Tides vary from place to place because the height and timing of tidal water depend on far more than just the moon’s pull. Local geography, ocean depth, the shape of the coastline, and even Earth’s rotation all reshape how tidal energy arrives at any given shore. That’s why the global average tidal range is about 3 feet, yet the Bay of Fundy in Nova Scotia sees swings of up to 53 feet.
How the Moon and Sun Create Tides
The moon is the primary driver. Its gravitational pull tugs on Earth’s oceans, creating a bulge of water on the side of the planet facing the moon and a second bulge on the opposite side. These bulges are the high tides; the low points between them are the low tides. As Earth rotates through these bulges, most coastlines experience two high tides and two low tides roughly every 24 hours and 50 minutes (one lunar day).
The sun matters too, though less than you might expect given its enormous mass. It sits about 390 times farther away than the moon, which leaves it with a little less than half the moon’s tide-generating force. Twice a month, when the Earth, sun, and moon line up during full and new moons, their combined pull produces especially large “spring tides.” When the sun sits at a right angle to the moon, during the first and last quarter phases, the forces partially cancel out, producing smaller “neap tides.” These cycles are the same everywhere on Earth. What makes tides different from one coast to the next is everything that happens after that gravitational energy enters the ocean.
Coastline Shape Funnels Tidal Energy
One of the biggest reasons tidal ranges differ is the geometry of the coast. A wide, open beach receives tidal water spread across a broad front, so the rise and fall stays modest. A funnel-shaped bay, on the other hand, forces the same volume of incoming water into a progressively narrower channel, and the water has nowhere to go but up.
The Bay of Fundy is the textbook example. Its funnel shape squeezes Atlantic tidal water into a channel that narrows dramatically as it reaches the Minas Basin. The result: water levels can climb more than 15 meters (about 53 feet) between low and high tide, the highest tidal range on the planet. By contrast, a coastline that faces open ocean without a narrowing inlet typically sees tides of 1 meter or less.
Coastal geomorphology matters at smaller scales too. Changes in the shape of a river delta, the width of an estuary, or the orientation of a headland can all alter how tidal waves propagate inland. These effects are region-specific, which is why two beaches only a few miles apart can have noticeably different tidal ranges.
Continental Shelves and Ocean Depth
Tidal waves (not tsunamis, but the slow, continent-spanning waves of water that produce tides) travel faster in deep water and slow down in shallow water. When a tidal wave crosses from the deep ocean onto a wide, shallow continental shelf, it decelerates and its energy compresses vertically, pushing the water higher. Coastlines bordered by broad continental shelves tend to experience larger tides than coastlines where the ocean floor drops off steeply just offshore.
Wide shelves can also create a phenomenon called tidal resonance. If the shelf’s dimensions happen to match the natural period of the incoming tidal wave, the wave bounces back and forth in a way that amplifies the tide, sometimes dramatically. Even minor changes to the geometry of a basin or its connected river channels can shift this resonance enough to alter tidal properties significantly. This is one reason sea level rise, by subtly changing water depth over a shelf, can change local tidal ranges over time.
Earth’s Rotation Deflects Tidal Currents
Earth doesn’t just sit still while the moon pulls on its oceans. Its rotation creates the Coriolis effect, which deflects moving water to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This deflection prevents tidal bulges from simply tracking straight beneath the moon. Instead, tidal waves curve and rotate around fixed points in the ocean called amphidromic points.
At an amphidromic point, the tidal range is essentially zero. The water pivots around these nodes like water swirling slowly in a basin. The farther you are from an amphidromic point, the larger the tidal range becomes. Dozens of these points are scattered across the world’s oceans, and their positions explain why some mid-ocean islands experience almost no tide while certain continental coasts see large swings. Continental landmasses also interfere with the westward movement of tidal bulges, forcing tidal energy to pile up and redistribute unevenly.
Why Some Places Get One Tide, Others Get Two
Not every coast follows the familiar pattern of two high tides and two low tides per day. The Gulf of Mexico, for instance, experiences a diurnal tide cycle: just one high and one low per lunar day. Much of the U.S. Atlantic coast sees a semidiurnal cycle, with two highs and two lows of roughly equal height. Many Pacific coastlines get mixed tides, where there are still two highs and two lows but with noticeably different heights.
These patterns depend on the local ocean basin’s shape, depth, and connection to the open ocean. A semi-enclosed basin like the Gulf of Mexico responds to tidal forcing differently than an open Atlantic coastline because the geometry of the basin filters out one of the two daily tidal pulses. The result is that the number of tides you see each day is determined as much by where you stand as by the moon overhead.
Enclosed Seas and Inland Bays
Seas that connect to the open ocean through narrow straits tend to have very small tides. The Mediterranean, for example, has a tidal range measured in centimeters across most of its coastline because tidal water from the Atlantic can only enter through the Strait of Gibraltar. The narrow opening throttles the volume of water that can flow in and out with each cycle.
Some coastal bays are so sheltered they’re classified as non-tidal entirely. Laguna Madre in Texas and Pamlico Sound in North Carolina both have ocean inlets, but their water levels are driven more by wind than by gravitational tides. In these locations, weather can matter more than the moon.
Wind and Atmospheric Pressure
Weather adds another layer of variation. Persistent winds blowing along a coastline can push water toward or away from shore, raising or lowering the observed tide by several inches or more. On the eastern margins of the Atlantic and Pacific, longshore winds are one of the primary forces behind shorter-term sea level changes. Research on historical tide records found that wind forcing along coastlines correlated with sea level shifts from annual to multi-decade timescales, with some coastal stations showing sea level drops of roughly 80 millimeters (about 3 inches) tied to changes in wind patterns between the late 1800s and early 1900s.
Atmospheric pressure plays a role too. High pressure pushes the sea surface down; low pressure lets it rise, a relationship known as the inverted barometer effect. A strong storm system passing through can add or subtract several centimeters from the predicted tide. When a storm surge coincides with a spring high tide, the combined effect can produce water levels far beyond what either factor alone would create. This is why two coastal cities at the same latitude, exposed to the same moon, can report very different water levels on any given day.