Learn · The four levers · 6 min
How tides actually work
Which high tide? And why is it always later than yesterday?
Example break
Mon Oct 19 · 6amTomorrow at Ocean Beach, the two high tides are 4.6 ft at 8:27am and 4.7 ft at 6:35pm: 0.2 ft apart. Tomorrow 6am, the water is 3.7 ft and rising.
- Low tide12:42am0.7 ft
- High tide8:27am4.6 ft
- Low tide1:31pm3.2 ft
- High tide6:35pm4.7 ft
Forecast built Today 6:47pm. Highs and lows are this spot’s tide table, in feet above NAVD88.
Most surfers read a tide chart the way you read a bus timetable — find the number, note the time, move on. It is worth ten minutes to understand what generates that curve, because three of its features decide whether your spot works, and all three are predictable weeks ahead.
The short version: the West Coast gets two high tides and two low tides a day, they are unequal, and the whole pattern slides later each day. Each of those facts has a cause, and each one has a consequence in the water.
Two highs a day, and they are different heights
Coasts get one of three tidal patterns. Some places get a single high and a single low each day. Much of the US East Coast gets two of each, roughly equal. The US West Coast gets what oceanographers call a mixed semidiurnal tide: two highs and two lows a day, of noticeably different sizes.
That difference is called the diurnal inequality, and it traces to the Moon's tilt relative to Earth's equator, filtered through the fact that continents block the tidal bulges from circling freely and force each ocean basin into its own pattern. On this coast the inequality is large — often more than a foot between the two highs, sometimes much more.
Ocean Beach, 48 hours of water level
Consecutive highs happen to be within 0.2 ft of each other in this window, which is unusual on this coast and will not hold next week.
This is the practical consequence: 'high tide at 4pm' is not a specification. If your spot drowns out above four feet, then a 3.9-foot high is a session and a 5.6-foot high is a wasted drive, and both of them appear on the chart under the same word. Read the height, not the label.
Why everything is about fifty minutes later tomorrow
The tide follows the Moon, and the Moon does not hold still. In the time the Earth takes to spin once, the Moon has moved along in its orbit, so a point on Earth has to turn a little further to catch up with it. That extra catching-up takes about fifty minutes, which makes a lunar day 24 hours and 50 minutes long.
So the tide runs on a slightly slower clock than you do. Everything arrives roughly fifty minutes later each day, which is about six hours a week. If dawn patrol was on a perfect dropping tide this Saturday, next Saturday it will be nowhere near it — and this is exactly why 'it was great last weekend' is such an unreliable guide to a spot.
Spring and neap: the fortnightly envelope
On top of the daily rhythm there is a slower one. When the Sun and Moon line up — at new and full moon — their pulls add and the tide swings through a bigger range. When they sit at right angles, around the quarter moons, the pulls partly cancel and the range shrinks. The cycle between them is about two weeks.
Spring tides are the big-range ones, and the name has nothing to do with the season. They matter for surf in two directions at once: you get access to lower lows, which can switch on a shallow outer bank that normally has too much water over it, and you also get higher highs that can drown that same bank a few hours later. Neap weeks are the opposite — less range, less change, and a spot that stays roughly the same all day.
The rates matter too. A big spring range means the water is moving fast between the extremes, so the window at a tide-sensitive spot is short and you need to be there for it. A neap tide gives you a longer, lazier window at a smaller range.
Using a tide chart like a forecaster
- Read the height, not the wordTwo highs today, different heights. Your spot cares about the number of feet of water, not the label on the peak.
- Check the range, not just the levelA spring week moves fast — short windows, sharper changes. A neap week is forgiving.
- Add fifty minutes per dayWhatever worked today happens about fifty minutes later tomorrow, and six hours later next week.
- Match it to the spot, not the coastThe tide is regional; the window is local. A shallow bar and a steep reef a mile apart want opposite tides on the same day.
Solved at real breaks: Ocean Beach, Pleasure Point, Bolinas (The Channel). The physics behind these figures, its sources and its limits are written up in the methodology.