A tide is the regular rise and fall of ocean water caused by the gravitational pull of the moon and sun on Earth. The global average tidal range (the difference between high and low water) is about 3 feet, but local geography can push that number dramatically higher, up to 53 feet in Canada’s Bay of Fundy. Tides follow predictable cycles, which is why coastal communities have been forecasting them for centuries.
How the Moon and Sun Create Tides
The moon’s gravity is the primary engine behind tides. On the side of Earth facing the moon, the ocean is pulled toward it, creating a bulge of higher water. On the opposite side of Earth, something less intuitive happens: inertia dominates over the moon’s weaker pull (weaker because that side is farther away), and water moves slightly outward, forming a second bulge. These two bulges are why most coastlines experience two high tides per day rather than one.
As Earth rotates, different parts of the planet pass through these bulges, cycling between high and low water roughly every 12 hours and 25 minutes. The extra 25 minutes comes from the moon’s own movement in orbit, which means high tide arrives a little later each day.
The sun also exerts a gravitational tug on Earth’s oceans, but because it’s so much farther away, its tide-generating force is less than half the moon’s. Still, the sun’s influence is strong enough to either amplify or dampen the moon’s effect depending on alignment, which creates the monthly rhythm of stronger and weaker tides.
Spring Tides and Neap Tides
Twice a month, the Earth, sun, and moon line up. This happens during both the full moon and the new moon. When they align, their gravitational forces combine to produce especially high high tides and especially low low tides. These are called spring tides (the name has nothing to do with the season; it comes from an old word meaning “to leap”).
About a week after each spring tide, the sun and moon sit at right angles to each other relative to Earth. In this position the sun’s pull partially cancels out the moon’s, producing a smaller tidal range. These moderate tides are called neap tides, and they occur around the first-quarter and last-quarter moon phases. The repeating pattern of spring and neap tides every two weeks shapes everything from fishing schedules to coastal construction timelines.
Three Types of Tidal Cycles
Not every coast experiences tides the same way. There are three main patterns found around the world:
- Semidiurnal tides produce two roughly equal high tides and two roughly equal low tides each day. This is the most common pattern, found along much of the U.S. Atlantic coast.
- Diurnal tides produce just one high tide and one low tide per day. Parts of the Gulf of Mexico follow this cycle.
- Mixed semidiurnal tides produce two high tides and two low tides per day, but they’re unequal in height. One high tide is noticeably higher than the other, and one low tide is noticeably lower. This pattern is common along the U.S. Pacific coast.
The type of tide a coastline gets depends on the shape of the ocean basin, the width of the continental shelf, and the latitude.
Why Tidal Range Varies So Much by Location
The 3-foot global average obscures enormous local variation. When oceanic tidal bulges hit wide continental shelves, the shallow water amplifies the tide’s height. Funnel-shaped bays concentrate incoming water into an ever-narrower space, pushing it higher still. The Bay of Fundy in Nova Scotia combines both effects: a broad continental shelf funnels water into a tapering bay, producing tidal swings over 15 meters (about 53 feet).
The opposite is also true. Mid-ocean islands far from continental margins typically see tides of 1 meter or less. And narrow inlets or very shallow water can dissipate tidal energy, producing smaller tides than you’d expect given the geography nearby.
Tidal Bores
In a few places around the world, an incoming tide funnels into a narrow, shallow river estuary and forms a visible wave that travels upstream. This wave is called a tidal bore. It typically requires a tidal range of at least 4 to 6 meters and a funnel-shaped estuary to form. Tidal bores occur on rivers like the Qiantang in China, the Severn in England, and the Petitcodiac in Canada. Some are large enough for surfers to ride for miles.
How Tides Shape Coastal Ecosystems
The strip of shoreline between high and low tide, called the intertidal zone, is one of the most challenging habitats on Earth. Organisms living there must survive being alternately submerged in saltwater and exposed to air, sun, and predators. Life in this zone arranges itself in four distinct bands based on how well each species tolerates these swings.
The spray zone, highest on the shore, is only submerged during the highest tides or storms. Below it, the high intertidal zone floods at the peak of daily high tides but stays dry for long stretches in between. Creatures here, like barnacles and periwinkles, survive by sealing moisture inside their shells. The middle intertidal zone is covered and uncovered twice a day, supporting a wider variety of life including sea stars and anemones. The low intertidal zone is almost always underwater, exposed to air only during the lowest spring tides, and hosts the most diverse community of plants and animals.
How to Read a Tide Table
Tide tables are published for thousands of coastal locations and are freely available online through national ocean agencies. A standard table lists the date, the times of high and low tides (usually split into a.m. and p.m.), and the predicted water height at each point, measured in feet or meters above or below a baseline sea level. Many tables also include sunrise and sunset times and the current moon phase, which helps you anticipate spring and neap cycles.
If you’re planning a beach visit, a fishing trip, or a boat launch, the key numbers are the time of low tide and the height at low tide. A negative number means the water will drop below the average low-water mark, exposing areas that are normally submerged. These extreme low tides, common during spring tide periods, are ideal for tidepooling but can make shallow harbors temporarily impassable for boats.
Tidal Energy
Because tides are predictable years in advance, they’re an attractive source of renewable energy. Two main technologies exist. Tidal range systems use a dam, called a barrage, built across an estuary. Water flows through turbines as the tide rises and falls, converting the height difference into electricity. Plants in France (240 megawatts), Canada (20 MW), and China (about 5 MW) have operated since the 1960s and 70s, proving the concept works at scale.
Tidal stream systems take a different approach, placing underwater turbines directly in the path of strong tidal currents to capture kinetic energy, much like underwater wind turbines. Dozens of new device designs emerged between 2006 and 2013. Both technologies face a common limitation: because tides ebb and flow on a fixed schedule rather than running continuously, the load factor (the percentage of time a plant generates at full capacity) sits around 25%, which raises the cost of the electricity produced.