This tidal glossary explains common tidal terms, ranges, and patterns, describing how the Moon and Sun influence tides around the world.
Common Types of Tides
- Spring tides: highest tidal range
- Neap tides: lowest tidal range
- Semidiurnal tides: two highs and lows daily
- Diurnal tides: one high and low daily
Tidal Glossary
Apogean Tide
A monthly tide of decreased range that occurs when the Moon is farthest from Earth (at apogee).
Diurnal
Applies to a location that normally experiences one high water and one low water during a tidal day of approximately 24 hours.
Mean Lower Low Water
The arithmetic mean of the lesser of a daily pair of low waters, observed over a specific 19-year cycle called the National Tidal Datum Epoch.
Neap Tide
A tide of decreased range occurring twice a month, when the Moon is in quadrature (during the first and last quarter Moons, when the Sun and the Moon are at right angles to each other relative to Earth).
Perigean Tide
A monthly tide of increased range that occurs when the Moon is closest to Earth (at perigee).
Semidiurnal
Having a period of half a tidal day. East Coast tides, for example, are semidiurnal, with two highs and two lows in approximately 24 hours.
Spring Tide
Named not for the season of spring, but from the German springen (to leap up). This tide of increased range occurs at times of syzygy (q.v.) each month. A spring tide also brings a lower low water.
Syzygy
Occurs twice a month, when the Sun and the Moon are in conjunction (lined up on the same side of Earth at the New Moon) and when they are in opposition (on opposite sides of Earth at theFull Moon, though usually not so directly in line as to produce an eclipse). In either case, the gravitational effects of the Sun and the Moon reinforce each other, and tidal range is increased.
Vanishing Tide
A mixed tide of considerable inequality in the two highs or two lows, so that the “high low” may become indistinguishable from the “low high.” The result is a vanishing tide, where no significant difference is apparent.
Learn More
See our Tide Charts to get local high and low tide times, and our Moon Phase Calendar to see when the Moon will next be full.
The tide never goes out so far but it always comes in again
–Cornish proverb
Which tidal term was new to you? Let us know in the comments.
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