Written by: Laura Rose, Virginia Sea Grant, Virginia Institute of Marine Science

Summary
Compare predicted and observed tides using data from NOAA.

Objectives

• Describe the forces causing tides.
• Appraise accuracy of tidal predictions.

Vocabulary
Gravitational pull, Centrifugal force

Introduction

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.

Tide predictions 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 area.

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 predictions.

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 site).

Data Activity
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 revolution?

(NOTE: These tidal predictions are not NOAA certified.)