The Ins and Outs of Tides
Compare predicted and observed tides using data from NOAA.
- Describe the forces causing tides.
- Appraise accuracy of tidal predictions.
Gravitational pull, Centrifugal force
The rhythmic ebb and flow of the oceans subject coastlines to constant
The rhythmic ebb and flow
of the oceans subject coastlines to constant change. Tides dictate the lives of the marine organisms which live within their reach, as
well as the plans of those who live, work, and play near the coast.
Understanding tides is crucial for safe maritime navigation, coastal zone
management, coastal engineering projects, fisherpeople,
boaters, surfers and other water sport enthusiasts. Tides vary widely from
place to place, so tide prediction data for specific areas are very useful.
This is especially true for the Bay of Fundy in Nova Scotia, Canada, which has the highest
tides in the world. The bay is very narrow, so water from the ocean rushes in
and out, changing the water level by up to 20 meters a day!
Tides are caused by two forces, one being the gravitational pull of the sun and the
moon on the earth. The moon has more influence on tides than the sun because it
is 400 times closer to the earth. The other factor is the centrifugal force
acting on the earth as it spins. This causes bulges in the ocean that follow
the moon as it revolves around the earth, one bulge directly under the moon and
the other on the opposite side of Earth. There are usually 2 high tides and 2
low tides with each complete revolution (every 24 hours 50 minutes). During
full moon and new moon phases, we experience spring tides, and during the moon's quarter phases we have neap tides.
provided by the National Oceanic and Atmospheric Administration (NOAA) are
based upon analyses of tidal observations for periods of at least one year and
are generated using a complex computer program. Predicted tidal heights are
those expected under average weather conditions since it would be impossible to
predict the effect that wind, rain, freshwater runoff, and other short-term
meteorological events will have on the tides. Generally, prolonged onshore
winds or a low atmospheric pressure can produce higher tides than predicted,
while the opposite conditions can result in lower tides than those predicted.
When planning around the tides, one must be aware that actual water levels may vary from those predicted when weather differs from what is considered average for
The accuracy of
tide predictions varies with location. Periodically NOAA does a comparison of
the predicted tides versus the observed tides for a calendar year. The
information generated is compiled in a Tide
Prediction Accuracy Table. Look at the average yearly differences in
estimated tidal times and heights to get an idea of the accuracy of
At the NOAA Tides Online site,
you can access near real-time data and see how close the predictions were to
the observed tides at various sites (select State Maps, then choose your
Suppose you want to photograph marine
organisms at low tide, or you are planning to scuba dive from a beach and need
to know what the tides will be doing. Use the NOAA Tide Predictions page to find
tidal predictions for the entire year at your site of interest.
Generate a graph of tidal predictions for a
location near you by using the WWW Tide and Current
Predictor, a site containing
different station locations. Once you have chosen your site and obtained the
data in tabular form, scroll down to Prediction Options. In the first
section of options, select Graphic Plot. In the next section, select 3 days for the Length of Time to Display. Start with the default
colors, and you can change them later if you wish. Label the horizontal lines
on your graph.
Notice the gradual shift in the times that
highs and lows occur from day to day. How does this correspond with the moon's
(NOTE: These tidal predictions are not NOAA