1. Lesson 10: Tides Physical Oceanography

2. Last lesson we learned about waves
How do you differentiate between a deepwater and shallow-water wave?
What is the period of a wave?
Teacher’s Note:
1. Deepwater waves – wave depth is greater than ½ its wavelength
Shallow-water waves – wave depth is about 1/20th of its wavelength or less
2. Period (T) refers to the time it takes for the same spot on two consecutive waves to pass the same point.

3. Figure 1: Tide predictions for Dauphin Island, 3/1/11
Water Level (ft)
Figure 1: Shows tidal predictions for Dauphin Island, AL 3/1/11 Local Standard Time (LST) – Data comes from station (
Y-axis shows water levels in feet and x-axis shows time in hourly increments over the course of a day.
Time

4. 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

5. 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!

6. 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)
For complete technical accuracy, the type of day referred to in the definitions for diurnal, semidiurnal, and mixed (semidiurnal) tides is a lunar day (also referred to as a tidal day). A lunar day is 24 hours and 50 minutes compared to the typical solar day which is 24 hours. It refers to the time required for a spot on earth to make a full rotation from a specific location under the moon to the same location (under the moon). For a more in depth explanation of a lunar day, see (Accessed, May 2011).

7. 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!)
Low tide
High tide
Photo: NASA
Photo:
Accessed: November 2010

8. What causes tides?
The gravitational pull of the sun and the moon causes “bulges” on Earth that move as we rotate
Gravitational pull
“Bulge”
of Earth
Sun
Earth
Moon
Teacher’s Note: The slide labels the gravitational pull and bulge of the Earth as you advance the slide.

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

10. 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

11. Different types of tide
Earth
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?
Teacher’s Note: Even though the sun is larger than the moon, the moon’s pull is always greater because the moon is much closer to the earth.
Sun
Moon

12. 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!

13. 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.
Good luck!

14. Figure 1: Tide predictions for Dauphin Island, 3/1/11
Water Level (ft)
Figure 1: Shows tidal predictions for Dauphin Island, AL 3/1/11 Local Standard Time (LST) – Data comes from station (
Y-axis shows water levels in feet and x-axis shows time in hourly increments over the course of a day
What time does the first low tide occur? (8:00 a.m.)
What time does the first high tide occur? (8:00 p.m.)
How many high tides and low tides occur? (one high tide and one low tide)
What type of tide do you think this is? (Diurnal)
Time

15. Figure 1 shows a diurnal tide: one high and one low tide
First high tide ~8pm
Water Level (ft)
Figure 1: Shows tidal predictions for Dauphin Island, AL 3/1/11 Local Standard Time (LST) – Data comes from station (
Y-axis shows water levels in feet and x-axis shows time in hourly increments over the course of a day
What time does the first low tide occur? (8:00 a.m.)
What time does the first high tide occur? (8:00 p.m.)
How many high tides and low tides occur? (one high tide and one low tide)
What type of tide do you think this is? (Diurnal)
First low tide ~8am
Time

16. Figure 2: Tide predictions for Woods Hole, MA, 8/25/10
Water Level (ft)
Figure 2: Shows tidal predictions for Woods Hole, MA 8/25/10 Local Standard Time (LST) – Data comes from station (
Y-axis shows water levels in feet and x-axis shows time in hourly increments over the course of a day
How many high tides are present? (2)
What times do they occur? (~9:00 a.m. and ~9:00 p.m.)
How many low tides are present? (2)
What times do they occur? (4:00 a.m. and 4:00 p.m.)
What is the tidal range (distance between high and low tide) for first high and low tide (round to nearest foot)? (2 feet)
What is the tidal range for the second high and low tide (round to nearest foot)? (2 feet)
What type of tide is this? (Semidiurnal – 2 high and low tides of about equal heights)
Time

17. Figure 2 shows a semidiurnal tide: two high and low tides of equal heights
First high tide ~9am
Second high tide ~9pm
Tidal range 2ft
Tidal range 2ft
Water Level (ft)
Figure 2: Shows tidal predictions for Woods Hole, MA 8/25/10 Local Standard Time (LST) – Data comes from station (
Y-axis shows water levels in feet and x-axis shows time in hourly increments over the course of a day
How many high tides are present? (2)
What times do they occur? (~9:00 a.m. and ~9:00 p.m.)
How many low tides are present? (2)
What times do they occur? (4:00 a.m. and 4:00 p.m.)
What is the tidal range (distance between high and low tide) for first high and low tide (round to nearest foot)? (2 feet)
What is the tidal range for the second high and low tide (round to nearest foot)? (2 feet)
What type of tide is this? (Semidiurnal – 2 high and low tides of about equal heights)
Second low tide ~4pm
First low tide ~4am
Time

18. Figure 3: Tide predictions for Seattle, WA (Puget Sound), 5/1/11
Water Level (ft)
Figure 3: Shows tidal predictions for Seattle, Washington (Puget Sound) 5/1/11 Local Standard Time (LST) – Data comes from station (
Y-axis shows water levels in feet and x-axis shows time hourly increments over the course of a day
How many high tides are present? (2)
What times do they occur? (~4:00 a.m. and ~6:00 p.m.)
How many low tides are present? (2)
What times do they occur? (11:00 a.m. and 11:00 p.m.)
What is the tidal range (distance between high and low tide) for first high and low tide (round to nearest foot)? (10 feet)
What is the tidal range for the second high and low tide (round to nearest foot)? (4 feet)
What type of tide is this? (Mixed Semidiurnal – 2 high and low tides of unequal heights)
Time

19. Figure 3 shows a mixed semidiurnal tide: two high and low tides of unequal heights
Second high tide ~6pm
First high tide ~4am
Tidal range 4ft
Tidal range 10ft
Second low tide ~11pm
Water Level (ft)
Figure 3: Shows tidal predictions for Seattle, Washington (Puget Sound) 5/1/11 Local Standard Time (LST) – Data comes from station (
Y-axis shows water levels in feet and x-axis shows time hourly increments over the course of a day
How many high tides are present? (2)
What times do they occur? (~4:00 a.m. and ~6:00 p.m.)
How many low tides are present? (2)
What times do they occur? (11:00 a.m. and 11:00 p.m.)
What is the tidal range (distance between high and low tide) for first high and low tide (round to nearest foot)? (10 feet)
What is the tidal range for the second high and low tide (round to nearest foot)? (4 feet)
What type of tide is this? (Mixed Semidiurnal – 2 high and low tides of unequal heights)
First low tide ~11am
Time

20. 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