High or Low - What Causes Tides?

Last lesson we learned about waves How do you differentiate between a deepwater and shallow-water wave? What is the period of a wave? 2

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What Are Tides? Tides are the daily rise and fall of Earth’s waters on its coastlines. As the tide comes in, the level of water on the beach rises, and as the tide goes out, the level of water on the beach goes down. Tides occur in all bodies of water, but they are most noticeable in the ocean and large lakes.

High Tides High tides are when the water reaches its highest point.

Low Tides Low tides are when the water reaches its lowest point.

Figure 1: Tide predictions for Dauphin Island, 3/1/11 7 Water Level (ft) Time

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 8

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 1/2 Wavelength The ocean would have to be deeper than 5,250 miles for an ocean tide to be a deepwater wave! 9

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) 10

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!) High tide Low tide Photo: NASA 11

What Causes Tides? Tides are caused by the interaction of Earth, the Moon, and the Sun. Gravity is the reason for tides. Gravity is the force exerted by an object that pulls other objects toward it.

Moon’s Gravity and Tides The Moon’s gravity affects the water on Earth’s surface. Since the Moon is close to the Earth, it has a strong gravitational pull on it (closer objects have stronger gravitational pull).

“Bulge” of Earth What causes tides? The gravitational pull of the sun and the moon causes “bulges” on Earth that move as we rotate Sun Earth Moon Gravitational pull 14

What causes tides? The tide rises when coastline enters the bulge and falls when it rotates out High tide 15

Moon’s Gravity – Tidal Bulges The Moon pulls on the water on the side nearest to it more strongly than it pulls on the center of the Earth. This pull creates a bulge of water, called a tide bulge, on the side of Earth facing the the Moon.

Moon’s Gravity – Tidal Bulges The water on the side of Earth facing away from the Moon has a less strong pull. This water is “left behind” and forms a second bulge. As Earth rotates, different places on the planet’s surface pass through the areas of the tidal bulges and have the change in water levels.

Tidal Bulges – High Tide In places where there are tidal bulges, high tide is occurring along the coastlines. High Tide High Tide

Tidal Bulges – Low Tide In places between the bulges, low tide is occurring. LOW TIDE

Sun’s Gravity and Tides The Sun is so large that its gravity also affects tides. At times, the Sun and Moon pull together on Earth’s waters in the same direction. At other times they pull in different directions.

Daily Tide Cycle Most seashores have four tides every day – two high tides and two low tides. A change of from low to high tide or vice versa takes about 6 hours and 12 minutes. Changes in tides can be drastic (can notice) or or less dramatic (can’t notice).

Monthly Tide Cycle Changes in the positions of Earth, the Moon, and Sun affect the height of tides during a month.

Spring Tides Spring tides occur 2 times a month, during a full and new moon when the Earth, Sun, and Moon are lined up. Spring tides are higher and lower than normal tides. “strong tides”

Different types of tide Sun Earth Moon Spring tides occur when the sun and moon are in a straight line The tidal range is typically highest during spring tides 24

Neap Tides Neap tides occur in between spring tides, at the first and third quarters of the Moon when the Sun and Moon pull at right angles to each other. Neap tides are not as high or low as normal tides. “weak tides”

Different types of tide 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? Earth Moon Sun 26

Tides affect marine life Organisms that live near the shore have adapted to the rise and fall of the tide We’ll meet some of these creatures in today’s activity! 28

Tide exercise: Can you identify the different types of tides? 29 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!

Figure 1: Tide predictions for Dauphin Island, 3/1/11 30 Water Level (ft) Time

Figure 1 shows a diurnal tide: one high and one low tide 31 Water Level (ft) Time First low tide ~8am First high tide ~8pm

Figure 2: Tide predictions for Woods Hole, MA, 8/25/10 32 Water Level (ft) Time

Figure 2 shows a semidiurnal tide: two high and low tides of equal heights 33 Water Level (ft) Time First low tide ~4am First high tide ~9am Second high tide ~9pm Second low tide ~4pm Tidal range 2ft

Figure 3: Tide predictions for Seattle, WA (Puget Sound), 5/1/11 34 Water Level (ft) Time

Figure 3 shows a mixed semidiurnal tide: two high and low tides of unequal heights 35 Water Level (ft) Time First high tide ~4am Second high tide ~6pm First low tide ~11am Second low tide ~11pm Tidal range 10ft Tidal range 4ft

Student activity In today’s 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 36

Review Tide Song 37