The Coast · The Tide Desk
Not Everyone Gets Two Tides a Day
The Moon pulls the same on every shore. Yet today one coast's sea climbs ten metres while another barely breathes — and on the Gulf of Mexico the water rises and falls just once a day, not twice. The tide isn't one thing; it's whatever the ocean basin does with the same small tug.
— reading the gauges —
metres of tidal swing — drawn to one shared scale↑ rising ↓ falling, right now
water now, within today's swing
the rest of today's range
strip below each gauge = today's rhythm: one hump or two
Each column is one tide gauge. Its height is the real thing — the metres between today's lowest and highest water, all drawn to the same scale, so a tall column genuinely holds more sea than a short one. The filled blue is where the water sits this minute; the arrow is which way it's going. The little curve under each gauge is today's tide traced out in time: most coasts draw two humps a day, the Gulf draws one.
Why one shore swings ten metres and another a few centimetres
The Moon's pull is almost the same everywhere — the difference between one coast and the next is a rounding error. What differs is the ocean. A tide is a wave the width of an ocean basin, and like water sloshing in a bathtub, each basin has a rhythm it prefers. Where the basin's natural sloshing period lines up with the Moon's roughly twelve-hour beat, the tide resonates and piles up; the Bay of Fundy and Alaska's Cook Inlet are tuned almost perfectly, and their water climbs by the height of a building twice a day. Where the basin is out of tune, the same Moon barely moves the surface.
Today the biggest swing on these gauges is — and the smallest is — — a factor of —, under the identical Moon. The water isn't being pulled harder in Alaska. The basin is just better at answering.
Stranger still is the count. Most coasts get two high tides and two lows a day — the Earth turns through two of the Moon's tidal bulges. But the Gulf of Mexico is a nearly closed basin whose resonance suppresses one of them, so places like Pensacola get a single, lazy rise and fall every twenty-four hours. Watch the strip under the Pensacola gauge: where the others draw two humps, it draws one. Same sky, half as many tides.
Source:
NOAA Tides & Currents (CO-OPS). Live six-minute water levels and today's high/low predictions for six gauges, datum MLLW, GMT. Fetched fresh on load and every five minutes; "today" is the next 24 hours.
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