Time
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Figure 3 shows a mixed semidiurnal tide two high
and low tides of unequal heights
Second high tide6pm
First high tide4am
Tidal range4ft
Tidal range10ft
Secondlow tide11pm
Water Level (ft)
First low tide11am
Time
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Student activity

• In todays 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