03Tide School · Tide cycles
Why California gets two unequal tides a day
Mixed semidiurnal tides, the 24 h 50 min lunar day, spring and neap cycles, king tides, and the tide wave that sweeps up the coast from San Diego to Crescent City.
Updated Oct 4, 2026 · 7 min read · 5 figures
The short answer
Most of the California coast has mixed semidiurnal tides: usually two high tides and two low tides every lunar day (about 24 hours 50 minutes), and the two highs and two lows are noticeably different heights. That is why tide times shift roughly 50 minutes later each day.
From the tide questions on the home page. The long answer follows.
On this page
Look at a week of California tides and two things stand out. There are usually two highs and two lows a day, and they are not alike: one high is much higher than the other, and one low dips far lower. Every day the whole pattern also shifts about 50 minutes later. This page explains both, and then follows the longer rhythms layered on top: spring and neap tides, king tides, and the tide wave that travels up the coast.
Why twice a day
The moon pulls on the whole Earth, but not evenly: the ocean on the side facing the moon is pulled a little harder than Earth’s center, and the ocean on the far side a little less. The result is two bulges of water, one under the moon and one on the opposite side. As Earth turns, any coastline passes through both bulges, and gets two high tides with two lows between them.
The lunar day
The moon is not standing still. It orbits Earth in the same direction Earth spins, moving about 12 to 13 degrees along its orbit each day, so Earth has to turn a little extra to bring a coastline back under it. That extra turn takes about 50 minutes. A lunar day is about 24 hours 50 minutes, the semidiurnal tide repeats every 12 hours 25 minutes, and each tide arrives roughly 50 minutes later than it did the day before.
The sun raises its own, smaller pair of bulges, a bit less than half as strong as the moon’s, on a cycle of exactly 24 hours. Most of what makes one week of tides different from the next is the moon and sun pulling together or apart.
Why the two tides are unequal
If the moon always sat over the equator, its two bulges would be the same size wherever you stood and the two daily highs would match. But the moon’s orbit is tilted, and over each month it swings from about 18 to 28.5 degrees north of the equator to the same distance south and back (the range itself shifts over an 18.6-year cycle). When the moon is far north, the bulge under it is centered well north of the equator and the opposite bulge well south. California, at 32 to 42 degrees north, passes through the thick part of one bulge and the thin edge of the other. One high is big, the other small; one low is deep, the other shallow.
That difference is the diurnal inequality, and it comes and goes on a two-week rhythm:
- Tropic tides, when the moon is near its farthest north or south, have the largest inequality: a huge higher high, a deep lower low, and a weak middle pair.
- Equatorial tides, when the moon crosses the equator, have the smallest: the two highs and the two lows come out nearly even.
Two waves added together
Tide scientists describe the same thing with harmonic constituents. The twice-daily constituents (M2 for the moon, S2 for the sun, and their relatives) make two equal bulges a day. The once-daily constituents (K1 and O1, mostly) raise one of the two and lower the other. Add them and you get the California tide.
The form factor
Oceanographers sort tides with one ratio, the form factor: F = (K1 + O1) ÷ (M2 + S2), the strength of the once-a-day constituents over the twice-a-day ones.
| Form factor | Tide type | Example |
|---|---|---|
| below 0.25 | Semidiurnal: two nearly equal tides a day | Much of the U.S. Atlantic coast |
| 0.25 to 1.5 | Mixed, mainly semidiurnal | The California coast |
| 1.5 to 3 | Mixed, mainly diurnal | Parts of the Gulf of Mexico |
| above 3 | Diurnal: one high and one low a day | Pensacola, Florida |
At Port San Luis, K1 is 1.17 ft and O1 0.73 ft; M2 is 1.61 ft and S2 0.49 ft. F = 1.90 ÷ 2.10 = 0.90: firmly mixed, with the once-a-day part almost as strong as the twice-a-day part. That is why the inequality is so striking here. San Francisco works out to 0.84. On days when the once-a-day part wins, usually around tropic tides, a California station can briefly show just one high and one low with a long, nearly flat stand between.
Spring and neap tides
Twice a month the sun and moon line up with Earth: at new moon, when they are on the same side, and at full moon, when they are on opposite sides. Their bulges stack, highs get higher and lows get lower, and the range is at its widest. These are spring tides (from an old sense of "spring" as "to leap", nothing to do with the season).
At the first and third quarter moons, the sun and moon pull at right angles. The sun’s bulges partly fill the moon’s troughs, and the range shrinks. These are neap tides: modest highs, modest lows, slow currents.
Spring tides usually peak a day or two after the new or full moon, not on it, because the ocean takes time to respond. That lag is called the age of the tide. On the West Coast the moon’s monthly north–south swing also matters, so the biggest day of a spring series can drift away from the moon phase. The 14-day calendar on Tide Intelligence calls SPRING and NEAP from each day’s actual range compared with the station’s average, not from the moon phase alone, for exactly this reason.
Perigee, apogee and king tides
The moon’s orbit is an ellipse. At perigee it is about 12% closer than at apogee, and its tidal pull is noticeably stronger. When perigee lands near a new or full moon, the result is a perigean spring tide, a few inches beyond a normal spring tide. Earth is also closest to the sun in early January (perihelion).
Put those together in winter and you get California’s king tides: the highest predicted tides of the year, usually in December and January, sometimes with matching minus tides in the afternoon. The California King Tides Project invites the public to photograph them as a preview of everyday water levels with future sea-level rise. A king tide with a winter storm on top is when low-lying streets, parking lots and beach access flood.
The tide travels north
The ocean does not simply bulge under the moon. Continents get in the way, so the tide moves around each ocean basin as a set of rotating waves pivoting on points of nearly zero range, called amphidromic points. Along the California coast, the tide behaves like a wave hugging the shore and travelling north with the coast on its right, a coastally trapped Kelvin wave.
You can see it in NOAA’s numbers. Each station has a high water interval: the average time from the moon crossing the Greenwich meridian to high water there.
On the open coast, high tide reaches La Jolla and San Diego first, Santa Barbara about half an hour later, Port San Luis almost an hour after San Diego, Monterey about 1⅓ hours after, Point Reyes about 1⅔, and Crescent City about 2⅓ hours after San Diego. Inside bays the tide arrives later still: San Francisco is nearly an hour behind Point Reyes and the bay entrance, because the tide has to squeeze through the Golden Gate first.
The range changes along the way too, from about 5¼ ft around Point Conception to almost 7 ft at Crescent City and Humboldt Bay.
The long cycles
A few slower rhythms sit underneath the daily and monthly ones:
- The seasons. Water expands as it warms, and the California Current shifts with the seasons, so mean sea level runs a few inches higher in late summer and fall than in spring. NOAA builds this into its predictions as the SA (solar annual) constituent; at Port San Luis it is worth about 0.23 ft.
- The nodal cycle. The tilt of the moon’s orbit swings over 18.6 years, which slowly widens and narrows the tropic tides. It is why tidal datums are averaged over a 19-year epoch (see MLLW and tide datums).
- Sea-level rise. Not a tide, but it moves the whole curve up a little every year.
Putting it to use
- Expect the lows to alternate. If this morning’s low is a deep minus tide, this evening’s will be shallow. The good tidepool low moves about 50 minutes later each day and eventually flips to the other half of the day.
- Watch the calendar for tropic and spring days together. That combination brings the deepest lows and highest highs of the month.
- Use the right station. Two beaches 100 miles apart can be half an hour or more apart in tide time.
- Treat the weird days as normal. A day with three turns, or a high and the next low only a foot apart, is how a mixed tide behaves, not an error.