Types of tides: diurnal, semidiurnal and mixed

The three types of tide

Coasts fall into three patterns: semidiurnal, diurnal and mixed. NOAA’s definitions are the ones everybody uses. A semidiurnal coast has two high and two low tides of approximately equal size every lunar day. A diurnal coast has one high and one low tide every lunar day. A mixed coast has two high and two low tides of different size. The lunar day is 24 hours 50 minutes long, which is why the pattern slides later on the clock every day rather than repeating at the same hour.

The three are easiest to tell apart side by side on one date, at three coasts that each show their type cleanly: Boston on the Atlantic, Pensacola on the Gulf of Mexico, and San Francisco at the Golden Gate.

Those three shapes are the whole classification. A coast keeps its shape: the type is set by the ocean basin it opens onto and by the size and depth of the shelf in front of it, so it does not change from week to week or with the seasons. What does change through the month is how pronounced it looks, because the daily part of the tide grows and shrinks with the Moon’s declination — its angle north or south of the equator.

Semidiurnal tides

A semidiurnal coast gets two high waters and two low waters a day of roughly equal height, about 12 hours 25 minutes apart. That interval is half a lunar day, and it means roughly 6 hours 12 minutes from each high to the low that follows it. Both of the day’s lows are usable and both of its highs are usable, which makes this the simplest pattern to plan around.

It is the pattern of the Atlantic: the coasts of western Europe, eastern North America and most of Africa. Boston is a textbook case: its two highs sit close to level with each other and its two lows are almost identical. Because the two halves of the day are so alike, a semidiurnal coast is where the rule of twelfths works best, and where the arithmetic of “six hours in, six hours out” is close enough to reality to use in your head.

Diurnal tides

A diurnal coast gets one high water and one low water in each lunar day, so there is a single low to plan around and about twelve hours of water going one way. That is why a diurnal coast feels slow: the same range is spread over twice the time a semidiurnal coast takes.

The classic diurnal basins are the Gulf of Mexico, the Gulf of Tonkin, the Gulf of Thailand and the Gulf of Carpentaria — all of them partly enclosed, all of them the wrong size to respond strongly to a twice-daily wave. Pensacola on the Gulf coast is the easiest one to check for yourself.

The daily part of the tide comes from the tilt of the Moon’s path against the equator, so a diurnal coast is at its most one-sided when the Moon is far north or south of the equator, and a weak second turn can appear in the day when the Moon crosses it. The Moon’s declination swings to about 28.6 degrees either side over an 18.6-year cycle, so the effect is always present and never the same twice.

Mixed tides

A mixed coast gets two highs and two lows a day of noticeably different heights — a higher high and a lower low, which is called the diurnal inequality. The four turning points get their own names on a chart: higher high water and lower high water, higher low water and lower low water. The gap between the two lows can be a large fraction of the day’s range, and on this pattern that gap is the number that matters.

Mixed tides are what you get when the daily and the twice-daily parts of the tide are both substantial, so neither wins outright. That describes the Pacific coast of North America, much of the Pacific and Indian Ocean coasts, and western Australia. San Francisco at the Golden Gate is the standard illustration: two highs that can differ by a third of the day’s range.

The inequality is not constant. It grows when the Moon is far from the equator and nearly vanishes when the Moon crosses it, so a mixed coast will run through days that look almost semidiurnal and then, a week later, days that look almost diurnal. The Moon and the tides guide follows that cycle in more detail.

Which coasts have which

Type follows the ocean basin, not the latitude: two coasts at the same latitude on opposite sides of an ocean can have different types, and one coast can keep its type from the tropics to the Arctic.

The United States has all three within one country, which is why American sources use it as the worked example: the east coast is semidiurnal, the Gulf coast is diurnal, and the west coast is mixed semidiurnal.

The form factor: how oceanographers decide

Oceanographers classify a coast by a form factor, the size of the daily constituents divided by the size of the twice-daily ones. A tide prediction is a sum of harmonic constituents, each one a cosine wave with a period fixed by astronomy and an amplitude that belongs to the place. Four of them carry most of the signal: M2 and S2 twice a day, K1 and O1 once a day. The form factor is the daily pair’s amplitudes over the twice-daily pair’s, at that one station.

F = (K1 + O1) / (M2 + S2)

It turns the question into a number, and the number falls into four bands rather than three.

The constituents in that formula are the same ones our predictions are built from: every station here is computed from the FES2022 global model, which supplies 34 constituents at the nearest ocean point, and we sum them ourselves — the method is on our how we compute page. We do not print a form factor on station pages, because the shape of the day’s curve says it more directly than the ratio does.

Why it matters when you read a table

On a diurnal coast there is one low water a day to work with, and on a mixed coast the day’s two lows are not interchangeable. That is the practical whole of it, and it changes what you look at first in a table.

• Semidiurnal — read the times. The heights of the day’s two lows are close enough that either one will do, so the decision is when you can get there.
• Diurnal — read the calendar. There is one low a day, and if it falls at three in the morning there is no second chance until tomorrow.
• Mixed — read the heights. Two lows a day, one of them clearly lower than the other; the flats that uncover on the lower low may not uncover at all on the higher one.

Two things are true whatever your type. The tides slide about 50 minutes later each day, so today’s convenient low is tomorrow’s slightly less convenient one. And the heights swing through the fortnight with the spring–neap cycle, so the same low water is lower at springs than at neaps — that part is tidal range, not type. The how to read a tide chart guide takes a table apart row by row.

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