Why are some tides huge?
The moon and sun raise the open ocean by only a foot or two. Whether the tide on a coast ends up tiny or enormous depends on the water it arrives in: a bay whose natural sloshing rhythm matches the tide's 12.4-hour beat, and that narrows and shallows as it goes, can multiply the tide ten times or more. That is why Anchorage can swing 30 feet in a day while Pensacola, on the Gulf of Mexico, moves a foot or so.
Today's tides, on the same scale
Live NOAA predictions for each place's local day, all drawn to the same vertical scale so you can compare them directly.
Try it: rock the bay
Every bay has a natural sloshing rhythm, set by its length and depth, like water rocking back and forth in a bathtub. The tide pushes on it twice a day. Slide the bay's length until its slosh matches the tide and watch what happens.
Tide at the head of the bay: about 1.0× the ocean tide. Out of rhythm: the pushes fight each other.
The three ingredients
1. The push: the moon and the sun
The moon's gravity pulls hardest on the side of Earth facing it and least on the far side, stretching the oceans into two bulges. Earth turns under them, so most coasts get two highs and two lows about every 24 hours and 50 minutes: a full cycle of 12 hours 25 minutes, with about 6 hours 12 minutes from high to low. The sun adds a smaller push. When the two line up at the new and full moon, tides are bigger (spring tides); at the quarter moons they partly cancel (neap tides).
2. The rhythm: resonance
That push is gentle, but it repeats like clockwork, and the ocean basins and bays it lands in have rhythms of their own. The Gulf of Maine and Bay of Fundy together slosh naturally about every 13 hours, very close to the tide's 12.4. Each tide arrives just in time to push the sloshing water higher, like pushing a child on a swing at the top of each arc. The result is the biggest tides on Earth: up to about 50 feet at the head of the bay in Nova Scotia, and 18 feet or more at Eastport, Maine.
3. The shape: funnels and shallows
As the tide moves into a bay that narrows and gets shallower, the same water is squeezed into less space and has to rise. Cook Inlet, Alaska, tapers for about 180 miles to Anchorage, and its tides grow all the way up. Wide, gently sloping continental shelves, like the one off South Carolina and Georgia, lift the tide more gently, which is why the Lowcountry sees 5 to 9 feet while southeast Florida, where the shelf is narrow, sees 2 to 3.
Why the Gulf of Mexico gets one small tide a day
The tide is really several rhythms added together: a strong twice-a-day part from the moon and sun, and a weaker once-a-day part that comes from the moon swinging north and south of the equator over the month. The Gulf of Mexico is almost enclosed, and its size and shape answer the once-a-day rhythm better than the twice-a-day one. So along much of the Gulf coast the tide rises once and falls once a day, and only by a foot or two. There, the wind often moves the water more than the tide does.
The Pacific coast sits in between: two highs a day, but of noticeably different heights. These “mixed” tides are why a Seattle tide table has a higher high, a lower high, a higher low and a lower low.
Common mix-ups
- “Tides come every 6 hours.” High to low takes about 6 hours 12 minutes, but the full cycle, high to high, is about 12 hours 25 minutes, and that is the beat bays resonate with.
- “The biggest tides are at the full moon.” Both the new and the full moon bring spring tides, and the biggest of all, king tides, come when one of them lines up with the moon's closest approach.
- “Big tides need a big bay.” Size alone isn't it. A bay's length and depth set its rhythm; a bay the wrong length for the tide can have small tides even if it is huge.
See it near you
- King tides for every coastal town: the year's highest and lowest tides, with dates.
- Every coastal town: today's tide, water temperature and the story of the place.
- Beach tides: the best low tides for walking and shelling this week.
Tide predictions from NOAA CO-OPS. The demo is a simplified model of a bay's first sloshing mode, not a forecast.