Guides to how tides work
Tide School
Every question in the home page’s Tide questions, answered at length. How to read a tide table, where the numbers come from, why California gets two unequal tides a day, what 0.0 ft really means, and what to do when the sea ignores the forecast. Real NOAA numbers, diagrams drawn from them, and links into the live data.
5 lessons · about 36 min of reading · start anywhere
- 01 · Tide times8 minHow to read a tide tableHighs, lows, heights, Pacific Time and slack water: what every column means, how the water moves between the turns, and how to plan a tidepool walk, a paddle or a harbor run around it.Read
- 02 · Data sources7 minWhere tide data comes fromFrom a radar gauge on a pier to a prediction on your phone: NOAA’s water-level network, harmonic constituents, subordinate stations, wave buoys and weather models.Read
- 03 · Tide cycles7 minWhy California gets two unequal tides a dayMixed 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.Read
- 04 · Datums7 minMLLW and tide datums, explainedWhy 0.0 ft is not sea level, what MLLW, MHHW and MSL measure, how minus tides happen, and the real datum ladder for every California station.Read
- 05 · Accuracy & safety7 minWhen the tide doesn’t match the predictionAir pressure, wind, swell, El Niño, tsunamis and sea-level rise: why the water runs above or below the forecast, how to read the residual, and how to stay safe on the shore.Read
Glossary the words on the console
- MLLW
- Mean Lower Low Water: the average of the lower of each day’s two low tides. Tide heights are measured from it, so 0.0 ft is an average lower low.
- minus tide
- A low tide below 0.0 ft MLLW, lower than an average lower low. The best tidepooling.
- MSL
- Mean Sea Level: the average water level over NOAA’s 19-year tidal epoch.
- MHHW
- Mean Higher High Water: the average of the higher of each day’s two high tides. Water above it is an unusually high tide.
- slack
- The pause around high or low tide when the water barely moves before it turns.
- residual
- Observed water level minus NOAA’s prediction. Wind, air pressure and swell can hold the real tide above (+) or below (−) the forecast.
- period
- Seconds between wave crests. Longer-period swell (13 s and up) carries more energy and breaks bigger than its height suggests.