What is a tide?

The Moon pulls, and the water bulges

The Moon’s gravity pulls hardest on the side of the Earth facing it and least on the far side. The oceans respond by bulging slightly at both ends of that line. The Earth then turns underneath those bulges, so a point on the coast passes through roughly two high waters and two low waters in each lunar day of 24 hours 50 minutes — which is why high tide slides about 50 minutes later each day.

What raises the water is the difference in that pull from one side of the planet to the other, not the pull itself. The near-side ocean is closer to the Moon than the Earth’s centre is, so it is drawn towards the Moon a little harder; the far-side ocean is further away, so it is drawn a little less hard and the planet is pulled out from under it. Both ends of the line end up standing high. Neither bulge is a heap of water travelling round the globe: what travels is a wave thousands of kilometres long and, in the open ocean, well under a metre tall.

That wave arriving at a coast is the tide you can see. High water is the moment the level stops rising, low water the moment it stops falling, and the vertical distance between one low and the next high is the tidal range. On a coast with two tides a day the interval from one high to the next is 12 hours 25 minutes — half a lunar day — and the water spends about six hours coming in and six hours going out.

The Sun matters too

The Sun raises its own tide, a little under half the size of the Moon’s. When the two line up the tides are large, and when they pull at right angles the tides are small. That interplay is the spring and neap cycle, and it repeats about every two weeks.

The Sun is vastly more massive than the Moon but it is also very much further away, and tide-raising force falls off with the cube of distance. What survives that trade is about 46 per cent of the Moon’s effect — enough to matter, not enough to take over. At new Moon and at full Moon the Sun, the Earth and the Moon lie on one line and the two tides add: the range swells to its spring value. At the quarters the Sun pulls across the Moon’s line and partly cancels it, and the range shrinks to its neap value. The full round trip takes 14.77 days, half the 29.53-day cycle of the Moon’s phases.

Distance moves the number as well. The Moon’s orbit is an ellipse — it averages 384,400 km from us and returns to its closest point about every 28 days — and by the same cube law a Moon at perigee pulls roughly a fifth harder than an average one. A spring tide that lands near perigee is one of the biggest of the year, which is what people mean by a king tide.

One caution about all of this: the biggest tide does not arrive on the day of the alignment. The ocean takes time to respond, and on most coasts the peak follows the exact new or full Moon by one to two days.

Semidiurnal, diurnal and mixed coasts

Most Atlantic coasts get two nearly equal highs and lows a day (semidiurnal). Parts of the Gulf of Mexico and South-East Asia get only one of each (diurnal). Much of the Pacific coast of the Americas gets two, but of noticeably different heights (mixed). Which one you have depends on the ocean basin you sit on, not on your latitude.

NOAA puts the three patterns like this: 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 east coast of the United States is semidiurnal, the Gulf of Mexico is diurnal, and the west coast is mixed semidiurnal. You can see the difference on a single day: Boston draws a clean, even double curve, while Pensacola on the Gulf coast draws one slow rise and one slow fall.

The pattern is a property of the place and it does not change from week to week, so it is worth knowing which one you are reading before you plan around a table. The types of tides guide takes each pattern apart, with three real gauges drawn side by side on the same day.

Why the local coast decides the size

The astronomical tide in the open ocean is under a metre. Everything above that is geography: a funnel-shaped bay amplifies it, a shallow shelf slows the wave down, a narrow entrance can delay high water by hours. That is why the Bay of Fundy sees over 15 m and the Mediterranean barely half a metre from the same Moon on the same day.

Three things do most of that work. The first is resonance: a bay has a natural sloshing period set by its length and depth, and where that period lands near 12 hours 25 minutes the tide drives the basin in step with itself and the range grows enormous. The second is the funnel: as a wave runs into water that is both narrowing and shallowing, the same volume has less room and must go up. The third is the shelf, which slows the wave and lets it steepen before it reaches land.

In the open ocean the tide is not a simple bulge sweeping past either. The Earth’s rotation steers it into rotating systems, each turning around a point where the two tides cancel and the range falls close to zero — an amphidromic point. Distance from one of those points matters more to your range than distance from the equator does.

The enclosed seas show the same arithmetic from the other end. The Mediterranean is fed through one narrow strait and is the wrong size to resonate, so most of it stays under half a metre; Venice reaches about a metre and the Gulf of Gabès about two, and those are the exceptions people name. The Baltic is measured in centimetres. For scale, Boston’s mean range is about 2.9 m. The tidal range guide has the table of the largest ranges in the world, and what a coast has to do to earn one.

What weather does to the prediction

Every tide prediction on this site — ours included — is astronomy only. Low air pressure lets the sea stand higher (roughly 1 cm per hectopascal), and wind can pile water onto a coast or push it off. In a storm the real water level can differ from the prediction by half a metre or more, which is why tide times are for planning, never for safety-critical decisions.

That gap has a name: storm surge. It is not a flaw in the calculation, it is a different phenomenon, and no tide table anywhere includes it. The astronomy tells you what the water would do on a still day; the weather then adds or subtracts its own contribution on top. The two combine most dangerously when a surge lands on a tide that was already going to be high.

How a tide is predicted

Because the tide is driven by orbits, it can be taken apart into a sum of pure cosine waves, one for each regular motion in the Earth–Moon–Sun system. Each of those is a harmonic constituent, with a fixed period set by astronomy and an amplitude and a phase that belong to the place. Work out the amplitudes and phases once, and you can add the waves back together for any date you like — a date next week and a date next year cost the same.

A handful of constituents carry most of the signal:

• M2, the principal lunar semidiurnal term: the twice-daily tide the Moon alone would raise, period 12 hours 25 minutes. On most coasts it is much the largest of them.
• S2, the principal solar semidiurnal term: the Sun’s twice-daily tide, period exactly 12 hours. It drifts in and out of step with M2, and that beat is the spring–neap cycle.
• N2, the lunar elliptic term, which carries the Moon’s changing distance and is why one spring tide is bigger than the next.
• K1 and O1, the two large daily terms, which come from the tilt of the Moon’s and the Sun’s paths against the equator. Where they are big compared with M2 and S2, the coast gets one tide a day instead of two.

Every station on this site is predicted from the FES2022 global tide model, which supplies 34 constituents at the nearest ocean point to the place you asked for; we add them up ourselves, along with Sun and Moon positions from the JPL DE421 ephemeris. Heights are given relative to mean sea level rather than a chart datum, so low waters print as negative numbers — the water is below the average, not below the sea bed. The full method, and what it does and does not claim, is on our how we compute page.

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