What is the tidal form factor?

The form factor F is the ratio of the two main diurnal constituents (K1 and O1) to the two main semidiurnal constituents (M2 and S2). It is the standard Courtier number used worldwide to classify whether a location has one or two tides per day.

What is the difference between semidiurnal and diurnal tides?

A semidiurnal regime (F below 0.25) has two nearly equal high and two low tides each lunar day. A diurnal regime (F above 3) has just one high and one low per day. Mixed regimes between them show two tides with noticeable inequality in height.

Why do spring and neap tides happen?

Spring tides occur at new and full moon when the Sun and Moon are aligned and their gravitational pulls add, giving the largest range. Neap tides occur at the quarter moons when the two bodies are at right angles and their pulls partly cancel, giving the smallest range.

A king tide is an especially high spring tide. It happens when a spring tide coincides with the Moon at perigee, its closest point to Earth, so the tidal range is even larger than usual. King tides preview the everyday water levels expected from sea-level rise.

Where do the constituent amplitudes come from?

They come from harmonic analysis of a tide gauge record, which separates the observed water level into dozens of constituents, each tied to an astronomical cycle. National hydrographic offices publish K1, O1, M2, and S2 amplitudes for ports, which you enter here.

Tidal Classification Reference

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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.

Spring, neap, and notes

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 tides are largest (spring), while at the quarter moons their pulls partly cancel and tides are smallest (neap). A spring tide near lunar perigee produces an exceptionally large king tide.

Constituent amplitudes come from harmonic analysis of a tide gauge and are published by national hydrographic offices. Local geography — funnel-shaped bays like the Bay of Fundy — can amplify the range dramatically beyond what F alone suggests.