Why do some coasts get two tides a day and others just one? The answer lies in the balance of astronomical tidal constituents. This tool classifies a tidal regime from the standard Courtier form factor and explains how the spring–neap cycle and king tides ride on top of the daily pattern, with real-world coastal examples.
How it works
Ocean tides are the sum of many constituents, each driven by a cycle of the Moon and Sun. Four dominate: the diurnal K1 and O1 (one cycle per day) and the semidiurnal M2 and S2 (two cycles per day). The form factor compares them:
F = (K1 + O1) / (M2 + S2)By the Courtier classification, F ≤ 0.25 is semidiurnal, 0.25–1.5 mixed mainly semidiurnal, 1.5–3.0 mixed mainly diurnal, and above 3.0 diurnal. Enter the four amplitudes from a port’s harmonic analysis and the tool names the regime.
The four regime types and where they occur
Semidiurnal (F ≤ 0.25): Two nearly equal high tides and two low tides each lunar day, roughly 12 hours 25 minutes apart. The Atlantic coasts of Europe and North America are textbook semidiurnal — ports like Southampton, Brest, and Halifax show this pattern. The M2 constituent dominates.
Mixed, mainly semidiurnal (0.25 < F ≤ 1.5): Two tides per day, but with noticeable inequality in height between the two highs or the two lows. Much of the US Pacific coast falls here. San Francisco has two high waters each day, but one is consistently higher than the other — the “higher high water” and “lower high water” you see in US tide tables.
Mixed, mainly diurnal (1.5 < F ≤ 3.0): The diurnal constituents dominate but the second tide each day is still present, just much smaller. This regime is common across the Gulf of Mexico and parts of Southeast Asia.
Diurnal (F > 3.0): One high and one low tide each lunar day, roughly 24 hours 50 minutes apart. The Gulf of Tonkin and parts of the Philippines and Indonesia show predominantly diurnal tides.
Spring, neap, and king tides
Independently of F, the spring–neap cycle modulates the tidal range over about 14 days: at new and full moon the Sun and Moon align and their gravitational pulls add, producing the largest ranges (spring tides). At the quarter moons the two bodies are at right angles and their pulls partly cancel, giving the smallest ranges (neap tides).
A king tide (perigean spring tide) occurs when a spring tide coincides with the Moon near perigee, its closest approach to Earth. The result is a larger-than-typical spring range. King tides are used by coastal planners to visualise the water levels that will become commonplace as sea level rises — they are often called “sea-level-rise previews.”
Where constituent amplitudes come from
Constituent amplitudes come from harmonic analysis of a long-running tide gauge record — typically one to several years of measurements — and are published by national hydrographic offices (NOAA in the US, the UK Hydrographic Office, SHOM in France, etc.). The K1, O1, M2, and S2 amplitudes are among the most commonly tabulated and are available for hundreds of ports worldwide. Local geography can then amplify or suppress the tidal range well beyond what the open-ocean constituents suggest — the Bay of Fundy’s funnel shape drives ranges over 15 metres while the Mediterranean’s enclosed basin has ranges under 0.5 metres despite similar constituent inputs.