Tides, Explained

Springs, neaps and the fortnightly heartbeat

The moon's gravity stretches the ocean into two bulges — one toward the moon, one away — and the spinning Earth carries each coast through them, which is why most places see two high waters and two low waters in a lunar day of about 24 hours 50 minutes (each day's tides arrive ~50 minutes later than yesterday's). The sun raises its own, smaller pair of bulges, and the two systems beat against each other: when moon and sun align — just after new and full moon — their bulges stack and the range peaks in spring tides (nothing to do with the season; think 'springing up'). When they sit at right angles, at the quarter moons, they partly cancel: neap tides, the fortnight's laziest water.

Geography then remixes everything. Semidiurnal coasts (two roughly equal tides — the Atlantic pattern) are the textbook case; parts of the Gulf of Mexico and Southeast Asia get one tide a day (diurnal), and much of the Pacific runs 'mixed' — two tides of clearly different sizes. Local shape rules local drama: funnel-shaped bays amplify range (the Bristol Channel and Bay of Fundy exceed 12–15 m on big springs) while enclosed seas like the Mediterranean barely see 30 cm.

Reading heights: everything hangs from chart datum

A tide table's numbers only mean something against a zero, and that zero is chart datum — Lowest Astronomical Tide on most of the world's charts, Mean Lower Low Water in the US (the full story lives in GPS vs the nautical chart). The working sum is one line: depth now ≈ charted depth + height of tide. A drying rock marked 1.8 m (underlined figures dry above datum) is covered by 2 m of water when the table says the tide stands at 3.8 m.

For the hours between high and low water, sailors estimate with the rule of twelfths: in the six hours of a (roughly sinusoidal) tide, the water moves 1, 2, 3, 3, 2, 1 twelfths of its range hour by hour — hardly anything in the first hour, half the range in the middle two. It's an approximation with real exceptions (some ports have double highs and long stands), but it turns a tide table into a running depth estimate with no electronics at all. This page deliberately carries no live tide feed — predictions belong to the official sources (national hydrographic offices such as the UKHO and NOAA publish them) and to your locality, not to a generic worldwide widget.

Streams, currents and the gift of slack water

The vertical rise and fall drives horizontal flow: tidal streams, which flood one way, ebb the other, and reverse on the tide's rhythm — unlike ocean currents (the Gulf Stream kind), which flow on regardless. Streams accelerate wherever the sea squeezes — headlands, channels, reef passes — and their speed scales with the tide's range, so a passage that runs at 2 knots on neaps can run at 5 on springs, which is the difference between an inconvenience and a conveyor belt for a diver or a small boat.

Slack water is the pause when the stream turns, and it is the most bookable gift in the sea's schedule: dive guides at current-swept sites (think Komodo's pinnacles or a spring-tide wreck like the Yongala in our destinations guide) plan entire days around it. The trap is assuming slack coincides with high and low water — in many places the stream keeps running well past the turn of height, offset by hours in some channels. Local tidal-stream atlases and dive operators know the offset; the tide table alone doesn't.

Two safety corollaries close the subject. Wind against stream stacks water into short, steep, dangerous seas — the classic overfalls off headlands. And on any shore, an incoming spring tide moves fastest in mid-flood (those middle twelfths): the sandbank route that took twenty dry minutes on the way out can be thigh-deep on the way back.