A CIESE Realtime Data Project

Tide Types

Can you determine a diurnal, semi diurnal or mixed tide?

Objectives
Students will be able to:

  • determine for a data source if a tide is a diurnal, semi-diurnal or a mixed tide
  • describe some of the causes of the different types of tides

Materials
Computers with Internet access
check these websites:

Rollover Pass, TX
Cape May, NJ

Graph paper for Rollover Pass (.pdf) Cape May (.pdf)
Student Worksheet

Background

The tides cycle with the 24-hour 50-minute lunar day. The lunar day is 50 minutes longer than a solar day because the moon revolves around the Earth in the same direction that the Earth rotates around its axis. So, it takes the Earth an extra 50 minutes to catch up to the moon. In most places, there are two high tides and two low tides every 24 hours and 50 minutes. This means that high tides occur 12 hours and 25 minutes apart. So for example, if the first high tide of the day occurs at 1:10 am, then next will occur at 2:00 pm. When the two highs and the two lows are about the same height, the pattern is called a semi-daily or semidiurnal tide.

If the Earth were a perfect sphere without large continents, all areas on the planet would experience two equally proportioned high and low tides every lunar day. The large continents on the planet, however, block the westward passage of the tidal bulges as the Earth rotates. Unable to move freely around the globe, these tides establish complex patterns within each ocean basin that often differ greatly from tidal patterns of adjacent ocean basins or other regions of the same ocean basin (Sumich, J.L., 1996).

Disruption of the water flow can cause areas to experience different heights between the high and low tides. When this pattern occurs, the pattern is called a mixed semidiurnal tide. For example, if the first high tide of the day occurs at 2:00 am, and is 1 ft, the second high tide of the day would occur at 2:50 pm, but may be 4 ft. The U.S. West Coast tends to have mixed semidiurnal tides.

Some areas, such as the Gulf of Mexico, have only one high and one low tide each day. This is called a diurnal tide. (Sumich, J.L., 1996; Thurman, H.V., 1994; Ross, D.A., 1995).

The shape of bays and estuaries also can magnify the intensity of tides. The Bay of Fundy in Nova Scotia is the classic example of this effect, and has the highest tides in the worldover 15 meters (Thurman, H.V., 1994). Narrow inlets and shallow water also tend to dissipate incoming tides.

Because tides can range in size and type, it has become extremely important to predict and measure tides. To measure the height of tides, it is necessary to figure out a starting point. The National Ocean Service (NOS) of the National Oceanic and Atmospheric Administration (NOAA) has been measuring tides for many years. They compile all of the information to determine tidal datums, used as a basis for measuring or calculating the heights of tides.

Tidal Datum Definitions
National Tidal Datum Epoch The specific 19-year period adopted by the National Ocean Service as the official time segment over which tide observations are taken and reduced to obtain mean values for tidal datums.

Tidal Datum Elevation
Mean Higher High Water (MHHW) The average of the higher high water height of each tidal day observed over the National Tidal Datum Epoch.
Mean High Water (MHW) The average of all of the high water heights of each tidal day observed over the NTDE.
   
Mean Sea Level (MSL) The arithmetic mean of hourly heights observed over the NTDE.
Mean Tide Level (MTL) The arithmetic mean of mean high water and mean low water.
   
Mean Low Water (MLW) The average of all of the low water heights of each tidal day observed over the NTDE.
Mean Lower Low Water (MLLW) The average of the lower low water height of each tidal day observed over the NTDE.

In this lesson, students will use real time data sets available from various tide monitoring stations. They will plot the water height information and determine if that area experiences diurnal, semi diurnal or mixed tides.

Procedure
1. Use the real time data available from the following tide monitoring stations. On the Student Worksheet, record the hourly reading for water height for the past 24 hours and determine if that location experiences diurnal, semi diurnal or mixed tides:

Rollover Pass, TX
Cape May, NJ

Note: Scroll to the bottom of the data page to find a definition for each data column

2. Based on the data collected, can you predict at what time the next high and low tide will occur for each location?

3. What was the predicted water height for the same time period? Record the information on the Student Worksheet.

Rollover Pass, TX
Cape May, NJ

Note: Scroll to the bottom of the data page to find a definition for each data column

4. Using a different color, plot the predicted water height for the past 24 hours on the same graph. Use your graph to answer the following questions.

5. Were there any differences between the predicted and actual water heights? Explain.

6. From the data you have collected, can you determine if tomorrow's predictions will hold true? Explain.

7. Why would it be important to know this information? Can you think of any vocations for which this information may be vital? Explain why.

8. What could some possible causes of the differences between predicted and observed height? List and explain.

Assessment
1. You are in charge of an upcoming Sandcastle Building event. Use the real time and predicted water heights for Galveston, TX and Atlantic City, NJ to determine the best time and location to hold the event. Explain your reasoning.