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Tides & Currents Education


What are Tides? - Tides are one of the most reliable phenomena in the world. As the sun rises in the east and the stars come out at night, ocean waters regularly rise and fall along our shores. Read more about "What are Tides?"

What Causes Tides? - Gravity is one major force that creates tides. In 1687, Sir Isaac Newton explained that ocean tides result from the gravitational attraction of the sun and moon on the oceans of the earth. Read more about "What Causes Tides?"

Gravity, Inertia, and the Two Bulges - Gravity is only one of the major forces responsible for creating tides. Another is inertia, which is the force that acts to counterbalance gravity. It is the tendency of moving objects to continue moving in a straight line. Together, gravity and inertia are responsible for the creation of two major tidal bulges on the earth. "Read more about "Gravity, Inertia, and the Two Bulges"

Changing Angles and Changing Tides - The earth's two tidal bulges are aligned with the positions of the moon and the sun. Over time, the positions of these celestial bodies change relative to the earth's equator. The changes in their relative positions have a direct effect on daily tidal heights and tidal current intensity. Read more about "Changing Angles and Changing Tides"

Frequency of Tides - Most coastal areas, with some exceptions, experience two high tides and two low tides every lunar day. Almost everyone is familiar with the concept of a 24-hour solar day, which is the time that it takes for a specific site on the earth to rotate from an exact point under the sun to the same point under the sun. Similarly, a lunar day is the time it takes for a specific site on the earth to rotate from an exact point under the moon to the same point under the moon. Read more about "Frequency of Tides"

Tidal Variations - The moon is a major influence on the earth's tides, but the sun also generates considerable tidal forces. Solar tides are about half as large as lunar tides and are expressed as a variation of lunar tidal patterns, not as a separate set of tides. When the sun, moon, and earth are in alignment (at the time of the new or full moon), the solar tide has an additive effect on the lunar tide, creating extra-high high tides, and very shallow low tides, both commonly called spring tides. Read more about "Tidal Variations"

Types and Causes of Tidal Cycles - 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 planet's large continents, 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. Read more about "Types and Causes of Tidal Cycles"

What Affects Tides? - The relative distances and positions of the sun, moon and earth all affect the size and magnitude of the earth's two tidal bulges. At a smaller scale, the magnitude of tides can be strongly influenced by the shape of the shoreline. When oceanic tidal bulges hit wide continental margins, the height of the tides can be magnified. Conversely, mid-oceanic islands not near continental margins typically experience very small tides of 1 meter or less. Read more about "What Affects Tides?"

Monitoring the Tides - Predicting tides has always been important to people who look to the sea for their livelihood. Commercial and recreational fishermen use their knowledge of the tides and tidal currents to help them improve their catches. Depending on the species and water depth in a particular area, fish may concentrate during ebb or flood tidal currents. In some areas, strong tidal currents concentrate bait and smaller fish, attracting larger fish. Knowledge of the tides is of interest to recreational beachgoers and surfers. Read more about "Monitoring the Tides"

How are Tides Measured? Part.I - Since the early 1800s, NOAA and its predecessor organizations have been measuring, describing and predicting tides along the coasts of the United States. The longest continuous sea-level records exist for the Presidio in San Francisco, California. Records for the area date back to June 30, 1854. Today, the Center for Operational Oceanographic Products and Services (CO-OPS), which is part of NOAA's National Ocean Service (NOS), is responsible for recording and disseminating water-level data. Read more about "How are Tides Measured? Part.I"

How are Tides Measured? Part.II - Advances in technology have helped solve many of the problems associated with the old tidal recording systems. Microprocessor-based technologies allow for customized data collection and have improved measurement accuracy. While older tidal measuring stations used mechanical floats and recorders, a new generation of monitoring stations uses advanced acoustics and electronics. Today's recorders send an audio signal down a half-inch-wide sounding tube and measure the time it takes for the reflected signal to travel back from the water's surface. The sounding tube is mounted inside a 6-inch-diameter protective well, which is similar to the old stilling well. Read more about "How are Tides Measured? Part.II"


When used in association with water, the term "current" describes the motion of the water. Some currents you may be familiar with are the motion of rainwater as it flows down the street, or the motion of the water in a creek, stream, or river flowing from higher elevation to lower elevation. This motion is caused by gravity. The speed and direction (velocity) of currents can be measured and recorded.

The Challenge of Measuring Water Currents
NOS Education - Currents

For Teachers & Students

A set of lesson plans on tides and water levels has been developed for students in grades 9-12, but are easily adaptable for students at the middle school or undergraduate level. Each lesson integrates information presented in the tides and water levels tutorial with data offerings from the tides and water levels "roadmap to resources." These lesson plans focus on the forces that cause and affect tides, analysis of the variations in tidal patterns, and the conditions that cause them. Lessons also address the effect of lunar cycles on living organisms.

Tides Lesson Plans
Tides Roadmap to Resources

Our Restless Tides

The Astronomical Tide-Producing Forces: General Considerations
Detailed Explanation of the Differential Tide Producing Forces
Variations in the Range of the Tides: Tidal Inequalities
Factors Influencing the Local Heights and Times of Arrival of the Tides
Prediction of the Tides

About Tides & Water Levels

History of Tidal Analysis and Prediction
Tide Predicting Machines
How Do We Measure Water Levels?
Why Measure Water Levels?

Revised: 10/15/2013
NOAA / National Ocean Service
Web site owner: Center for Operational Oceanographic Products and Services