TIDES

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TIDES

• periodic, short term changes in the height of the
  ocean surface at a particular place

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What causes tides?

• a combination of the gravitational force of the
  moon and sun and the rotation of the earth
• moon exerts greater effect than the sun

3
Tides are waves

• longest waves with wavelengths of half the
  Earths circumference
• High tide is the crest of the wave and low tide
  is the trough

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Equilibrium Theory of Tides

• Isaac Newton, 1600s
• Would accurately describe tides on a planet
  uniformly covered with water
• Does not consider the effects of continents,
  ocean basin shape and ocean depth
• Explains the effects of the gravitational
  attractions of Earth, moon and sun

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Fig. 10-2, p. 229
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Fig. 10-2, p. 229
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Fig. 10-2, p. 229
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Fig. 10-3, p. 230
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Fig. 10-4, p. 230
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Fig. 10-5, p. 230
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Fig. 10-6, p. 231
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How tides occur...

• The bulges stayed aligned with the moon as the
  earth rotates on its axis,
• as points on the earth pass through the bulges,
  they experience high tide

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Fig. 10-7a, p. 231
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Fig. 10-7b, p. 231
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Other factors affecting tides...

• the moon rises 50 min. later each day
• high tides occur about 50 min later each day.

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Figure 9.8
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Other factors affecting tides...

• the moon moves each month from a position 28.5o
  above to 28.5o below the equator
• leads to unequal tides a high high and a lower
  high that same day.

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Fig. 10-9, p. 232
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Fig. 10-10, p. 233
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Figure 9.14
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Influence of the Sun

• only about 46 of that of the moon due to the
  large distance b/w the earth sun

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Influence of the Sun

• The effects of the sun and moon cause wave
  interference
• constructive interferencehigher high tides and
  lower low tides spring tides
• Maximum tidal range
• destructive interferencelower high tides and
  higher low tides neap tides
• Minimum tidal range

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Figure 9.11
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Figure 9.10
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Fig. 10-12, p. 234
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Dynamic Theory of tides

• Laplace, 1775
• considers ocean depth, continents, the shape of
  the basin

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Tidal regimes

• Semidiurnal tides two high tides and two low
  tides of nearly equal level each lunar day
• Diurnal tides one high and one low tide each
  lunar day
• Mixed tides when successive high or low tides
  are significantly different in height

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Figure 9.16
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Tidal currents

• Rise and fall in sea-level as the tidal crest
  passes will cause currents
• Water rushing toward shore due to rising
  sea-level is called a flood current
• Water rushing away from shore is called an ebb
  current
• Slack water a time of no current occurs at high
  or low tide

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Figure 9.19
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Figure 9.18 Bay of Fundy Largest Tidal Range
Extreme Tides Max tidal range about 17m or 56 ft
at northern end of Minas Basin