Tides
The water level under the wave
Tides move the water level up and down through the day. The same spot can be perfect at one tide and unsurfable two hours later.
The tide cycle
Most coastlines see two high tides and two low tides per day, roughly six hours apart. The water level oscillates smoothly between them, which means there are always two transition windows on every tide: rising and falling.
Low tide: shallower, sharper waves
At low tide there is less water over the sandbar or reef. The wave has to compress into a smaller column, so it stands up faster and breaks with more punch. Some spots become heavy or closeout-prone; others come alive only at low tide.
High tide: deeper, fuller waves
At high tide the water is deeper, so the wave does not feel the bottom as much. Waves come in fuller and softer. Some spots go totally flat at high tide; others get gentle and beginner-friendly.
Rising vs falling tide
A rising tide is adding water; a falling tide is draining it. The same mid-tide level can feel very different on the way up versus the way down. Rising tides often produce fuller, friendlier waves. Falling tides can sharpen the face but also speed up current near jetties, river mouths and passes.