Tides and Water Levels

Types and Causes of Tidal Cycles – Diurnal, Semidiurnal, Mixed Semidiurnal; Continental Interference

Depending upon your location on the Earth you may experience Diurnal, Semidiurnal or Mixed Semidiurnal tidal cycles. Click the image for a larger view.

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 large continents on the planet, 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 (Sumich, J.L., 1996).

Three basic tidal patterns occur along the Earth’s major shorelines. In general, most areas have two high tides and two low tides each day. When the two highs and the two lows are about the same height, the pattern is called a semi-

This map shows the geographic distribution of different tidal cycles. Coastal areas experiencing diurnal tides are yellow, areas experiencing semidiurnal tides are red and regions with mixed semidiurnal tides are outlined in blue. Click the image for a larger view.

daily or semidiurnal tide. If the high and low tides differ in height, the pattern is called a mixed semidiurnal tide. Some areas, such as the Gulf of Mexico, have only one high and one low tide each day. This is called a diurnal tide. The U.S. West Coast tends to have mixed semidiurnal tides, whereas a semidiurnal pattern is more typical of the East Coast (Sumich, J.L., 1996; Thurman, H.V., 1994; Ross, D.A., 1995).

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