Useful terms

Age of Moon

The time elapsed since the preceding New Moon.

Bench mark (BM)

A fixed physical object or mark used as reference for a horizontal or vertical datum. A tidal bench mark is one near a tide station to which the tide staff and tidal datums are referred. See also: Datum

Crest

The highest point in a propagating wave.

Current

Generally, a horizontal movement of water. Currents may be classified as tidal and nontidal. Tidal currents are caused by gravitational interactions between the Sun, Moon, and Earth and are part of the same general movement of the sea that is manifested in the vertical rise and fall, called tide. See also: Tide

Datum

For marine applications, a base elevation used as a reference from which to reckon heights or depths. It is called a tidal datum when defined in terms of a certain phase of the tide.

Diurnal

Having a period or cycle of approximately one tidal day. Thus, the tide is said to be diurnal when only one high water and one low water occur during a tidal day. See also: High Tide, Low Tide

High Tide

The maximum height reached by a rising tide.

Intertidal Zone

The zone between the mean higher high water and mean lower low water lines. See also: High tide, Low tide

Low Tide

The minimum height reached by a falling tide.

Marigram

A graphic record of the rise and fall of water level. The record is in the form of a curve in which time is generally represented on the x-coordinate and the height of the water level on the y-coordinate.

Mean Sealevel (MSL)

A tidal datum. The arithmetic mean of hourly heights observed over the National Tidal Datum Epoch. Shorter series are specified in the name; e.g., monthly mean sea level and yearly mean sea level. See also: Datum

Neap Tides

Tides of decreased range or tidal currents of decreased speed occurring semimonthly as the result of the Moon being in quadrature. See also: Quadrature of Moon

Quadrature of Moon

Position of the Moon when its longitude differs by 90° from the longitude of the Sun. The corresponding phases are known as first quarter and last quarter.

Range of Tide

The difference in height between consecutive high and low waters. The mean range is the difference in height between mean high water and mean low water. See also: High tide, Low tide

Relative Mean Sea Level Change

A local change in mean sea level relative to a network of bench marks established in the most stable and permanent material available (bedrock, if possible) on the land adjacent to the tide station location. See also: Mean Sealevel (MSL), Bench mark (BM)

Semidiurnal

Having a period or cycle of approximately one-half of a tidal day. The predominant type of tide throughout the world is semidiurnal, with two high waters and two low waters each tidal day. The tidal current is said to be semidiurnal when there are two flood and two ebb periods each day. A semidiurnal constituent has two maxima and two minima each constituent day, and its symbol is the subscript 2. See also: High tide, Low tide

Sequence of Tide

The order in which the four tides of a day occur, with special reference as to whether the higher high water immediately precedes or follows the lower low water. See also: High tide, Low tide

Spring Tides

Tides of increased range or tidal currents of increased speed occurring semimonthly as the result of the Moon being new or full.

Tidal Wave

A shallow water wave caused by the gravitational interactions between the Sun, Moon, and Earth. Essentially, high water is the crest of a tidal wave and low water, the trough. Tidal current is the horizontal component of the particulate motion, while tide is manifested by the vertical component. The observed tide and tidal current can be considered the result of the combination of several tidal waves, each of which may vary from nearly pure progressive to nearly pure standing and with differing periods, heights, phase relationships, and direction. See also: High tide, Low tide, Crest, Trough, Current, Tide

Tide

The periodic rise and fall of a body of water resulting from gravitational interactions between Sun, Moon, and Earth. The vertical component of the particulate motion of a tidal wave. Although the accompanying horizontal movement of the water is part of the same phenomenon, it is preferable to designate this motion as tidal current.

Tide Curve

A graphic representation of the rise and fall of the tide in which time is usually represented by the x-coordinate and height by the y-coordinate. For a semidiurnal tide with little diurnal inequality, the graphic representation approximates a cosine curve. See also: Diurnal, Semidiurnal

Tide Station

The geographic location at which tidal observations are conducted. Also, the facilities used to make tidal observations. These may include a tide house, tide (water level) gauge, tide staff, and tidal bench marks. See also: Bench mark (BM)

Trough

The lowest point in a propagating wave.

Type of Tide

A classification based on characteristic forms of a tide curve. Qualitatively, when the two high waters and two low waters of each tidal day are approximately equal in height, the tide is said to be semidiurnal; when there is a relatively large diurnal inequality in the high or low waters or both, it is said to be mixed; and when there is only one high water and one low water in each tidal day, it is said to be diurnal. See also: Diurnal, Semidiurnal, Tide curve