Tides are the regular rise and fall of sea level caused mainly by the Moon's gravitational influence on Earth, with the Sun also playing an important role. These forces create very long-period waves that move through the oceans and appear along coastlines as high and low tides. The basic astronomical cycle is predictable, but the timing and size of real tides also depend strongly on ocean depth, the shape of ocean basins and local coastline geography.
What are tides?
A tide is a periodic change in sea level. At the coast, we experience this as the water rising toward high tide and falling toward low tide, but the process operates across entire oceans.
Tides are best understood as extremely long-period waves. Instead of the short waves you see breaking on a beach, tidal waves extend across huge areas of ocean and develop in response to the gravitational relationship between Earth, the Moon and the Sun.
What causes tides?
Tides are generated primarily by differences in the Moon's gravitational pull across Earth. The Sun also creates tides, but the Moon has the stronger tide-generating influence because it is much closer to Earth.
The important part is that the Moon does not pull equally on every part of Earth. The side facing the Moon is closer and experiences a stronger gravitational attraction than Earth's centre. The far side is farther away and experiences a weaker attraction.
These differences in gravitational force stretch the Earth and its oceans along the Earth-Moon line. In a simplified model, this produces two tidal bulges: one on the side facing the Moon and another on the opposite side.
Why are there two tidal bulges?
The bulge facing the Moon is relatively intuitive. The Moon's gravitational attraction is stronger there, helping pull ocean water outward relative to Earth's centre.
The second bulge is on the opposite side of Earth. In a simplified Earth-Moon model, Earth and the Moon orbit their common centre of mass. The combination of this motion and the weaker lunar gravitational attraction on Earth's far side produces a second outward tidal effect.
The result is a tendency for the oceans to stretch along the line between Earth and the Moon rather than simply forming one pile of water directly underneath the Moon.
How do high and low tides work?
As Earth rotates relative to the tidal forcing, different coastal regions experience rising and falling water levels. High tide occurs when the tidal wave produces a higher sea level at a location, while low tide occurs around the lower part of the tidal cycle.
In a simplified model, two tidal bulges help explain why many coastlines experience two high tides and two low tides during a lunar day.
Real tides are more complicated. The oceans are interrupted by continents, vary enormously in depth and have coastlines and basins of different shapes. Tidal waves therefore form complex patterns through the ocean basins rather than following the Moon as two perfectly symmetrical bulges.
For a closer look at what the changing water level means at the coast, see high tide vs low tide.
How often do tides occur?
Many coastal locations experience two high tides and two low tides during a lunar day. A lunar day lasts approximately 24 hours and 50 minutes, slightly longer than the 24-hour solar day used by our clocks.
In a typical semidiurnal tidal pattern, consecutive high tides are therefore roughly 12 hours and 25 minutes apart. The exact timing varies by location.
Not every coast follows this pattern. Some locations experience one dominant high and one low tide each lunar day, while others have two high and two low tides of noticeably different heights.
Why are tides later each day?
The Moon moves around Earth in the same general direction that Earth rotates. After Earth completes a 24-hour rotation, the Moon has moved farther along its orbit. Earth therefore needs to rotate for roughly another 50 minutes before the same location reaches a similar position relative to the Moon again.
This is why the astronomical tidal cycle follows a lunar day of about 24 hours and 50 minutes rather than an exact 24-hour day.
What role does the Sun play in tides?
The Sun also exerts a tide-generating force on Earth's oceans. Its effect is weaker than the Moon's tidal influence, but the relative positions of the Sun, Moon and Earth change the tidal range throughout the lunar month.
Around new moon and full moon, the Sun, Moon and Earth are approximately aligned. Their tidal effects reinforce each other, generally producing a larger difference between high and low water. These are called spring tides.
Around the first and third quarter moon, the Sun and Moon act from roughly right angles relative to Earth. Their tidal effects partially counteract each other, generally producing a smaller tidal range. These are called neap tides.
Despite the name, spring tides have nothing to do with the season of spring. The complete cycle is explained in spring tide vs neap tide.
What is tidal range?
Tidal range is the vertical difference in water level between high tide and low tide. A coast with a large tidal range experiences a much greater change in water level than a coast with a small tidal range.
The range changes through the lunar cycle and can vary dramatically between locations. Local geography plays a major role in determining how strongly a coastline responds to the astronomical tide.
See what tidal range means for a closer look at why the difference between high and low water varies so much between coastlines.
Why are tides different around the world?
If Earth were covered by a uniform ocean of constant depth, tides would be much easier to describe. The real planet is divided by continents and contains ocean basins, shelves, channels, bays and estuaries of very different shapes and depths.
As tidal waves move through these basins, their speed and behaviour are influenced by water depth, the shape of the seabed and the boundaries formed by land. Coastlines can amplify, reduce or shift the timing of the tidal response.
This is why the Moon's position alone cannot tell you the exact time of high tide at a particular beach. Two places relatively close to each other can experience different tidal timing and range because the tidal wave interacts differently with their local geography.
Are there always two high tides and two low tides?
No. Two high and two low tides per lunar day are common, but they are not universal. Tidal patterns are generally described as semidiurnal, diurnal or mixed.
- Semidiurnal: usually two high tides and two low tides of broadly similar height during a lunar day.
- Diurnal: usually one high tide and one low tide during a lunar day.
- Mixed: usually two high and two low tides, but with significant differences in the heights of successive tides.
Which pattern dominates depends on the astronomical forcing and the way a particular ocean basin and coastline respond to it.
Where does the water go when the tide goes out?
The water does not disappear from the ocean. A falling tide represents water being redistributed as tidal waves move through the ocean system. While sea level is falling along one coastline, the tidal cycle is producing different water levels elsewhere.
On shallow beaches, mudflats and estuaries, even a modest vertical change in sea level can expose a large horizontal area of seabed. This makes it look as though an enormous volume of water has vanished from the coast, while in reality the water is being redistributed through the wider tidal system.
What is the difference between a tide and a tidal current?
A tide describes the periodic rise and fall of water level. A tidal current describes the horizontal movement of water associated with tidal forcing.
As water moves into an area during a rising tide, the associated current is commonly called a flood current. As water moves out during a falling tide, it is called an ebb current. The relationship between the timing of high or low water and the strongest or weakest tidal current is not identical at every location.
This distinction matters around channels, estuaries and other areas where moving water can be significant. Knowing that it is high tide does not automatically tell you the exact strength or direction of the local current.
Tidal currents are also only one type of ocean current. Wind, differences in water density and large-scale ocean circulation can move water for different reasons. See what causes ocean currents for the broader picture.
Why do tides matter at the coast?
Tides continuously change water depth and the position of the shoreline. That can alter which rocks, sandbanks and reefs are submerged, how much beach is exposed and how water moves through coastal channels and estuaries.
For surfing, the changing water depth can alter how waves interact with the seabed. Beach breaks, reef breaks and point breaks can therefore respond differently as the tide rises or falls. See beach break vs reef break vs point break for how the structure beneath and around a wave affects the way it breaks.
Tide is also an important part of interpreting a surf forecast alongside swell height, period, direction and wind. Understanding how to read swell and surf forecasts helps put those different variables together.
The astronomical forces behind tides are global and highly predictable. What happens when those forces interact with a particular stretch of coast is local. That combination is why tide predictions can calculate expected water levels far into the future, while the timing and range of the tide can still differ substantially from one coastline to another.