tides

TIDES!

What causes the tides? Can they be predicted? To begin discovering the answers to these questions, let's collect and analyze data on tides. (Instructors: see notes on the Tides Main Page.)

This page guides you through steps in collecting your own, real-world data on tides. If you work with several other people, each person (or pair) collecting data from a different place in the world, you can compare and contrast your results with each other.

The steps outlined below refer to MSWord and Excel, but you probably can use whatever word processing and spreadsheet software you're used to.

Warning! This is not a pre-packaged set of numbers that have been cleaned up to make sure they work. (What fun is that?!) This is real science, using real-world data, so it's going to be messy. And who knows? You just might discover something new!

STEP 1: Collect tide data.
Go to the NOAA site that has tide data. Under "Verified / Historical Water Level Data" click on "US and Global Coastal Stations". Select a station. Hint: Not every station has data that can be displayed easily. In fact, most do not. To find the best ones, select a station and then click "View Plot". If you get an error message, select a different station and try again. If you get a graph, great! That means you've picked a good station and should be able to find data for it. If you'd like to work with a data set that has already been chosen for you, click here.
Once you find a station that works, click "Back" on your browser to go back to the "Retrieve Verified / Historic (Tides) Water Level Data" page.
Now take these steps:
* Enter beginning and ending dates--ask it for no more than a year's worth of data at a time, as it seems to get overwhelmed easily. Note that some stations may have been operating for only a short period of time, or may not be in service at present, so if you get an error message, that could be the reason. Just change the dates and try again. Try starting with one month and then gradually expanding the time period to see what works.
* Make sure the time interval is what you want. If you're not sure, just leave it at the default of "Hourly Heights".
* If you'd like to get a general idea of what the graph will look like, click "View Plot". Otherwise, go directly to the next step.
* Click "View Data".
When you get a table of data, copy and paste it into an MSWord file. Click "File" and "Save As", give the title, and then (this is important!) select file type "Text file".
Close the file. Start Excel, and find the file you just closed (note that you'll have to choose file type "All Files"). Open it. The Text Wizard should begin. Click "Delimited", then "Next", then click "Space", then "Next", then "Finished". You now have an Excel file of hourly water levels. At the top, type the station number and location (including latitude and longitude), and save the file as an Excel file.

STEP 2: Plot the tide data.
Now the fun really begins! Start by plotting water level versus time for a period of three days, on an Excel "Line" type chart. Plot time on the horizontal (x-axis) and water level on the vertical (y-axis). Be sure to label the axes, giving the units of measurement. Also put a title on your graph, giving the location of the measuring station, including the name of the place from which the data were taken, the latitude and longitude, and dates of the three-day time period you chose. Print the graph and post it on the wall.

STEP 3: Analyze the tide data.
Examine your graph. Does water level change in some regular fashion? Describe what you see. How long are the cycles? (Answer in terms of hours or days.) How high are they, from peak to trough? (Answer in terms of feet or meters.) Are the cycles regular and symmetrical, or are they mixed?
Now compare your graph with those of the researchers you are working with. How are the graphs different? How are they similar? What if you and the others all printed out graphs from the exact same time period--how would the graphs compare?
Next, construct and print out a graph of a one-month time period. What trends do you see that you did not observe on the three-day graph? Compare with your classmates. Do their graphs reveal similar patterns? How are they the same, and how are they different?
Finally, construct a graph of one whole year's worth of data. This one may be too large to print out! Stretch it out on your computer screen so that you can see the individual ups and downs of data. What larger patterns do you notice? Describe those trends, and then compare what you discovered with what your colleagues found.

STEP 4: Explain your observations.
How can we explain the ups and downs of the tides? The position of the moon is the biggest control on the height of tides, and the position of the sun has an effect too. But many other factors come into play as well. Click here for a page with some websites that might help as you try to explain the graphs.

Tides Main Page | Tide Links for Educators (and others)

Northeastern Illinois University | The NEIU NASA/UNCF Project | Department of Earth Science