How to Read a Tide Chart
A tide chart is the single most useful tool for a San Diego beach day, and the easiest one to misread. Here is what the numbers mean and how to plan around them.
A mixed semidiurnal day, translated
San Diego has a mixed semidiurnal tide: two highs and two lows in roughly 24 hours, with pairs that almost never match. A morning high might reach 4.6 feet while the same afternoon's high stops near 2.9 feet. The lows behave the same way, and the second low of the day is often the lower one, so the chart itself tells you which low is worth the drive.
What the numbers are measured against
Heights on a NOAA chart for the California coast are measured against mean lower low water, written MLLW. It is the average of the lower of the two daily lows, the reference NOAA picked so that most lows land above zero. A negative number means the water sits below that line.
Negative lows open the widest beach and the most reef. San Diego gets them in most months, with the deepest ones in winter. A minus 1.2 foot low uncovers ground that stays underwater the rest of the year.
Spring tides, neap tides, and where the big lows cluster
The range between high and low grows and shrinks on a two-week cycle. Spring tides, the larger range, come near the new moon and the full moon. Neap tides, the smaller range, come near the quarter moons. The same beach can offer a generous low one week and a thin one the next, at the same hour.
The deepest daylight lows of the year cluster in winter, which is also when the coast is coldest and the swells are largest. Summer brings friendlier morning lows with less range, so a summer negative tide is a small event, not a given.
Which station, and which clock
Use the NOAA station closest to the beach you are actually visiting. This is the mistake that ruins the most trips. A prediction for La Jolla does not describe San Diego Bay, and a prediction for the bay does not describe the open coast. An app that says only "San Diego" is pulling from one station, and it might not be yours. Look the station up by name, check where it sits, and use it consistently.
Charts also come in two clock formats, 12 hour local and 24 hour. The numbers are the same but easy to mix up. A 0.3 foot high at 06:00 and a minus 1.0 foot low at 18:00 fall on the same day and belong to completely different trips. Before leaving the house:
- Name the station and confirm it matches the beach.
- Find the one low you are aiming for, and note whether it is the first or second of the day.
- Write the time down as a target, not a suggestion.
- Find the next high. That is your deadline.
Reading it for a real trip
Rocky intertidal areas such as the Point Loma shoreline are workable roughly an hour either side of low tide, and they open up properly only below about zero feet. A plus 2 foot low and a minus 1.5 foot low are not versions of the same trip; the second is a different place. On rocks, the height matters more than the hour.
A tide chart is not a surf report. Swell, wind, and tide decide conditions together, and a low tide under a big west swell exposes rocks in the middle of water you were planning to enjoy. Cold water and cold air stack up too, which is why the coast runs cooler than the rest of the county.
Plan the return leg before low water, not after. On a long beach walk or a route along the rocks, an incoming tide can close the way back sooner than expected, and the ankle deep stretch you crossed an hour ago becomes the part you cannot cross. The same caution applies on a bluff walk, where the turnaround time that leaves daylight to spare is the one to take.
Tide pool hours and any seasonal closures around Point Loma are set by Cabrillo National Monument, and fees or seasonal rules at state and city beaches come from the agency that runs each site. Check the current notice for the specific place before counting on it. A chart read once for one trip is a plan; a chart saved and reused for six months is a guess.