Reading the tide
Why the tide range differs so much from coast to coast
The moon pulls on every coast alike, yet the water rises nearly thirty feet in one place and less than one in another.
The tide range is the height difference between low and high water, and in the United States it runs from 28.1 ft on an average day at Anchorage to 0.6 ft at Destin, Florida, on the stations of this site. The moon and sun raise only a small bulge in the open ocean, about two feet; what makes a coast's tide big or small is the shape of the water it sits on. A wide, shallow continental shelf slows the tide wave and lets it grow. A long bay that narrows toward its head, like Cook Inlet, funnels it higher. A basin whose natural period matches the tide, like the Gulf of Maine and the Bay of Fundy, resonates with it. And some places sit near an amphidromic point, where the tide wave rotates around a center of almost no rise. Barrier islands and narrow inlets then cut what reaches the bays behind them.
Compare the tide range of two stations
Myrtle Beach vs San Diego, average daily range
5.3 / 5.6 ft
| Ratio | 0.9 to 1 |
| Water moves per hour at Myrtle Beach | up to 1.3 ft |
| At San Diego | up to 1.4 ft |
Per-hour figure: a quarter of the range, the rule-of-twelfths peak (3 twelfths) in the middle hours.
Published by Radif Partners · How the tides are computed · predictions downloaded October 13, 2026
The same moon, very different tides
The table ranks a sample of the site's stations by their average daily range, measured from NOAA's predictions over October 1, 2026 to December 31, 2027, with the range reached on one day in ten. Every one of them feels the same gravitational pull of the moon and sun. The differences come from the ocean basins, the shelf and the shape of the coast.
| Station | Tide type | Average daily range | Spring range |
|---|---|---|---|
| Anchorage, AK | semidiurnal | 28.1 ft | 33.6 ft |
| Homer, AK | semidiurnal | 18.2 ft | 24.7 ft |
| Seattle, WA | mixed | 11.3 ft | 14.3 ft |
| Bar Harbor, ME | semidiurnal | 11.1 ft | 13.5 ft |
| Plymouth, MA | semidiurnal | 10.0 ft | 12.2 ft |
| Tybee Island, GA | semidiurnal | 7.1 ft | 8.8 ft |
| San Diego, CA | mixed | 5.6 ft | 7.7 ft |
| Hyannis, MA | semidiurnal | 3.6 ft | 4.5 ft |
| Honolulu, HI | mixed | 1.9 ft | 2.5 ft |
| Annapolis, MD | mixed | 1.2 ft | 1.5 ft |
| Destin, FL | diurnal | 0.6 ft | 0.9 ft |
A wave, not a bulge
School diagrams show two bulges of water following the moon around a smooth Earth. Real oceans have continents in the way, so the tide behaves instead as a set of very long waves that slosh around each ocean basin, steered by the Earth's rotation. In most basins these waves rotate around points of nearly zero range, called amphidromic points. Near such a point the tide hardly rises; far from it, along the edges of the basin, the range grows. Nantucket Sound lies close to one, which is why Hyannis averages 3.6 ft while Plymouth, less than twenty miles away on Cape Cod Bay, averages 10.0 ft.
Shelves that slow the wave and lift it
A tide wave travels fast in deep water and slows on a shallow shelf. As it slows, its energy concentrates in a shorter, taller wave, the way an ocean swell steepens before breaking. Where the continental shelf is broad, the tide grows. The Georgia Bight, between the Carolinas and northern Florida, has a wide shelf, and Tybee Island averages 7.1 ft. Southern Florida has a narrow shelf with the Gulf Stream close to shore, and Miami averages 2.5 ft. Hawaii, rising from deep water with almost no shelf, keeps the small open-ocean tide.
Funnels and resonance
A bay that narrows and shallows toward its head squeezes the incoming tide into a smaller cross-section, so the water rises higher. Cook Inlet in Alaska is the extreme case in the United States: by the time the tide reaches Anchorage, the average range is 28.1 ft, and a tidal bore runs up Turnagain Arm on the biggest tides. Resonance adds to the effect. Every basin has a natural period at which water sloshes back and forth; when it is close to the tide's 12 hours 25 minutes, each cycle reinforces the last. The Gulf of Maine and the Bay of Fundy form such a system, and the range grows steadily along the Maine coast toward Canada, reaching 11.1 ft at Bar Harbor.
Inlets and lagoons that cut the tide
Much of the Atlantic and Gulf coast is lined with barrier islands. The bays behind them exchange water with the sea only through a few inlets, and an inlet can pass only so much water in six hours. Behind a narrow inlet the tide shrinks and lags. Barnegat Bay in New Jersey is a textbook case: the inlet station at Long Beach Island averages 2.6 ft, and NOAA predicts the bay shore at about a tenth of the ocean's range, three hours late. The North Carolina sounds and Florida's Indian River lagoon behave the same way, and in all of them the wind often moves the water more than the moon.
Why the Gulf of Mexico tide is small and daily
The Gulf of Mexico is a nearly enclosed basin that responds strongly to the once-daily tidal forces and weakly to the twice-daily ones. On much of its northern coast the result is a small diurnal tide, often a single high and low a day, with ranges of one to two feet: Destin averages 0.6 ft. The Chesapeake Bay shows yet another pattern: its long, shallow body takes the tide most of a cycle to cross, and by Annapolis little height is left.
What this means for you
Range sets the pace. On a big-range coast, water climbs two or three feet an hour in the middle of the flood, flats that look endless at low tide fill fast, and currents run hard; plan walks and launches with a margin. On a small-range coast, the timing matters less than the weather, and a day of strong wind can do what the moon does in a week. The spring and neap tides guide explains why the range also changes from week to week at the same place.