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Tales of the Coast
Coastal Dynamics

Tidal Inequality

The ranges of two successive tides at a given location are generally not identical but exhibit differences in height. These differences are referred to as the diurnal inequality of the tide and result from the relative position of the sun and moon as well as the specific location of an observer on the earth.

The tidal bulge is centered along a line from the center of the moon or sun to the center of the earth. The tidal bulge at a given sublunar or subsolar location (a location on the earth nearest the moon or sun) has an equivalent bulge on the opposite side of the earth. If the sublunar or subsolar point appears at a given north latitude, the peak of the corresponding tidal bulge on the opposite side of the equator will appear at a corresponding south latitude. Thus, a point on the same north latitude but 180 degrees in longitude from the sublunar or subsolar point will show a reduced amplitude.

The combinations of astronomical forcing that define spring and neap cycles and diurnal inequalities are further modified by local bathymetry and shoreline boundary influences. All of these factors combine to produce tidal envelopes that vary from location to location. The result is site-specific tidal signatures, which can be classified as semidiurnal, diurnal, or mixed.


Tidal Inequality

Coastal Characteristics: Tides along the Atlantic coast are generally semidiurnal with a small diurnal inequality.

Tides inside the Gulf of Mexico range from semidiurnal at Key West, Florida, to diurnal at Pensacola, Florida, to mixed at Galveston, Texas. Tides in the Gulf of Mexico are more complex than open ocean stations because astronomical forcing is modified by geometrically forced nodes and antinodes.

Pacific coast tides are generally of larger amplitude than Atlantic and Gulf coast tides and often have a decided diurnal inequality.