Tides and Currents: Motion in the Ocean

Subject: Earth Science
Grade Level: 9–12

Lesson Goal

Students will explore what causes ocean currents and waves.

Learning Objectives

By the end of this lesson, students will be able to:

• Identify the main causes of ocean currents and waves.
• Explain how and why ocean currents differ depending on latitude (distance from the equator).
• Explain the cause of the Coriolis Effect — the apparent curving of moving objects, like water or wind, caused by Earth's rotation — and describe how it influences ocean currents.
• Calculate the speed of ocean currents using data collected from drifter studies (devices that float with the current to track its movement).

Background: Why This Matters

More than 98% of all cargo shipped into and out of the United States travels by water. Safely navigating coastal waters requires more than accurate maps — sailors also need up-to-the-minute information about weather and sea conditions, which can change quickly and vary widely from place to place. Without accurate, real-time data, ships risk running aground or colliding with one another.

NOAA's Center for Operational Oceanographic Products and Services (CO-OPS) exists to prevent these problems. This center collects and shares information about water levels, coastal currents, and tides to keep maritime transportation safe, efficient, and environmentally responsible. Specifically, CO-OPS:

• Provides data on water levels, coastal currents, and tides.
• Sets standards for how this data should be collected and processed.
• Researches new and improved ways to observe ocean conditions.
• Designs software to better process this data.
• Regularly checks and verifies the quality of the data.
• Shares all of this information with the public.

CO-OPS also runs a nationwide network called PORTS® (Physical Oceanographic Real-Time Systems), installed in major U.S. harbors. PORTS® delivers real-time data — including water levels, currents, "air gap" (the space between the water's surface and the underside of a bridge), and weather conditions — helping ships avoid accidents.

While CO-OPS focuses mainly on coastal and estuary currents (estuaries are areas where rivers meet the sea), other NOAA offices study currents in the open ocean. NOAA's National Oceanographic Data Center (NODC) gathers data from current meters and drifters used in ocean research. Through NODC's website, you can access datasets covering beach temperatures, buoy readings, global temperature and salinity levels, and more. For current data gathered by satellites, NOAA also offers an Ocean Surface Current Analyses website.

In this lesson, students will investigate how currents, winds, and ocean waves are connected.

Procedures

1. Prepare for the lesson by reviewing background readings on tides and currents, a "Currents Subject Review," and a worksheet titled "Problems on Winds, Waves, and Currents." An optional tutorial on tides and water levels is also available, along with additional lesson plans for grades 9–12.

2. Start a class discussion about why marine navigation still matters today. Even with air travel and space technology, oceans remain essential for shipping goods, producing energy, and recreation. Talk about why real-time information is critical for safe navigation, and ask students to brainstorm what kinds of data a sailor today would find useful. Explain that their task is to learn the basics of "ocean motion" and apply that knowledge to solve problems involving wind, waves, and currents.

3. Have students complete the Currents Subject Review, either individually or in groups (you might assign different sections to different groups). This can be done as homework to save class time. Afterward, discuss their answers and highlight why understanding tides is useful in real life.

4. Distribute the "Problems on Winds, Waves, and Currents" worksheet and have students complete it.

5. Discuss the worksheet answers as a class, making sure students grasp the connections between wind, waves, and currents.

Worksheet Answer Key

1. 3 feet
2. Increasing wind speed by 60 knots raises wave height to about 12 feet, while increasing the fetch (the distance the wind travels over water) by 60 nautical miles raises it to less than 6 feet. Wind speed has the bigger effect.
3. A 60-knot wind would need to blow over roughly 9 miles of fetch to create a 10-foot wave.
4. The distance between the two points is 524.6 nautical miles. The total time elapsed is 6 days, 10.25 hours (154.25 hours). Dividing distance by time:

524.6 nm ÷ 154.25 hr = 3.40 knots, heading northeast.

1. The distance between the two points is 1,443.68 kilometers (1.444 × 10⁸ cm). The total time elapsed is 14 days, 2.92 hours (1.220 × 10⁶ seconds). Dividing distance by time:

1.444 × 10⁸ cm ÷ 1.220 × 10⁶ sec = 118.4 cm/sec, heading slightly east of due south.

1. Since the latitude at the equator is 0°, the Coriolis acceleration formula shows that the Coriolis effect is zero at the equator.
2. Tijuana sits at about 32.5° N latitude. A speed of 10 meters/second equals 1,000 centimeters/second. Plugging into the formula:

(sin 32.5° × 1.5 × 10⁻⁴ × 1,000) = 0.537 × 1.5 × 10⁻⁴ × 1,000 = 0.081 cm/sec² — an extremely small effect.

1. Even though the Coriolis effect is too tiny to notice on small scales — like a soccer ball or a walking person — it becomes significant when acting on enormous masses of water or air over very long distances, like ocean currents.

Extension Activities

• Personal connection assignment: Have students write a short essay explaining how the Coriolis force affects them personally, even though its effects are only noticeable on very large scales.
• Explore additional lesson plans and resources on tides, currents, and marine science through NOAA's education resources and university-hosted map libraries.
• Try interactive multimedia lessons on ocean currents and waves, including an activity where students must account for the Coriolis Effect to safely land a plane on an aircraft carrier.

Assessment

Students will complete the worksheet, demonstrating their understanding of how wind speed and fetch affect wave height, how to calculate current speed and direction from drifter data, and how the Coriolis Effect works mathematically and physically.