Understanding California Tides

California's tides are mixed. Most days bring two high tides and two low tides, and one of each is much bigger than the other. At Port San Luis an average day swings 5.32 ft from the higher high to the lower low. That is NOAA's great diurnal range for the station, and it runs from 5.24 ft at San Simeon to 6.87 ft at Crescent City (the full table is in MLLW and tide datums).

What Causes Tides

Tides come from the gravitational pull of the moon and sun on the oceans, and most of the work is the moon's. Although the sun is far larger, it is also much farther away, so its tidal force is only about 46% of the moon's (NOAA rounds it to about half). When the sun and moon line up, at new and full moon, you get spring tides, with higher highs and lower lows.

Why the Two Lows Differ

Because the moon's orbit is tilted, for much of each month it sits well north or south of the equator. California then passes through the thick part of one tidal bulge and the thin edge of the other, and one low each day drops far below the other. Why California gets two unequal tides a day has the full explanation, with Port San Luis's own harmonic constituents.

See It on This Site

For the next two weeks of highs and lows, with the spring and neap tides marked, open the Port San Luis tide calendar. On the Coast Map you can watch high tide move up the coast. To read the numbers in either, start with How to read a tide table.