Tide terms, clearly explained

The point in the Moon’s orbit when it is farthest from Earth. The Moon’s tide-producing influence is somewhat weaker near apogee.

The reference level used for water depths on a nautical chart. In U.S. coastal waters, this is usually Mean Lower Low Water.

Water moving horizontally. A current can be driven by tides, wind, differences in water density, or the larger circulation of the ocean.

A fixed reference level used to compare water heights or depths. Tidal datums are calculated from long-term observations at a particular place.

The difference in height between the two high waters or the two low waters that occur during one tidal day.

A tide pattern with one high water and one low water during a tidal day.

The speed of a current, usually stated in knots. It is commonly paired with set, the direction toward which the current flows.

A tidal current flowing away from shore, out of a harbor, or down a tidal river as the tide falls.

The part of the tidal cycle when the water level is falling from high water toward low water. Often shortened to ebb.

A tidal current flowing toward shore, into a harbor, or up a tidal river as the tide rises.

The part of the tidal cycle when the water level is rising from low water toward high water. Often shortened to flood.

One repeating astronomical rhythm used to describe and predict the tide. Each constituent has its own period, strength, and timing.

A tide prediction made by combining the repeating astronomical constituents measured for a station.

Shallow coastal flooding that can occur when unusually high tides push water onto roads, docks, and other low-lying places, even without a storm.

The highest water level reached between a rising tide and the following falling tide. It is commonly called high tide.

The higher of the two high waters on a day when the highs are unequal.

The higher of the two low waters on a day when the lows are unequal.

An informal name for an especially high predicted tide, often occurring when the Sun, Moon, and Earth are strongly aligned and the Moon is relatively close to Earth.

The lowest water level reached between a falling tide and the following rising tide. It is commonly called low tide.

The lower of the two high waters on a day when the highs are unequal.

The lower of the two low waters on a day when the lows are unequal.

The time it takes a point on Earth to rotate back beneath the Moon: about 24 hours and 50 minutes. This helps explain why tide times usually shift later each day.

The long-term average height of all high waters observed at a station.

The long-term average of the higher high water height from each tidal day at a station.

The long-term average height of all low waters observed at a station.

The long-term average of the lower low water height from each tidal day. It is the standard chart datum for most U.S. coastal waters.

The average level of the sea over a long period, with the regular rise and fall of tides and shorter weather effects averaged out.

The average of Mean High Water and Mean Low Water at a station. It is a calculated tidal datum, not the same measurement as mean sea level.

A tide pattern with two highs and two lows of noticeably different heights during a tidal day.

The changing sunlit shape of the Moon seen from Earth. New and full moons tend to produce spring tides; quarter moons tend to produce neap tides.

The standard 19-year period NOAA uses to calculate official tidal datums so stations can be compared on a consistent basis.

A tide with a smaller-than-usual range. Neap tides occur near the first- and third-quarter Moon, when solar and lunar tidal forces partly offset each other.

The water height actually measured by a station. It includes the astronomical tide plus weather, river flow, and other local effects.

A spring tide that occurs near lunar perigee. The combination can produce higher high waters and lower low waters than a typical spring tide.

The point in the Moon’s orbit when it is closest to Earth. The Moon’s tide-producing influence is somewhat stronger near perigee.

A water height calculated from the known astronomical tide at a station. It does not include unexpected effects from wind, air pressure, river flow, or storms.

A tide or current station with enough observations to support direct predictions. Nearby subordinate stations are related back to it using time and height adjustments.

The difference between an observed water level and the predicted astronomical tide. Weather and river flow are common causes.

A narrow, fast-moving flow of water traveling away from shore through the surf zone. Rip currents are not tides, though tide stage can affect where and when they form.

A standing wave that makes water rock back and forth in an enclosed or partly enclosed basin, such as a harbor, bay, or lake.

A tide pattern with two high waters and two low waters of roughly similar height during a tidal day.

The compass direction toward which a current flows. This is paired with drift, the current’s speed.

A short period when a tidal current is very weak as it changes between flood and ebb. It does not always happen at the same time as high or low water.

A tide with a larger-than-usual range. Spring tides occur near new and full moons, when the Sun’s and Moon’s tide-producing forces reinforce each other.

An abnormal rise of water generated by a storm, above the predicted astronomical tide. Wind and low air pressure are the main drivers.

The actual coastal water level during a storm, combining the astronomical tide, storm surge, and other effects.

A location whose predictions are calculated by applying time and height corrections to a nearby reference station.

A wave or wall of water that travels upstream when a strong incoming tide enters a shallow, narrowing river or estuary.

The horizontal movement of water caused by the same astronomical forces that create the vertical rise and fall of tides.

A standard elevation based on a particular phase of the tide, such as Mean High Water or Mean Lower Low Water. It is local to the area where it was measured.

The typical 24-hour, 50-minute cycle used to describe the daily pattern of tides at a location.

The vertical difference in height between a high water and the following or preceding low water.

The regular rise and fall of the ocean caused mainly by the gravitational pull of the Moon and Sun, together with Earth’s rotation.

An instrument that continuously measures water level at a tide station. Modern gauges often send observations in near real time.

A fixed location where water levels are measured over time. Station observations help establish datums, verify predictions, and monitor coastal conditions.

A schedule of predicted high and low water times and heights for a location, usually organized by day.

A series of long ocean waves caused by a sudden displacement of water, often from an undersea earthquake. A tsunami is not a tide.

The height of the water surface relative to a chosen datum at a particular time and place.