Ocean tides are the regular rise and fall of sea levels caused primarily by the gravitational pull of the moon and, to a lesser extent, the sun. Most coastlines experience two high tides and two low tides roughly every 24 hours and 50 minutes, a period known as a lunar day. The difference between high and low water can range from less than a meter on mid-ocean islands to over 16 meters in extreme locations like Canada’s Bay of Fundy.
How Gravity Creates Two Tidal Bulges
The moon’s gravity is the main engine behind tides. On the side of Earth facing the moon, the ocean is pulled toward it because the gravitational attraction is strongest at that closest point. This creates a bulge of water that we experience as high tide.
What’s less intuitive is the second bulge on the opposite side of the planet. Because the moon’s pull is weakest at the far side of Earth, water there isn’t held as tightly. Inertia, the tendency of water to keep moving in a straight line, wins out over gravity and pushes water outward, away from Earth’s center. The result is a second bulge directly opposite the first. Between these two bulges, water levels drop, creating the low tides. As Earth rotates through these bulges over the course of a day, most coastal areas pass through two highs and two lows.
Three Tidal Patterns Around the World
Not every coastline experiences tides the same way. Geography, ocean depth, and the shape of the seafloor create three distinct patterns:
- Semidiurnal: Two high tides and two low tides of roughly equal height each lunar day. This is the pattern along much of the U.S. East Coast.
- Diurnal: Only one high tide and one low tide per lunar day. Parts of the Gulf of Mexico follow this cycle.
- Mixed semidiurnal: Two highs and two lows per day, but with noticeably different heights. One high tide may be significantly taller than the other. This is common along the U.S. West Coast.
Spring Tides and Neap Tides
The sun also tugs on Earth’s oceans, though with less force than the moon due to its much greater distance. Twice a month, the sun, moon, and Earth line up. This happens during both full moons and new moons. When they align, their gravitational forces combine to produce especially high highs and especially low lows, called spring tides. The name has nothing to do with the season; it comes from the idea of water “springing” upward.
About a week after each spring tide, the sun and moon sit at right angles relative to Earth. In this position, the sun’s pull partially cancels the moon’s, producing neap tides with a much smaller difference between high and low water. These occur around the first-quarter and last-quarter moon phases. So the tidal range at any given coast swells and shrinks on a roughly two-week cycle, entirely driven by the geometry of these three bodies.
Why Some Places Have Extreme Tides
The open ocean actually has fairly modest tides. What creates dramatic swings at certain coastlines is the shape of the land and seafloor. When tidal bulges hit wide continental shelves, the shallow water forces the tide higher. Funnel-shaped bays amplify this effect even further, squeezing a large volume of water into a narrowing space.
The Bay of Fundy in New Brunswick, Canada, holds the record for the world’s highest tidal range: water levels there swing between 3.5 meters (11 feet) at their calmest and 16 meters (53 feet) at their most extreme. This happens because of a phenomenon called tidal resonance. The bay’s length happens to be close to one-quarter of the tidal wave’s wavelength, so incoming water and reflected water reinforce each other, much like pushing a child on a swing at exactly the right moment. Each cycle adds energy, building the tide higher and higher.
The reverse is also true. Narrow inlets and very shallow water tend to dissipate tidal energy, and mid-ocean islands far from continental margins typically see tides of one meter or less.
How Tides Are Predicted
Modern tide predictions are remarkably accurate, which matters for shipping, fishing, and coastal safety. The predictions work by breaking the complex motion of tides into 37 individual cycles, each representing a different gravitational influence: the moon’s distance, the sun’s position, the tilt of the moon’s orbit, and dozens of other periodic motions. Each cycle is modeled as a simple wave with its own height and timing. Stacking all 37 waves on top of each other produces the detailed tide curve for a given location.
To calibrate these predictions, at least 30 days of water-level measurements are needed at a station to capture most lunar and solar cycles. A full year of data is required to observe all 37 components directly. This is why established tide stations, some of which have been collecting data for over a century, produce the most reliable forecasts. NOAA publishes these predictions as tide tables that mariners, surfers, and coastal planners use daily.
Life in the Intertidal Zone
The strip of shoreline between high and low tide marks, the intertidal zone, is one of the most physically demanding habitats on Earth. Organisms there are alternately submerged in saltwater and exposed to air, sun, and predators, sometimes within hours. The species that thrive here have evolved specific survival strategies.
Barnacles, mussels, and limpets cement themselves to rocks and seal their shells shut when the tide drops, trapping moisture inside. Shore crabs and hermit crabs are mobile enough to follow the retreating waterline downward. In the lower intertidal zone, which is exposed only during the lowest tides, organisms like seaweeds and sea anemones anchor firmly and flex with wave energy rather than resisting it. This gradient of exposure creates visible bands of life along rocky shores, with the hardiest species living highest up and more delicate communities living closer to the permanent waterline.
Tidal Energy as a Power Source
Because tides are predictable and relentless, they represent a potential source of renewable energy. Two main technologies exist. Tidal barrages work like hydroelectric dams: a barrier across a bay or estuary controls water flow through turbines as tides rise and fall. Tidal stream turbines are more like underwater wind turbines, with blades that spin as tidal currents flow past them. These can be installed individually or in connected rows called arrays.
Only a handful of commercial tidal power plants operate worldwide. The largest is the Sihwa Lake Tidal Power Station in South Korea. Off Scotland’s Orkney Islands, a 600-ton turbine called the O2 is projected to generate enough electricity for about 2,000 homes over its 15-year lifespan. The technology works, but high construction costs and limited suitable locations have kept tidal energy a small fraction of the global power mix. Sites need consistently strong tidal currents or large tidal ranges to justify the investment.