Tide Glossary

A more comprehensive Tide & Currents Glossary is available from NOAA as a 29-page PDF document.  

agger -- See double tide.

chart datum -- (or datum, datum plane, hydrographic datum, plane of reference, reference plane, tidal datum, tidal datum plane). The permanently established surface from which soundings or tide heights are referenced (usually low water). The surface is called a tidal datum when referred to a certain phase of the tide. In order to provide a factor of safety, some level lower than mean sea level is generally selected, such as mean low water or mean lower low water.

conjunction -- The situation of two celestial bodies with the same celestial longitude (the angular distance measured east of the vernal equinox along the ecliptic); for example, conjunction occurs when the moon and the sun are directly in line with the earth and the moon is between the earth and the sun.

Coordinated Universal Time -- (abbreviated UTC; also called zulu time) Local mean time at the Greenwich meridian.  UTC has generally replaced Greenwich Mean Time or GMT, though for most purposes the two can be considered equivalent. UTC achieves higher accuracy by adding leap seconds irregular intervals to compensate for the Earth's slowing rotation.
Local note: To convert UTC or GMT (zulu) to Central Standard Time (CST), subtract 6 hours. Subtract 5 hours during Daylight Saving Time.


datum -- 1. Any numerical or geometrical quantity or set of such quantities which may serve as a reference or base for other quantities. 2. See chart datum

datum plane -- See chart datum.

diurnal tide -- A tide in which there is only one high water and one low water each lunar day.

double high water -- See double tide.

double tide -- (or agger, double high water, gulder). A high water consisting of two maximums of nearly the same height separated by a relatively small depression, or a low water consisting of two minimums separated by a relatively small elevation.

flood tide -- See rising tide.

GMT -- See Greenwich mean time.

Greenwich mean time -- (abbreviated GMT; also called zulu time) Local mean time at the Greenwich meridian.  GMT has been replaced by Coordinated Universal Time or UTC. UTC achieves higher accuracy by adding leap seconds irregular intervals to compensate for the Earth's slowing rotation. For most purposes, GMT and UTC may be considered equivalent.
Local note: To convert GMT or UTC (zulu) to Central Standard Time (CST), subtract 6 hours ( subtract 5 hours during Daylight Saving Time).


gulder -- A double low water occurring on the south coast of England. See double tide.

HHW -- See higher high water

high tide -- See high water.

higher high water -- (abbreviated HHW). The higher of two high waters occurring during a tidal day where the tide exhibits mixed characteristics. See mixed tide.

high water -- (abbreviated HW; also called high tide). The highest limit of the surface water level reached by the rising tide. High water is caused by the astronomic tide-producing forces and/or the effects of meteorological, hydrologic and/or oceanographic conditions.

HW -- See high water

hydrographic datum -- See chart datum.

limb -- The outer edge of the apparent disk of a celestial body.

LLW -- See lower low water

lower low water -- (abbreviated LLW). The lower of two low waters of any tidal day where the tide exhibits mixed characteristics. See mixed tide.

low tide -- See low water.

low water -- (abbreviated LW; or low tide). The lowest limit of the surface water level reached by the lowering tide. Low water is caused by the astronomic tide-producing forces and/or the effects of meteorological, hydrologic and/or oceanographic conditions.

lunar day -- (or tidal day). The interval between two successive upper transits of the moon over a local meridian. The period of the mean lunar day, approximately 24.84 solar hours, is derived from the rotation of the earth on its axis relative to the movement of the moon about the earth.

LW -- See low water

mean higher high water -- (abbreviated MHHW). The average height of all the daily higher high waters recorded over a 19-year period, or a computed equivalent period. It is usually associated with a tide exhibiting mixed characteristics. See mixed tide.

mean high water -- (abbreviated MHW). The average height of all the high waters recorded over a 19-year period, or a computed equivalent period.

mean lower low water -- (abbreviated MLLW). The average height of all the lower low waters recorded over a 19-year period, or a computed equivalent period. It is usually associated with a tide exhibiting mixed characteristics. See mixed tide.

mean sea level -- (abbreviated MSL; or sea level datum). The mean surface water level determined by averaging heights at all stages of the tide over a 19-year period. Mean sea level is usually determined from hourly height readings measured from a fixed predetermined reference level (chart datum).

MHW -- See mean high water

MHHW -- See mean higher high water

mixed tide -- The type of tide in which a diurnal wave produces large inequalities in heights and/or durations of successive high and/or low waters. This term applies to the tides intermediate to those predominantly semidiurnal and those predominantly diurnal.

MLLW -- See mean lower low water

moonrise -- The time when the upper limb of the moon appears above the sensible horizon (no adjustment made for elevation of observer or atmospheric refraction).

moonset -- The time when the upper limb of the moon disappears below the sensible horizon (no adjustment made for elevation of observer or atmospheric refraction).

MSL -- See mean sea level

National Tidal Datum Epoch -- (abbreviated NTDE). A period of 19 years adopted by the National Ocean Service as the period over which observations of tides are to be taken and reduced to average values for tidal datums. This 19-year period covers all variations in the path of the moon about the sun (regression of the moon’s nodes). The most recent NTDE of 1983-2001 was implemented in April 2003 to reflect the latest variations in Mean Sea Level (MSL) along the nation's coasts.

neaps -- See neap tide.

neap tide -- (or neaps). Tide of decreased range which occurs about every two weeks when the moon is in quadrature.

NTDE -- See National Tidal Datum Epoch

opposition -- The situation of two celestial bodies with their celestial longitudes (the angular distance measured east of the vernal equinox along the ecliptic) differing by 180 degrees; for example, opposition occurs when the moon and the sun are directly in line with the earth and on opposite sides of the earth.

plane of reference -- See chart datum.

quadrature -- The position of the phase cycle when the two principal tide producing bodies (moon and sun) are nearly at a right angle to the earth; the moon is then in quadrature in its first quarter or last quarter.

reference level -- See chart datum.

reference plane -- See chart datum.

reference station -- (or standard station, standard port). A place where tide or tidal current constants have been determined from observations, and which is used as a standard for the comparison of simultaneous observations at a subordinate station. It is also a place for which independent daily predictions are obtained for other locations by means of differences or factors.

rising tide -- (sometimes called flood tide). The portion of the tide cycle between low water and the following high water.

sea level datum -- See mean sea level.

semidiurnal tide -- The type of tide having two high waters and two low waters each tidal day with small inequalities between successive high and successive low water heights and durations.

sensible horizon -- the plane tangent to the earth's surface at an observers position

springs -- See spring tide.

spring tide -- (or springs). Tide of increased range which occurs about every two weeks when the moon is at new moon or full moon (syzygy).

stand -- See stand of tide.

standard port -- British term for reference station.

standard station -- See reference station.

stand of tide -- (or stand, tidal stand). The interval at high or low water when there is no appreciable change in the height of the tide; its duration will depend on the range of the tide, being longer when the tide range is small and shorter when the tide range is large. Where a double tide occurs, the stand may last for several hours even with a large range of tide.

subordinate station -- 1. One of the places for which tide or tidal current predictions are determined by applying a correction to the predictions of a reference station. 2. A tide or tidal current station at which a short series of observations has been made, which are reduced by comparison with simultaneous observations at a reference station.

sunrise -- The time when the upper limb of the sun appears above the sensible horizon as a result of the diurnal rotation of the earth (no adjustment made for elevation of observer or atmospheric refraction).

sunset -- The time when the upper limb of the sun disappears below the sensible horizon as a result of the diurnal rotation of the earth (no adjustment made for elevation of observer or atmospheric refraction).

syzygy -- The two points in the moon's orbit when the moon is in conjunction or opposition to the sun relative to the earth; time of new moon or full moon in the cycle of phases.

tidal datum -- See chart datum.

tidal datum plane -- See chart datum.

tidal day -- See lunar day.

tidal difference -- The difference in time or height of a high or low water between a subordinate station and a reference station. The difference is applied to the prediction at the reference station to obtain the time or height of the tide at a subordinate station. These differences are available in tide tables.

tidal stand -- See stand of tide.

tide -- The periodic rising and falling of the earth's oceans. It results from the tide- producing forces of the moon and sun acting upon the rotating earth. This disturbance actually propagates as a wave through the surface layer of the oceans.

tide prediction -- Predetermined time and height of high or low water at a reference station. May be computed in advance by mechanically summing the harmonic constituents of which the tide is composed. Used to compile tide tables.

tide range -- The difference in height between consecutive high and low waters. 

tide-producing force(s) -- The slight local difference between the gravitational attraction of two astronomical bodies and the centrifugal force that holds them apart. These forces are exactly equal and opposite at the center of gravity of either of the bodies, but, since gravitational attraction is inversely proportional to the square of the distance, it varies from point to point on the surface of the bodies. Therefore, gravitational attraction predominates at the surface point nearest to the other body, while centrifugal "repulsion" predominates at the surface point farthest from the other body. Hence there are two regions where tide- producing forces are at a maximum, and normally there are two tides each lunar day and solar day.

tide tables -- Tables which give daily predictions, usually a year in advance, of the times and heights of the tide. These predictions are usually supplemented by tidal differences and constants by means of which additional predictions can be obtained for numerous other places. See tide prediction, tidal difference.

transit -- The passage of the moon over the local meridian; it is designated as upper transit when it crosses the observers meridian and as lower transit when it crosses the same meridian but 180 degrees from the observer's location. When specified, transit may be referred to the Greenwich meridian.

UTC -- See Coordinated Universal Time.

zulu time -- See Greenwich Mean Time and Coordinated Universal Time.