High tide and low tide

Two bulges and a planet turning through them

The moon pulls hardest on the side of the Earth that faces it and least on the far side. That difference stretches the oceans into two bulges, one under the moon and one opposite. As the planet spins, a given harbor passes through both bulges and both troughs between them, which gives the familiar pair of highs and pair of lows. The sun does the same thing with a bit less than half the moon's strength, because it is so much farther away, and the two effects add or partly cancel depending on how the bodies line up.

That picture is the textbook start. Real oceans are bounded by continents, so the tide behaves more like a wave sloshing around each ocean basin than like a fixed bulge. NOAA's tides and water levels tutorial describes both views; for reading a tide page, the useful consequence is that every coast has its own timing and its own height.

Why tomorrow's tides come later

The moon travels along its orbit in the same direction the Earth turns, so the planet needs extra time each day to bring a harbor back under it: about 24 hours and 50 minutes. Two highs share that lunar day. At Boston the average gap between one high and the next over the prediction period is 12 hours 25 minutes, which is why a high at 7 AM on Monday drifts to around 8 AM by Tuesday morning and past 1 PM by the weekend.

Some places break the two-a-day habit. At Grand Isle the once-daily constituents dominate, and 93% of days bring a single high. A single tide still slips later each day, but by the full 50 minutes on one event instead of splitting it between two.

High water rarely comes when the moon is overhead

If the bulge sat right under the moon, high tide would match the moon's passage everywhere. It does not. Friction with the seabed and the time a tide wave needs to cross a continental shelf, enter a bay or climb a river shift the arrival by hours, and the delay is different at every gauge. Two harbors an hour's drive apart can have tides several hours out of step. That is the reason tide times are always given for a named station, and why the corrections NOAA publishes for subordinate stations are counted in minutes from a reference gauge.

How high is high tide?

There is no single answer, even at one place. At Boston an ordinary high reaches 9.7 ft above Mean Lower Low Water, NOAA's Mean High Water datum, the average of every high over its 19-year epoch, stands at 9.8 ft, and Mean Higher High Water, the average of the taller high of each day, at 10.3 ft. Around a new or full moon the water climbs further; the tallest prediction of this period is 11.8 ft on November 26, 2026. Those biggest days are the subject of the king tides guide.

The swing between high and low follows a two-week pulse as well. When the sun and moon line up, at new and full moon, Boston's range opens to about 12.1 ft on the biggest days; at the quarter moons, when they pull at right angles, it closes to about 7.9 ft. So the same harbor can offer a high that floods the lowest parking lot by the seawall one week and one that stays politely below the docks the next. The spring and neap tides guide follows that pulse with a daily coefficient, and a glance at it tells you whether today's numbers sit at the top or the bottom of the month.

On the Pacific side, consecutive highs are often unequal. In Seattle the two highs of a day differ by 1.8 ft on average, so "high tide today" may mean a modest morning peak followed by a much fuller evening one, or the reverse a week later.

Low tide, the half people plan around

Beachcombers, clammers and anyone launching a deep keel care more about lows than highs. Boston's ordinary low sits near 0.6 ft, close to the chart zero. Because that zero is an average, many lows dip below it: the lowest prediction there is -1.8 ft on January 23, 2027. Seattle's lows are even more unequal than its highs, 6.3 ft apart on an average day, so one low barely uncovers the beach while the next drains it. The negative low tides guide explains when the deepest ones come.

Rising and falling do not always take the same time

In open water the climb and the drop last about the same. In shallow estuaries the tide wave deforms: its crest travels faster than its trough, so the water can rise in less time than it falls. Anchorage, at the head of Cook Inlet, shows it clearly. Its average rise takes 5 hours 45 minutes, its average fall 6 hours 41 minutes. The opposite happens at Hyannis on Nantucket Sound, where the rise lasts 6 hours 48 minutes and the fall 5 hours 37 minutes. A lopsided tide means the incoming water arrives faster than a symmetric picture would suggest, which matters on flats.

Getting today's high and low tide times

The quickest route is the box above: choose a station and it gives the height right now, whether the water is rising or falling and how long until the next high and the next low. Every station near you has its own page with seven days of times, sunrise and sunset, and a planner. Three habits keep those numbers useful:

• Pick the gauge on the same water as you, ocean side or sound side, since inlets delay and shrink the tide.
• Read the times in the station's zone, which the page states, especially when planning from another state.
• Treat the heights as the astronomical prediction. Onshore wind and falling pressure pile water up; offshore wind and high pressure hold it down.

These are predictions, not live measurements, and they are not for navigation. The tide chart guide shows how to read the curve between the times.