How Tide Predictions Work: Reading the Next High and Low

A tide table can tell you the exact minute of next Tuesday's high tide, weeks before it happens — and be right. That's only possible because tides, unlike weather, are driven by something almost perfectly predictable: the clockwork pull of the moon and sun on the ocean. The tide layer puts the next high or low for major coastal stations on the map, so you can read the rhythm of the sea at a glance.

Why tides can be predicted so far ahead

The moon's gravity (and, to a lesser degree, the sun's) tugs the world's oceans into bulges that the spinning Earth carries past every coastline. Because the motions of the moon and sun are known with extraordinary precision, the resulting rise and fall at any given harbor can be calculated years in advance.

Each location has its own fingerprint — a set of harmonic constituents that capture how that particular bay or inlet responds to the astronomical pull. Feed those into the math and out come tide tables far into the future. That's why the tide layer shows a prediction, not a live gauge reading: the timing is astronomy, dependable enough to schedule a boat launch around. (Real-world wind and storm surge can raise or lower the actual water a bit, but they don't change the underlying clock.)

High, low, and the range between

Most coasts see two high tides and two low tides a day — a semidiurnal rhythm, each full cycle about 6 hours and 12 minutes apart. The difference in height between high and low is the tidal range, and it's the number that matters most in practice:

• A large range means powerful tidal currents and mudflats or rocks that cover fast — important for boaters, anglers, surfers and anyone walking a beach.
• A small range means gentler, slower change.

On the map, each station's dot shows whether the tide is rising toward the next high (blue) or falling toward the next low (green), with the time of that next turning point.

Why some coasts have wild tides

The same astronomical pull produces dramatically different tides depending on local geography. The Bay of Fundy funnels water into a narrowing bay and sees ranges over 15 meters — the biggest on Earth. The nearly enclosed Mediterranean barely moves a foot. Coastline shape, water depth and basin resonance all amplify or damp the tide, which is exactly why every harbor needs its own prediction.

Spring and neap tides

There's a slower rhythm on top of the daily one. At new and full moon, the sun and moon line up and their pulls combine into the largest ranges — spring tides (the word means "springing forth," not the season). Around the half moons, the pulls work against each other and ranges shrink into neap tides. So the tidal range itself swings over a roughly two-week cycle following the moon's phase.

Reading it on the map

The tide layer pairs naturally with the other ocean layers:

• Add Sea State (Waves) to see how swell and tide combine along a coast — big surf on a rising spring tide is a very different beach than calm water at neap low.
• Add Sea Surface Temperature for the broader state of the coastal ocean.
• Click any station for the next high or low, its height, and a link to NOAA's full tide table for that harbor.

It's one of the calmest layers on the map — no drama, just the steady breathing of the sea, scheduled to the minute.