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Tides

What does this mean?


Tidal action results when the waters of oceans and large lakes are affected by the gravitational pull of our sun and moon. The rotation of the earth causes this gravitational effect to vary geographically in a cyclic way, with a period corresponding to a "tidal day," the time of rotation of the earth with respect to the moon, approximately 24.84 hours. The term "tides" is a generic one and refers to the alternating rise and fall of the oceans with respect to the land. Although tidal action causes water to move horizontally as well as vertically, we measure tides as the height of the water surface above a reference point (mean sea level) at a particular geographic point on a shoreline.

Water levels fluctuate in a consistent way in response to tidal movement, but they are modulated by a number of other factors and so are not perfectly regular. In addition to the gravitational pull of the sun and moon, tides also are influenced by coastline configuration, water depth, and seafloor topography, as well as hydrographic and meteorological conditions.

Tides also vary by geographical location and by season. Tides are higher when the Earth is closest to the moon and strongest overall when the moon and sun are aligned, increasing the total gravitational pull. In the northern hemisphere, tides are lowest during the winter when the Earth is farthest from the sun. The tidal range (difference between high and low tide) also varies throughout the year and is largest during the spring.

There are three basic types of tides: semidiurnal (twice-daily), mixed (also twice-daily), and diurnal (daily). A semidiurnal tide has two high waters (high tides) and two low waters (low tides) each tidal day, with the two high waters approximately equal in height, and the two low waters also approximately equal in height. A mixed tide is similar to the semidiurnal except that the two high waters and the two low waters of each tidal day typically have marked differences in their heights. A diurnal tide has one high water and one low water each tidal day.


How are the data collected? (Methods)

Tide data are collected at fixed monitoring stations along the coast. Each station is identified by name, located by latitude/longitude coordinates, and uses a gauge to record the water level with respect to a fixed reference elevation, or datum, that is called "mean sea level." Tide gauging stations are maintained by the National Oceanic and Atmospheric Administration?s (NOAA) National Weather Service.

Tide gauges are programmed to take measurements at 6-minute intervals, with each measurement consisting of a set of 181 water level samples taken at one-second intervals. The samples are averaged, a three standard deviation outlier rejection test is applied, and the mean and standard deviation are recalculated and reported along with the number outliers. The reported measurements have 0.01 foot resolution and are stored in the tide station system's memory. Timing of the system is controlled by an oscillator located in the Geostationary Operational Environmental Satellite (GOES) transmitter which is accurate to 2 seconds per month. Every three hours the data are transmitted via GOES to the satellite downlink. In addition, telephone connections can be used to retrieve data and to interact with the system. Laptop computers are used by field personnel to check and maintain the system. The backup water level sensor is a strain gauge pressure transducer that records data on a separate data collection platform which is optically coupled to the satellite radio. Yearly second-order class I geodetic levels are run from the primary sensor reference point to the local benchmark network.


Calculations

Tide predictions: Because of the numerous factors that affect tides, it is not feasible to predict them purely from knowledge of the positions and movements of the moon and sun obtained from astronomical tables. On October 1 of each year, NOAA releases tide predictions for the following calendar year. Its computations employ a partially empirical approach based upon actual observations of tides in many areas over an extended period of time. To achieve maximum accuracy in prediction, a series of tidal observations at one location ranging over at least a full 18.6-year tidal cycle is required. Within this period, all significant astronomical modifications of tides are expected to occur. Because of the unforeseeable conditions that can affect tides, predictions are, by their very nature, imperfect best-estimates.

For detailed information on methods for data processing read Computational Techniques for Tidal Datums Handbook, U.S. Dept. of Commerce, NOAA, National Ocean Service, Center for Operational Oceanographic Products & Services.


Caveats and Limitations

The National Tidal Benchmark System (NTBMS) provides datum information for previously and currently occupied tidal measurement locations. The number of stations in the NTBMS is approximately 6000. Benchmarks may become invalid due to movement of the earth's crust, and may also be invalidated by changes in local tidal characteristics due to nearby dredging, erosion, and accretion. In many cases, benchmarks in the NTBMS have not been releveled in many years, resulting in some uncertainty in their validity. At present, about 2000 stations have benchmarks with valid published elevations.

» Benchmarks for Florida Tidal Stations


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