Tidal waves are formed by the gravitational pull of the moon and sun on Earth’s oceans. The moon’s gravity tugs on seawater, creating bulges of water that travel around the planet as it rotates, producing the regular rise and fall of tides you see at the coast. These are true tidal waves in the scientific sense, though many people use the term when they actually mean tsunamis, which are caused by something entirely different.
Tidal Waves vs. Tsunamis
The U.S. Geological Survey draws a clear line between these two phenomena. A tidal wave is a shallow water wave driven by gravitational interactions between the sun, moon, and Earth. A tsunami is triggered by sudden geological events: underwater earthquakes, volcanic eruptions, or submarine landslides. The two are completely unrelated. Scientists generally avoid the term “tidal wave” altogether because it was historically misapplied to tsunamis, creating lasting confusion. If you searched this term wondering about the massive, destructive waves that hit coastlines after earthquakes, those are tsunamis. What follows explains how the actual tide-driven waves work.
How the Moon Creates Two Bulges
The moon is the primary driver of tides, and the mechanism is surprisingly intuitive. The moon’s gravitational pull is strongest on the side of Earth closest to it. Water, being fluid and free to move, gets pulled toward the moon, forming a bulge of higher water on the near side. That bulge is a high tide.
A second bulge forms simultaneously on the far side of Earth, directly opposite the moon. Here, the moon’s gravitational pull is at its weakest. Inertia dominates: the water on that far side tends to keep moving in a straight line, pulling slightly away from Earth and creating another bulge. So at any given moment, there are two high-tide bulges on opposite sides of the planet and two low-tide troughs in between.
As Earth rotates through these bulges over the course of a day, most coastlines experience two high tides and two low tides. This is why tides follow a roughly predictable schedule, shifting about 50 minutes later each day as the moon advances in its orbit.
The Sun’s Supporting Role
The sun also exerts a tidal force on Earth’s oceans, but because tidal force depends on distance (not just mass), the sun’s contribution is only about 40% as strong as the moon’s. That still matters. When the sun and moon work together, their combined pull produces noticeably bigger tides. When they work against each other, tides become more moderate.
Spring Tides and Neap Tides
Twice a month, during the new moon and full moon, the sun, moon, and Earth line up. The sun’s gravitational pull adds to the moon’s, creating spring tides (sometimes called “king tides”). During spring tides, high tides are higher and low tides are lower than average. The name has nothing to do with the season; it comes from the idea of the tide “springing forth.”
During the first and third quarter moons, when the moon appears half full, the sun and moon sit at right angles relative to Earth. The sun’s tidal bulge partially cancels out the moon’s, producing neap tides. High tides are lower and low tides are higher than usual, creating a narrower overall range. This alternating pattern of spring and neap tides repeats in a reliable two-week cycle.
Three Global Tidal Patterns
Not every coast experiences tides the same way. There are three basic patterns distributed across the world’s oceans:
- Semidiurnal tides: Two highs and two lows each day, with relatively similar heights. This is the most common pattern and dominates much of the east coast of North America.
- Diurnal tides: A single high and a single low per day. These occur in places like the Gulf of Mexico, the Java Sea, and the Tonkin Gulf.
- Mixed tides: Two highs and two lows per day, but with significant differences in height between successive highs or lows. Parts of the U.S. West Coast experience this pattern.
Which pattern a location gets depends on the shape of the ocean basin, the latitude, and how the tidal energy resonates in that particular body of water.
Why Some Places Have Extreme Tides
Earth’s average tidal range is about 3 feet. But local geography can amplify that dramatically. When oceanic tidal bulges hit wide continental shelves, the shallow water forces the wave energy upward, increasing the tide’s height. Funnel-shaped bays are especially effective at this: as the incoming tide is squeezed into a narrowing channel, the water has nowhere to go but up.
The Bay of Fundy in Nova Scotia is the most extreme example on the planet. Near Wolfville, in the Minas Basin, the difference between high and low tide can reach 53 feet (16 meters). That’s a five-story building’s worth of vertical water movement, driven entirely by the same gravitational forces that produce a modest 3-foot tide elsewhere. The bay’s funnel shape and the natural resonance of water sloshing back and forth within it create this extraordinary range.
Tidal Bores: When Tides Become Visible Waves
In certain rivers and estuaries, an incoming tide can actually form a visible wave that travels upstream against the current. This is called a tidal bore, and it’s the closest thing to a literal “tidal wave” you can witness. Tidal bores form when a large tidal range (typically exceeding 4 to 6 meters) pushes into a narrow, funnel-shaped estuary. The incoming tide overtakes the slower-moving river water and rolls upstream as a defined wave front, sometimes for miles.
Famous tidal bores include those on the Severn River in England, the Qiantang River in China, and the Petitcodiac River in New Brunswick, Canada. Some are gentle ripples; others can reach several feet in height and attract surfers.
Storm Surge Is Something Different
One more distinction worth knowing: storm surge is not a tidal wave either. Storm surge is the abnormal rise in sea level caused by a storm’s winds and low atmospheric pressure pushing water toward shore. When a storm surge coincides with a high astronomical tide, the combined water level is called a storm tide, and the flooding potential increases significantly. But the surge itself has nothing to do with the moon or sun’s gravity.