Created by Michael Kramer
What is a Tide?
 Tides are periodic, short term changes in height of the
ocean surface at a particular place, caused by a
combination of the gravitational force of the moon
and un and the motion of Earth.
 The wavelength of an average tide can be up to 17,500
km
 They are the longest of all ocean waves
 They are also called forced waves
Why are Tides Important?
 They’re in the ocean, which means it’s in oceans bowl,
and oceans bowl is very important!
 Actually, tides play an important role in the life cycle of
many organisms
 Gives information necessary for underwater military
engineer use
 Helps the construction of bridges, docks, and
breakwaters
Characteristics of Tides
 Crest of the wave from is high tide and the trough is
low tide
 The vertical difference between high tide and low tide
is the tidal range
 The tidal period is the time between consecutive
high/low tides
More Characteristics
 There are three basic daily tides
 1. Semidiurnal tides- Two high tides and two low
tides of nearly equal level each lunar day.
 2. Diurnal tides- One high and one low
 3. Mixed (also known as semidiurnal mixed)successive high tides or low tides are of significantly
different heights throughout the cycle. This is caused
by blending diurnal and semidiurnal tides.
 In the United States, Mixed tides are prevalent in the
West Coast, Diurnal in the Gulf, and Semidiurnal in
the East Coast
Diagrams of the Tides
Tides around the world
Ok that’s cool, but what makes
tides?
 Tides result from gravitational attraction and the centrifugal effect
 What’s the centrifugal effect?
 It’s when Gravity tends to pull Earth and the moon toward each other,
but inertia keeps them apart.
 Another way of saying what causes tides is that the gravitational pull of
the sun and moon causes “bulges” on Earth that move as we rotate
Gravitational pull
Sun
Moon
Earth
More diagrams
Origin of Tides
 The latitude of the tidal bulge is determined by the
declination, the angle between Earth’s axis and the lunar
and solar orbital plane.
 A spring tide occurs when the sun, moon, and Earth are
all in a straight line. During spring tides, high tides are very
very high and low tides are very very low. These usually
occur at two-week intervals corresponding to the new and
full moons. Spring tides go with the Full and New Moon.
 A neap tide occurs when the sun, moon, and Earth are
aligned in a right angle. During neap tides, high tides aren’t
that high and low tides aren’t that low. These also occur at
two-week intervals. Neap tides go with the 1st and 3rd
quarter of the moon.
Diagrams of Spring and Neap Tides
Equilibrium Theory of Tides
 The equilibrium theory of tides is an idealized model of
tides that considers Earth to be covered by an ocean of
great and uniform depth capable of instantaneous response
to the gravitational and inertial forces of the sun and
moon.
 This theory assumes that the seafloor does not influence
the tides and that the ocean conforms instantly to the
forces that affect the position of its surface.
 It also treats tides as shallow-water waves. As Earth turns,
landmasses divert, slow, and complicate movements of
tidal crests.
Amphidromic Point
 The node near the center of an ocean basin is called the
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amphidromic point.
This is a no-tide point in the ocean, around which the tidal crest
rotates through one tidal cycle.
The crests sweep around aphidromic points like a wheel spokes
from a rotating hub.
The Coriolis effect influence these tide waves because a large
volume of water moves with waves.
The amphidromic point moves counter-clockwise in the Northern
Hemisphere and clockwise in the Southern Hemisphere.
The height of tides increase with distance from an amphidromic
point.
There are about a dozen of these points in the world, with five in the
Pacific.
Tidal Datum
 The tidal datum is the reference level to which tidal
height is compared to, similar to an index fossil.
 The tidal datum is the zero point seen in tide graphs.
 This reference is not always set at mean sea level,
which is the height of the ocean surface averaged over
a few years’ time.
 On coasts with mixed tides, the zero tide level is the
average level of the lower of the two daily tides. On
coasts with diurnal and semidiurnal tides, the zero
tide level is the average level of all low tides.
Tidal Range
 The tidal range is the high-water to low-water height
difference that varies with basin configuration. In
small areas such as lakes, the tidal range is small.
 In larger areas, like the Baltic and Mediterranean seas,
the tidal range is moderate. However, the tidal range is
not the same over a whole ocean basin.
 The largest tidal ranges are at the edges of largest
ocean basins, like bays or inlets.
Tidal Currents
 A tidal current is a mass flow of water induced by the
raising or lowering of sea level owing to passage of tidal
crests or troughs.
 The rise or fall in sea level as a tide crest approaches and
passes will cause a tidal current of water to flow into or out
of bays and harbors.
 Water rushing into an enclosed area because of the rise in
sea level as a tide crest approaches is a flood current.
Water rushing out due to the fall as the tide trough
approaches is called an ebb current.
 Slack water, a time of no currents, occurs at high and low
tides when the current changes direction.
Solar and Lunar Tides
 Solar tides are caused by the gravitational and inertial
interaction of the sun and Earth.
 The sun’s tractive forces develop in the same way as the
moon’s, and the smaller solar bulges tend to follow the
sun through the day.
 Lunar tides are caused by gravitational and inertial
interaction of the moon and Earth, and complete their
cycle in a tidal day (known as a lunar day).
 A complete tidal day is 24 hours and 50 minutes.
Ocean Bowl Quiz Time!
Short Answer:
The bulges are the troughs of the planet-sized waves that
cause what type of tides?
 Answer: Low Tides
 Multiple Choice:
 Semidiurnal Tides are most commonly found in what
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part of the United States?
W: West Coast
X: Gulf of Mexico
Y: East Coast
Z: Semidiurnal Tides are not found in the US
 Multiple Choice:
 Semidiurnal Tides are most commonly found in what
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part of the United States?
W: West Coast
X: Gulf of Mexico
Y: East Coast
Z: Semidiurnal Tides are not found in the US
 Multiple Choice
 Each time a tidal current changes directions there is a
period of no current known as…?
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W. Flood current
X. Ebb current
Y. Reversed current
Z. Slack current
 Multiple Choice
 Each time a tidal current changes directions there is a
period of no current known as…?
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W. Flood current
X. Ebb current
Y. Reversed current
Z. Slack current
 Short Answer
 What explains many characteristics of ocean tides by
examining the balance and effects of the forces that
allow a planet to stay in a stable orbit around the sun,
or the moon to orbit the Earth?
 Answer: Equilibrium Theory of Tides
 Multiple Choice
 The highest and lowest tides which occur when the
moon and sun are lined up with the Earth are
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W. Neap tides
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X. Winter tides
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Y. Spring tides
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Z. Summer tides
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 ANSWER: Y
 Multiple Choice
 Which of the following best describes the impact of
freshwater flow into an estuary?
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W. The speed of flood tidal currents increases
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X. Ebb tide is prolonged
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Y. Flood tide is prolonged
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Z. The tidal range is increased
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 ANSWER: X
GOOD JOB!!!