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TIDES. 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?

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what is a tide
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!
slide3

Tides are generated by:

  • Gravitational pull of the moon and sun
  • Centripetal force of the rotating Earth
slide4

Tides are generated by:

  • the gravitational pull of the moon and sun
  • - moon has 2x greater gravitational pull than the
  • sun
  • - sun is 10 million x more massive than the
  • moon and is 390 times farther away
what causes tides

Gravitational pull

“Bulge”

of Earth

Sun

Moon

Earth

What causes tides?

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

slide7

GRAVITATIONAL FORCE

CENTRIPETAL

GRAVITATIONAL & CENTRIPETAL

what causes tides1

High tide

What causes tides?

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

slide9

Description of tides

  • High water: a water level maximum ("high tide")
  • Low water: a water level minimum ("low tide")
  • Tidal range: the difference between high and low tide

Intertidal zone

High tide

Low tide

slide10

Alma at High Tide

Alma at Low Tide

earth moon sun positions and the monthly tidal cycle
Earth-Moon-Sun positions and the monthly tidal cycle

Spring Tide

Highest high tide and lowest low tide

Neap Tide

Moderate tidal range

different types of tide

Sun

Moon

Earth

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
different types of tide1
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?

Different types of tide

Earth

Sun

Moon

tides are also important physical forces in our ocean
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
slide17

Tidal Range

56 ft

6 ft

the bay of fundy site of the world s largest tidal range
The Bay of Fundy: Site of the world’s largest tidal range
  • Tidal energy is focused by shape and shallowness of bay
  • Maximum spring tidal range in Minas Basin = 17 meters (56 feet)
tidal extremes the bay of fundy

Low tide

High tide

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

tidal patterns vary around the world
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)
tide exercise can you identify the different types of tides
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.
figure 1 shows a diurnal tide one high and one low tide
Figure 1 shows a diurnal tide: one high and one low tide

First hightide~8pm

Water Level (ft)

First low tide~8am

Time

figure 2 shows a semidiurnal tide two high and low tides of equal heights
Figure 2 shows a semidiurnal tide: two high and low tides of equal heights

First high tide~9am

Secondhigh tide~9pm

Tidal range2ft

Tidal range2ft

Water Level (ft)

Second low tide~4pm

First low tide~4am

Time

figure 3 shows a mixed semidiurnal tide two high and low tides of unequal heights
Figure 3 shows a mixed semidiurnal tide: two high and low tides of unequal heights

Second high tide~6pm

First high tide~4am

Tidal range4ft

Tidal range10ft

Secondlow tide~11pm

Water Level (ft)

First low tide~11am

Time

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