Tides

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Tides

• Chap 11

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Tides

• Tides in the eastern Bay of Fundy on the Atlantic
  coast of Canada. (f.11.31)
• Tidal range is 15 meters (50 feet).
• Water rises 1 meter (3.3 ft.) in 23 minutes.

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• The tides are the regular rising and falling of
  sea level.
• They are caused by the gravitational attraction
  of the moon and the sun.
• These celestial bodies pull on the Earth.
• The solid Earth moves only a tad toward the moon
  and sun, but the liquid ocean moves more.

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• Let's look at the effect of the moon, first.
• It has a much stronger effect on the tides
  because it is so much closer
• The pull of the moon causes the ocean to 'bulge'
  out away from Earth and toward the moon

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• The water level is also low along the equator in
  the direction toward you and away from you.
• Where the bulge occurs is high tide where the
  low points occur (toward you and away from you)
  is low tide.
• The Earth rotates every 24 hours, so points on
  Earth rotate into the high tide, then into the
  low tide.

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• Now here comes one of the many bizarre aspects of
  the tides there is also a tidal bulge on the
  opposite side of the planet
• How is this possible? Centrifugal force.
• Swing an object attached to a string in a circle.
  
• Your hand is pulling the object toward you, but
  another force must be pushing the object away
  from you! That force is centrifugal force
  (centrifugal means 'fleeing the center').

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• The tide is a standing wave it has two crests
  (high tide), and two troughs (low tide).
  (f.11.24)
• The Earth rotates once every 24 hours.

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• If you were to plot sea level over this 24 hour
  period, it would look like, f.11.25.

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• Such a tide is called semidiurnal, because there
  are 2 high tides and 2 low tides every day.
• The difference in height between high tide and
  low tide is called the tidal range.

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• How high the high tide gets, and how low the low
  tide gets, depends on the other celestial body we
  have ignored up until now the sun.
• The effects of the sun are to raise high tides to
  a maximum, when the sun's gravity pulls in the
  same direction as the moon's, and to lower high
  tides to a minimum, when the sun's gravity pulls
  in opposition to the moon's.

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Three types of tides

1. Semidiurnal two high tides and two low tides
   each day, both about the same height
2. Mixed two high tides and two low tides each day,
   but one high tide is higher than the other
3. Diurnal one high tide and one low tide each day.

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Tidal Bore

• In macrotidal areas, the high tide may come in as
  a visible wave called the tidal bore.
• A tide crest encounter a narrow river passage,
  and it rushes forward to cause the bore (f.11.32)

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Tidal Currents

• When a point on Earth passes through the tidal
  crest, high tide is coming, and water moves
  onshore.
• It may move up rivers as a visible bore, or it
  may just rise almost imperceptibly.
• Either way, the water moves onshore and this is
  called the flood tide or flood current.

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• At the center of the crest, water stops moving
  onshore and stands still for a little while.
• This is a stage of the tide called slack water or
  slack tide.
• Now the point rotates into the trough of the
  tide, and low tide is coming.
• Water changes direction now and moves offshore,
  back out to sea.

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• This is called the ebb tide or ebb current.
• Once the center of the trough is reached, there
  is no more water movement offshore and the water
  lingers again.
• This is also called slack water or slack tide.