How tides actually work

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.

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.

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.

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.