Understanding Tides

Tim Cook,Worcester State University, timothy.cook@worcester.edu
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In this activity students investigate tidal phenomena by exploring water level observational (or predicted tidal) data from several locations around the world that provide examples of semi-diurnal, diurnal, and mixed tides. Students are asked to identify patterns of variability and differences among the sites on time scales of just a few days and over a period of a couple months. The activity is designed more to get students thinking about tides, asking questions about the causes of tidal variations, and thinking about ways to answer these questions, as opposed to providing an explanation of tidal processes. The activity leads to a body of observations that generate numerous questions about tides. The goal is to capture student's interest before spending subsequent class time developing a conceptual/theoretical model of how tides work.

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This activity is used in a 200-level undergraduate oceanography course and is a required core course for Geography majors. The course is considered an intermediate level course in that students are required to have taken at least one of the following courses as a prerequisite: Physical Geography, Physical Geology, or Meteorology. While skills and concepts are geared towards students with some background in the geosciences and those that are expected to take additional courses in the major, the content is an introduction to ocean science.

Skills and concepts that students must have mastered

This activity is designed as an introduction to the subject of tides, and as such, no background understanding is expected. However, the activity does start by asking students questions about the origin of tides in order to identify any existing assumptions or misconceptions they might have and to get them thinking about ways to investigate the origin of tides in more detail. Knowing that tides exist is useful, but not required.

How the activity is situated in the course

This activity is used to introduce the subject of tides. It takes the form a think-pair-share activity where students think about questions individually, discuss ideas with their neighbors, and eventually leads to a group discussion. This process is repeated multiple times through the course of the activity. The activity itself is designed more to raise interest in the subject and generate questions about tides. Subsequent class time is then used to fill in the details of a conceptual/theoretical model of tides.


Content/concepts goals for this activity

The goal of this activity is for students to recognize patterns in tidal data that reflect diurnal, semi-diurnal, and mixed tides, fortnightly (spring-neap) tidal variations, define the periodicity of tidal variations and identify both spatial and temporal differences in tidal period and tidal range.

Higher order thinking skills goals for this activity

Students must interpret time-series observations of water level variations and recognize patterns. One of the primary goals of this activity is to force students to explore graphical representations of time-series data in detail. Students are asked to develop questions that arise from their observations and to propose a means of investigating these questions (typically through comparison of water level data with other data sets).

Other skills goals for this activity

The activity is also designed to introduce students to the availability of tidal data through the NOAA Tides and Currents website. This aspect of the activity may be emphasized by asking to students to go online and obtain their own versions of the graphs which are included the activity posted here.

Description and Teaching Materials

This activity includes two handouts. The first is a short questionnaire to get students thinking about tides and generating questions which lead to more in-depth investigation of the topic. The second handout provides tidal data that students use as the basis of their investigation. A series of questions are included to help guide students observations. The ultimate goal is for students to generate a description of various tidal phenomena and to generate questions regarding the origin of these phenomena. The explanation of these phenomena comes from subsequent class discussions and lecture.

Teaching Notes and Tips

This activity starts by having students complete a short questionnaire that explores their existing understanding of tides. Most students have at least some idea about the existence of tides from having visited a coastal region and many have some idea about the connection between tides and the moon. When pressed to think about some of the details about tides, most students then realize that whatever existing ideas they have about tides cannot adequately explain all of the details. When asked to think about ways to investigate the cause of tides students generally come to the idea of collecting observations of water level variations and comparing those variations with some sort of astronomical data. Once the class discussion reaches that point the second handout is distributed to students which has a set of questions designed to focus students observations toward some of the key characteristics of tidal variations. Students generally work in small groups to explore the various tidal records that are provided, answer the specified questions, and generate their own list of questions that have evolved from their observations. Subsequent class time is then used to fill in the details and provide a conceptual/theoretical model that explains the observed tidal phenomena in the various records.

I use tidal predictions, as opposed to actual observations in this activity, simply due to the accessibility of the data, but the activity could be easily modified to utilize actual observations. I typically bring display a record of tidal observations that is compared with predicted water level variations to demonstrate the accuracy of tidal predictions and also as a means for discussing causes of non-tidal variations in water level.

Given adequate time I prefer to have students access and download tidal records directly for specified locations and time periods (as opposed to just including the graphs as done here) as I like to continuously emphasize the availability of oceanographic data to students and get them used to the idea of utilizing existing observational data to investigate ocean processes.


Students should be able to correctly define the tidal period, describe the differences in the pattern between locations, and identify the fortnightly pattern of tidal variation in the 2 month data sets.

Students are expected to come up with questions along the lines of the following:
Why are there two tides per day in some locations? Why is the tidal period not exactly 12 or 24 hours in length? Why does the tidal range vary from day to day and from week to week and from one location to another?

References and Resources

NOAA Tides and Currents website: https://tidesandcurrents.noaa.gov/

Data used in this activity are derived from the NOAA Tides & Currents website. Additional background information on tidal processes is also available from this site.