Tides Explained

Tides are caused by the gravitational pull of the Moon and the Sun on Earth's oceans, combined with the orbits of those bodies and our planet's rotation. The Moon is the main driver, with roughly twice the influence of the Sun despite being much smaller. This is because it's so much closer to us.

These forces work in opposition. The Moon's gravity pulls everything on Earth towards it, with the force being stronger on the side nearest the Moon. But the Earth and Moon rotate together (once a lunar month) around a shared centre of mass, and this pushes everything on Earth away from the Moon. The combination of these forces pulls water towards the Moon on one side of the Earth, and pushes it away the other side.

The schematic diagram above depicts the interaction between the Earth and the moon which explains the lunar tides. The gravitational influence of the sun on the Earth’s surface manifests itself in a similar way giving rise to solar tides.

However, these forces are constantly changing as the Earth turns each day. The speed at which water can move at any location is limited by the depth of the ocean there, meaning that water cannot move quickly enough to keep up with the changing forces.

The water doesn't disappear. As the tide ebbs at one location, that water simply moves across the ocean to wherever high tide is happening. The bulges created by the Moon and the Sun constantly shift around Earth as our planet rotates, and the water follows.

In shallow coastal waters and estuaries, large areas of seabed become exposed during low tide as the tide “goes out”, and water moves to other parts of the ocean.

In most locations, high tides occur roughly every 12 hours and 25 minutes. This timing relates to the lunar day, which is how long it takes for a specific spot on Earth to rotate back to the same position relative to the Moon.

The extra 25 minutes accounts for the Moon's own movement. While Earth completes one spin, the Moon has moved a little further along in its orbit. Earth needs to rotate that bit extra to catch up.

As global sea levels rise due to warming oceans and melting ice, tidal flooding patterns are changing. According to the Intergovernmental Panel on Climate Change, global sea level rose around 20 cm between 1901 and 2018. Sea-level rise is accelerating and is expected to rise by at least 60 centimetres in the 21st century. [Citation Palmer et al 2026].

As the baseline sea level rises, the same tides reach higher up the shore. Areas that previously flooded only during extreme events may start flooding during ordinary high tides. What were once extreme events can become routine.

Recent research from the National Oceanography Centre shows that UK sea level is rising faster than the global average, with two-thirds of the observed sea level rise since the 1900s happening in just the last three decades. This accelerating rise is increasing both the frequency and magnitude of extreme water levels around the UK coastline, meaning higher tides and extreme still water levels will become more common and raising the baseline for coastal flood risk.

Rising seas can also change tidal ranges in some locations by altering water depths and allowing tides to push further inland up rivers and estuaries.

Tides follow predictable patterns, but they never repeat exactly. This is because the Moon and the Sun are constantly shifting their positions relative to Earth. Lunar months don't divide neatly into calendar years, and the Moon's orbit itself wobbles over time. Every 18.6 years, for instance, we experience larger than average tides due to a pattern called the lunar nodal cycle.

While the underlying physics means tides could theoretically repeat over extremely long periods, by then Earth's own orbital characteristics would have changed. So in practical terms, each tidal cycle is unique.

If you check tide times regularly, you'll notice they shift by roughly 50 minutes each day. This happens because the Moon orbits Earth, moving a little further along each day. After Earth completes one full spin, it needs to rotate a bit more (about 50 minutes' worth) to catch up with where the Moon has moved to.

Although tides never repeat exactly, scientists can still predict them with remarkable accuracy many years in advance. This is because the movements of the Moon and the Sun follow well-understood orbital mechanics. The National Oceanography Centre  produces tide tables extending years into the future, or into the past, using these calculations.

However, actual sea levels can differ from predictions due to weather conditions such as air pressure, wind and storm surges. The Met Office models these factors, while the Environment Agency monitors coastal conditions and issues flood warnings when needed.

The actual physics of tides and surges has been well understood for decades, there haven't been many changes in the physics. But the weather changes, the coast evolves, sand moves, and sea-level rises. These all impact on tidal prediction. So, we need to make good quality measurements of tides in the changing conditions to constantly update our information and make accurate predictions.

Coastal geography creates complex tidal patterns. Land gets in the way of moving water, and coastline shape and bathymetry create delays. Bathymetry refers to the underwater equivalent of hills and valleys: the varying depths and contours of the seabed.

Just as landscape shapes how water flows across land, the underwater terrain influences how tides move through our oceans. Every stretch of coast has unique features, producing unique tidal patterns.

When tides move from deep ocean onto shallower coastal shelves, they slow down and their height increases. Some locations experience dramatic amplification because of a phenomenon called resonance, where the natural rhythm of a bay or inlet happens to match the tidal rhythm.

The Bay of Fundy in Canada provides the most spectacular example, boasting the world's highest tides. The bay's funnel shape and length happen to match the Atlantic tidal cycle almost perfectly, creating tidal ranges of up to 16 metres. That's roughly the height of a five-storey building.

The "Coriolis effect" from Earth's rotation also influences tidal currents. This effect is a consequence of living on a spinning planet: as water moves across Earth's surface, it curves rather than travelling in a straight line. In the northern hemisphere, moving water curves to the right; in the southern hemisphere, it curves to the left. This helps create the complex, swirling tidal patterns we observe in our oceans.

The forces that create tidal bulges need a large body of water to produce visible effects. In the vast oceans, the difference in gravitational pull across the water is significant. In smaller bodies like lakes, this difference is too small to notice.

The Great Lakes of North America do technically have tides, but they measure only a few centimetres. Another factor is that for tides to build up, a water body's natural rhythm must match the tidal rhythm of about 12 hours. Most lakes have much shorter natural rhythms, so tides can't gain momentum.

Tides can significantly affect estuaries and rivers, sometimes pushing their influence many kilometres inland. During flood tide, seawater moves upstream, raising water levels and sometimes reversing the river's flow entirely. During ebb tide, the river flow combines with the outgoing tide, creating strong seaward currents.

Some rivers experience a remarkable phenomenon called a tidal bore, where the incoming tide forms a wave that travels upstream. The Severn Bore in the UK is famous for this, attracting surfers who ride the wave for miles. The Pororoca on the Amazon River is another dramatic example. Research published in the Journal of Fluid Mechanics has extensively documented the complex hydraulics of these fascinating phenomena.

Tidal influence also creates zones of brackish water (a mix of fresh and salt water) that support unique wildlife communities.

Tides work the same way in the northern and southern hemispheres. The gravitational pull of the Moon and the Sun affects both equally. However, specific tidal patterns depend on local coastal geography and underwater terrain rather than which hemisphere you're in.

Tides shape life along our coastlines in remarkable ways.

The intertidal zone, that strip of shore between high and low tide marks, is one of the most dynamic habitats on Earth. Creatures living here must cope with being submerged in seawater one moment and exposed to air, sun and predators the next. Species such as barnacles, mussels and seaweeds have evolved remarkable ways to thrive in these ever-changing conditions.

Beyond the intertidal zone, tides influence marine life in several important ways:

• Feeding patterns: Many marine animals time their activities to the tidal cycle, feeding most actively when prey becomes accessible or currents bring food their way.
• Reproduction: Some species synchronise their breeding with particular moon phases and spring tides, releasing eggs or larvae when conditions give their offspring the best chance of survival.
• Nutrient transport: Tidal currents act as a delivery system, carrying nutrients and food throughout coastal waters and boosting the productivity of these vital habitats.

According to NOAA, understanding tidal influences is essential for effective marine conservation. The Marine Conservation Society emphasises that protecting intertidal habitats is crucial for maintaining biodiversity along UK coastlines.

A tidal bore is a sudden, steep-fronted surge of water that travels upstream in a river or estuary when an incoming tide forces seawater into a narrowing, shallow channel. Instead of the tide rising gradually, it forms a moving "wall" or wave of water. One of the best-known UK tidal bores is on the river Severn. The Severn Bore can reach up to 2 metres high. It travels at around 10 to 15 mph and can even be surfed under the right conditions! It occurs because the Severn Estuary has one of the largest tidal ranges in the world.

Tidal wave refers to the movement of water associated with the regular tidal cycle. These aren't visible as individual waves in the open ocean but manifest as the gradual rise and fall in sea level that we recognise as tides. The tidal wave is not a tsunami, but these two names are often confused in the media.

As tides approach the UK from the south-west, it splits into 3 branches, one west of Ireland, one up the Irish sea, and one into the English Channel. After the western branch reaches Scotland it causes another to sweep anticlockwise round the North Sea, so the tide arrives several hours later in East Anglia than in Edinburgh.

This means that despite the underlying gravitational drivers of the tide being similar, times and heights of tides can be completely different along different parts of the UK: usually when the tide is high in Cornwall, it's low in Liverpool, although they're only a few hundred miles distant.

Tides and other variations in the height of the sea are recorded using tide gauges – more properly called water level recorders as they can also be used in lakes and rivers. The technologies used have changed over the years, but can be separated into two categories.

Firstly, something can be placed in the water that measures the quantity of water above it. Most simply, this can be a simple tide staff which an observer can read off the sea surface height (much harder than in sounds in rough conditions). More modern approaches revolve around placing a sensor on the sea bed which measures the pressure exerted by the column of water above it, which can be converted into a height.

The second approach is to measure the distance down to the water from a spot on land. For many years the standard technique was the float gauge, where the height of an object floating on the surface of the water was monitored by connecting it to a pen on a rotating chart by means of a wire. More modern approaches rely on radar or acoustic sensors which measure the time taken to bounce a pulse off the water, and convert that to a height.

Image: UK Tide Gauge Network (please note: Port Ellen tide gauge decomissioned December 2018, Moray Firth tide gauge decomissioned August 2017, Harwich tide gauge superseded Felixstowe January 2016, Portbury (south of River Avon) replaced Avonmouth in 2011).