Not every coast has two tides a day. See the three main tide rhythms as 72-hour curves, compare them side by side, and learn which one your own water runs on.
Pick a tide type to see its three-day curve and what it means for you.
How it works
Why coasts differ
Why coasts differ
The tide is the sum of many astronomical waves. On some coasts the 12.42 h lunar wave dominates (semi-diurnal); on others the longer 24.8 h waves win (diurnal); on most Pacific shores they mix, producing two unequal highs a day (mixed).
The shapes you see
The semi-diurnal curve shows a neat, repeating double-up-and-down every day. The diurnal curve makes one long rise and fall per day. The mixed curve wiggles twice a day, but the two peaks are clearly different heights.
How to know your own coast
Record the high-tide times from a local table for three days. Two similar highs a day = semi-diurnal. One high a day, later by ~50 min each day = diurnal. Two unequal highs = mixed. Your answer decides whether you plan on a 12-hour or a 24-hour clock.
⚠️
Safety first. Tide predictions here use a simplified harmonic model for planning only and may differ from official charts. Always check the official local tide table, lifeguard advice and on-site signs before you go. The sea can turn quickly — if in doubt, turn back.