If you live near the coast or have ever visited the beach, you are probably aware of tides. But did you know that tides are really big waves that move through the ocean in response to the forces of the Moon and the Sun? Tides appear as the regular rise and fall of the sea surface. How much the water level changes over the day varies depending on where you are and what day it is.

NOAA's National Ocean Service has been measuring and predicting tides since the 1800s using engineered systems that automatically detect and record changes in water levels. These early sets of data were collected mechanically on drums of paper which had to be frequently attended to and adjusted by scientists on a regular basis. Today data are recorded electronically, transmitted via satellite, and made available online. At the backbone of this system is a network of long-term, continuously operating water-level stations known as the National Water Level Observation Network. There are real-time tidal observations and prediction stations around the United States. If the level of water is constantly changing, how do we know how much water levels have risen or fallen from normal? To define normal, scientists use a reference, or datum, as a starting point from which all measurements are made. The numbers that appear on a nautical chart represent water depths measured relative to such a datum.

Physics of Tides

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Tides occur as a response to the gravitational attraction of the Sun and Moon and the rotation of Earth. There are three basic tidal patterns - diurnal, semidiurnal, and mixed. Diurnal tides are characterized by one high and low tide a day. On the other hand, semidiurnal tides are characterized by two high and two low tides per day at approximately regular time intervals and are similar in height. Mixed tides are similar to semidiurnal tides in that they have two high and low tides but different in that the height of successive high (and low) tides may differ substantially. Semidiurnal tides are frequently found on the east coast of the United States, and mixed tides on the west coast of the U.S. Diurnal tides are mostly found in the Gulf of Mexico. The National Oceanic and Atmospheric Administration's (NOAA) Center for Operational Oceanographic Products and Services (CO-OPS) and other organizations have established a system of real-time monitoring of tides that can now be measured to centimeter accuracy.

Tides on the ocean can vary by tens of meters (vertically) between coastlines. The gravitational attraction on the portion of the ocean that is directly aligned with the Sun and/or Moon actually pull a bulge of water away from Earth. As the direct alignment of the Sun and Moon with Earth changes due to the rotation and revolution of these three celestial bodies, the bulge of water moves. The pull of water away from Earth toward the Sun and/or Moon draws water away from other portions of the ocean. Centrifugal acceleration actually causes another bulge of water on the opposite side of Earth. This movement of water up and down relative to the average sea level is by definition high and low tide. The timing of the high and low tides is easily predicted knowing the positions of the Sun and Moon.

Coastal Tides

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Tides are a regular event on every coastline and their timing is predictable. Their occurrence is of great importance to all marine activities, including the natural daily flooding and drying of coastal wetlands, where animals and plants have evolved to thrive in these unique environments. Many coastal wetland areas are also the outlet to terrestrial streams and rivers that flow to the ocean. In these locations, organisms need to be able to tolerate a range of environmental conditions including large variations in salinity that occur in relation to the level of freshwater input. Natural and human changes to these regular cycles can have devastating effects on these organisms. Some of these changes are rapid and some occur more slowly as coastlines are reformed over millions of years.

Tidal ranges can be affected by many factors including latitude, bathymetry, and the shape of the coastline. On coastlines with long shallow gradients, the high and low tide marks may be tens of meters apart in horizontal distance.

In other areas, water may be funneled up through estuaries and embayments by unique coastal structures, greatly increasing the size of the tide. The tidal range in the Bay of Fundy, New Brunswick, Canada can be almost 20 meters. The view during low tide in the Bay of Fundy can be a dramatic sight as bays that were filled with water earlier the day may be emptied for a few hours until high tide.

This link, http://www.srh.weather.gov/jetstream/ocean/fundy_max.htm, displays a chart of the tidal range in the Bay of Fundy.

Tidal Currents

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Oceanic currents are driven by many factors including wind, salinity and temperature differences. The rise and fall of the tides, which are driven by the gravitational attraction of the Sun and Moon on Earth's ocean are another cause of currents. Tidal currents are present in the ocean, near the shore, in bays and estuaries, and along the coast. The speed and direction of tidal currents are predictable.

Tidal currents are the only type of current affected by the interactions of Earth, the Sun, and the Moon. The Moon's force is much greater than that of the Sun because the moon is approximately 390 times closer to Earth than the Sun is. Tidal currents, just like tides, are affected by the different phases of the Moon. During a new, or full Moon, tidal current velocities are strongest and are called spring currents. When the Moon is at first or third quarter phases, tidal current velocities are weakest and are called neap currents.

Tides: Measurements and Data

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Measuring, collecting, and analyzing data on tides has become very sophisticated with modern advances in technology. Today there are vast numbers of monitoring stations transmitting data in real time.

This network of coastal monitoring stations are placed in major navigable harbors and along most coastlines of the United States. The rise and fall of tides greatly affect all marine travel. Bridges and other structures that extend across navigable rivers limit the size of vessels that can pass under them. The distance between the water and the bottom of the structure is called the air gap. If a 50 meter tall vessel tries to pass under a bridge with an air gap of only 45 meters, the result will be damage to the vessel and the bridge. Sometimes the passage under a bridge is calculated so closely that a vessel can only pass under a bridge at low tide - which increases the air gap. Or, if the air gap is too small for vessel to pass, the bridge must be movable, such as a drawbridge.

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